<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:2b6255ed-2b10-4d22-9a68-28bc00154e60,g:a3fdbcfb-3a82-46d3-bc0c-2b8bb92a0a18-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:biib="http://www.biogenidec.com/20231231" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types1="http://www.xbrl.org/dtr/type/2020-01-21" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2022-03-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.biogenidec.com/20231231">
  <xs:import namespace="http://fasb.org/srt/2023" schemaLocation="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2023" schemaLocation="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd"/>
  <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="http://www.xbrl.org/2003/xlink-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2020-01-21" schemaLocation="https://www.xbrl.org/dtr/type/2020-01-21/types.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2022-03-31" schemaLocation="https://www.xbrl.org/dtr/type/2022-03-31/types.xsd"/>
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/country/2023" schemaLocation="https://xbrl.sec.gov/country/2023/country-2023.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/currency/2023" schemaLocation="https://xbrl.sec.gov/currency/2023/currency-2023.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2023" schemaLocation="https://xbrl.sec.gov/dei/2023/dei-2023.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/ecd/2023" schemaLocation="https://xbrl.sec.gov/ecd/2023/ecd-2023.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="biib-20231231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="biib-20231231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="biib-20231231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="biib-20231231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="CoverPage" roleURI="http://www.biogenidec.com/role/CoverPage">
        <link:definition>0000001 - Document - Cover Page</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AuditInformation" roleURI="http://www.biogenidec.com/role/AuditInformation">
        <link:definition>0000002 - Document - Audit Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofIncome" roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsofIncome">
        <link:definition>0000003 - Statement - Consolidated Statements of Income</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofComprehensiveIncome" roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsofComprehensiveIncome">
        <link:definition>0000004 - Statement - Consolidated Statements of Comprehensive Income</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofComprehensiveIncome_1" roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsofComprehensiveIncome_1">
        <link:definition>0000004 - Statement - Consolidated Statements of Comprehensive Income</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheets" roleURI="http://www.biogenidec.com/role/ConsolidatedBalanceSheets">
        <link:definition>0000005 - Statement - Consolidated Balance Sheets</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheetsParenthetical" roleURI="http://www.biogenidec.com/role/ConsolidatedBalanceSheetsParenthetical">
        <link:definition>0000006 - Statement - Consolidated Balance Sheets (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofCashFlows" roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsofCashFlows">
        <link:definition>0000007 - Statement - Consolidated Statements of Cash Flows</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofEquity" roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsofEquity">
        <link:definition>0000008 - Statement - Consolidated Statements of Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPolicies" roleURI="http://www.biogenidec.com/role/SummaryofSignificantAccountingPolicies">
        <link:definition>0000009 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Acquisitions" roleURI="http://www.biogenidec.com/role/Acquisitions">
        <link:definition>0000010 - Disclosure - Acquisitions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Dispositions" roleURI="http://www.biogenidec.com/role/Dispositions">
        <link:definition>0000011 - Disclosure - Dispositions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Restructuring" roleURI="http://www.biogenidec.com/role/Restructuring">
        <link:definition>0000012 - Disclosure - Restructuring</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Revenue" roleURI="http://www.biogenidec.com/role/Revenue">
        <link:definition>0000013 - Disclosure - Revenue</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Inventory" roleURI="http://www.biogenidec.com/role/Inventory">
        <link:definition>0000014 - Disclosure - Inventory</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwill" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwill">
        <link:definition>0000015 - Disclosure - Intangible Assets and Goodwill</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurements" roleURI="http://www.biogenidec.com/role/FairValueMeasurements">
        <link:definition>0000016 - Disclosure - Fair Value Measurements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstruments" roleURI="http://www.biogenidec.com/role/FinancialInstruments">
        <link:definition>0000017 - Disclosure - Financial Instruments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstruments" roleURI="http://www.biogenidec.com/role/DerivativeInstruments">
        <link:definition>0000018 - Disclosure - Derivative Instruments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantandEquipment" roleURI="http://www.biogenidec.com/role/PropertyPlantandEquipment">
        <link:definition>0000019 - Disclosure - Property, Plant and Equipment</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Leases" roleURI="http://www.biogenidec.com/role/Leases">
        <link:definition>0000020 - Disclosure - Leases</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Indebtedness" roleURI="http://www.biogenidec.com/role/Indebtedness">
        <link:definition>0000021 - Disclosure - Indebtedness</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Equity" roleURI="http://www.biogenidec.com/role/Equity">
        <link:definition>0000022 - Disclosure - Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EarningsperShare" roleURI="http://www.biogenidec.com/role/EarningsperShare">
        <link:definition>0000023 - Disclosure - Earnings per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPayments" roleURI="http://www.biogenidec.com/role/ShareBasedPayments">
        <link:definition>0000024 - Disclosure - Share-Based Payments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxes" roleURI="http://www.biogenidec.com/role/IncomeTaxes">
        <link:definition>0000025 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetail" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail">
        <link:definition>0000026 - Disclosure - Other Consolidated Financial Statement Detail</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationships" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationships">
        <link:definition>0000027 - Disclosure - Collaborative and Other Relationships</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinVariableInterestEntities" roleURI="http://www.biogenidec.com/role/InvestmentsinVariableInterestEntities">
        <link:definition>0000028 - Disclosure - Investments in Variable Interest Entities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Litigation" roleURI="http://www.biogenidec.com/role/Litigation">
        <link:definition>0000029 - Disclosure - Litigation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingencies" roleURI="http://www.biogenidec.com/role/CommitmentsandContingencies">
        <link:definition>0000030 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Guarantees" roleURI="http://www.biogenidec.com/role/Guarantees">
        <link:definition>0000031 - Disclosure - Guarantees</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EmployeeBenefitPlans" roleURI="http://www.biogenidec.com/role/EmployeeBenefitPlans">
        <link:definition>0000032 - Disclosure - Employee Benefit Plans</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformation" roleURI="http://www.biogenidec.com/role/SegmentInformation">
        <link:definition>0000033 - Disclosure - Segment Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPolicies" roleURI="http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesPolicies">
        <link:definition>9954471 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesTables" roleURI="http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesTables">
        <link:definition>9954472 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AcquisitionsTables" roleURI="http://www.biogenidec.com/role/AcquisitionsTables">
        <link:definition>9954473 - Disclosure - Acquisitions (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringTables" roleURI="http://www.biogenidec.com/role/RestructuringTables">
        <link:definition>9954474 - Disclosure - Restructuring (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueTables" roleURI="http://www.biogenidec.com/role/RevenueTables">
        <link:definition>9954475 - Disclosure - Revenue (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryTables" roleURI="http://www.biogenidec.com/role/InventoryTables">
        <link:definition>9954476 - Disclosure - Inventory (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillTables" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillTables">
        <link:definition>9954477 - Disclosure - Intangible Assets and Goodwill (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsTables" roleURI="http://www.biogenidec.com/role/FairValueMeasurementsTables">
        <link:definition>9954478 - Disclosure - Fair Value Measurements (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsTables" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsTables">
        <link:definition>9954479 - Disclosure - Financial Instruments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsTables" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsTables">
        <link:definition>9954480 - Disclosure - Derivative Instruments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantandEquipmentTables" roleURI="http://www.biogenidec.com/role/PropertyPlantandEquipmentTables">
        <link:definition>9954481 - Disclosure - Property, Plant and Equipment (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesTables" roleURI="http://www.biogenidec.com/role/LeasesTables">
        <link:definition>9954482 - Disclosure - Leases (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IndebtednessTables" roleURI="http://www.biogenidec.com/role/IndebtednessTables">
        <link:definition>9954483 - Disclosure - Indebtedness (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityTables" roleURI="http://www.biogenidec.com/role/EquityTables">
        <link:definition>9954484 - Disclosure - Equity (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EarningsperShareTables" roleURI="http://www.biogenidec.com/role/EarningsperShareTables">
        <link:definition>9954485 - Disclosure - Earnings per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsTables" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsTables">
        <link:definition>9954486 - Disclosure - Share-Based Payments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesTables" roleURI="http://www.biogenidec.com/role/IncomeTaxesTables">
        <link:definition>9954487 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailTables" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables">
        <link:definition>9954488 - Disclosure - Other Consolidated Financial Statement Detail (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsTables" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsTables">
        <link:definition>9954489 - Disclosure - Collaborative and Other Relationships (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformationTables" roleURI="http://www.biogenidec.com/role/SegmentInformationTables">
        <link:definition>9954490 - Disclosure - Segment Information (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesNarrativeDetails" roleURI="http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails">
        <link:definition>9954491 - Disclosure - Summary of Significant Accounting Policies - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesSummaryofPropertyPlantandEquipmentEstimatedUsefulLivesDetails" roleURI="http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesSummaryofPropertyPlantandEquipmentEstimatedUsefulLivesDetails">
        <link:definition>9954492 - Disclosure - Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment Estimated Useful Lives (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AcquisitionsNarrativeDetails" roleURI="http://www.biogenidec.com/role/AcquisitionsNarrativeDetails">
        <link:definition>9954493 - Disclosure - Acquisitions -Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AcquisitionsPurchasePriceConsiderationDetails" roleURI="http://www.biogenidec.com/role/AcquisitionsPurchasePriceConsiderationDetails">
        <link:definition>9954494 - Disclosure - Acquisitions - Purchase Price Consideration (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AcquisitionsPreliminaryPurchasePriceAllocationDetails" roleURI="http://www.biogenidec.com/role/AcquisitionsPreliminaryPurchasePriceAllocationDetails">
        <link:definition>9954495 - Disclosure - Acquisitions Preliminary Purchase Price Allocation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DispositionsDetails" roleURI="http://www.biogenidec.com/role/DispositionsDetails">
        <link:definition>9954496 - Disclosure - Dispositions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringNarrativeDetails" roleURI="http://www.biogenidec.com/role/RestructuringNarrativeDetails">
        <link:definition>9954497 - Disclosure - Restructuring - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringBusinessTransformationandOtherCostSavingInitiativesFitforGrowthInitiativeDetails" roleURI="http://www.biogenidec.com/role/RestructuringBusinessTransformationandOtherCostSavingInitiativesFitforGrowthInitiativeDetails">
        <link:definition>9954498 - Disclosure - Restructuring, Business Transformation and Other Cost Saving Initiatives - Fit for Growth Initiative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringBusinessTransformationandOtherCostSavingInitiativesTotalChargescostsavinginitiativeDetails" roleURI="http://www.biogenidec.com/role/RestructuringBusinessTransformationandOtherCostSavingInitiativesTotalChargescostsavinginitiativeDetails">
        <link:definition>9954499 - Disclosure - Restructuring, Business Transformation and Other Cost Saving Initiatives - Total Charges cost saving initiative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringRestructuringReserveRollForwardDetails" roleURI="http://www.biogenidec.com/role/RestructuringRestructuringReserveRollForwardDetails">
        <link:definition>9954500 - Disclosure - Restructuring - Restructuring Reserve Roll Forward (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueRevenuesbyProductDetails" roleURI="http://www.biogenidec.com/role/RevenueRevenuesbyProductDetails">
        <link:definition>9954501 - Disclosure - Revenue - Revenues by Product (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueNarrativeDetails" roleURI="http://www.biogenidec.com/role/RevenueNarrativeDetails">
        <link:definition>9954502 - Disclosure - Revenue - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueReservesforDiscountsandAllowancesDetails" roleURI="http://www.biogenidec.com/role/RevenueReservesforDiscountsandAllowancesDetails">
        <link:definition>9954503 - Disclosure - Revenue - Reserves for Discounts and Allowances (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueTotalReservesforDiscountsandAllowancesDetails" roleURI="http://www.biogenidec.com/role/RevenueTotalReservesforDiscountsandAllowancesDetails">
        <link:definition>9954504 - Disclosure - Revenue - Total Reserves for Discounts and Allowances (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueRevenuesfromAntiCD20TherapeuticProgramsDetails" roleURI="http://www.biogenidec.com/role/RevenueRevenuesfromAntiCD20TherapeuticProgramsDetails">
        <link:definition>9954505 - Disclosure - Revenue - Revenues from Anti-CD20 Therapeutic Programs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueOtherRevenuesDetails" roleURI="http://www.biogenidec.com/role/RevenueOtherRevenuesDetails">
        <link:definition>9954506 - Disclosure - Revenue - Other Revenues (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryComponentsofInventoryDetails" roleURI="http://www.biogenidec.com/role/InventoryComponentsofInventoryDetails">
        <link:definition>9954507 - Disclosure - Inventory - Components of Inventory (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryNarrativeDetails" roleURI="http://www.biogenidec.com/role/InventoryNarrativeDetails">
        <link:definition>9954508 - Disclosure - Inventory - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillSummaryofIntangibleAssetsDetails" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillSummaryofIntangibleAssetsDetails">
        <link:definition>9954509 - Disclosure - Intangible Assets and Goodwill - Summary of Intangible Assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillSummaryofIntangibleAssetsDetails_1" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillSummaryofIntangibleAssetsDetails_1">
        <link:definition>9954509 - Disclosure - Intangible Assets and Goodwill - Summary of Intangible Assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillNarrativeDetails" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillNarrativeDetails">
        <link:definition>9954510 - Disclosure - Intangible Assets and Goodwill - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillEstimatedFutureAmortizationofIntangibleAssetsDetails" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillEstimatedFutureAmortizationofIntangibleAssetsDetails">
        <link:definition>9954511 - Disclosure - Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleAssetsandGoodwillChangesinGoodwillDetails" roleURI="http://www.biogenidec.com/role/IntangibleAssetsandGoodwillChangesinGoodwillDetails">
        <link:definition>9954512 - Disclosure - Intangible Assets and Goodwill - Changes in Goodwill (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsSummaryofAssetsandLiabilitiesRecordedatFairValueDetails" roleURI="http://www.biogenidec.com/role/FairValueMeasurementsSummaryofAssetsandLiabilitiesRecordedatFairValueDetails">
        <link:definition>9954513 - Disclosure - Fair Value Measurements - Summary of Assets and Liabilities Recorded at Fair Value (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsNarrativeDetails" roleURI="http://www.biogenidec.com/role/FairValueMeasurementsNarrativeDetails">
        <link:definition>9954514 - Disclosure - Fair Value Measurements - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsFairValueofContingentConsiderationObligationsDetails" roleURI="http://www.biogenidec.com/role/FairValueMeasurementsFairValueofContingentConsiderationObligationsDetails">
        <link:definition>9954515 - Disclosure - Fair Value Measurements - Fair Value of Contingent Consideration Obligations (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsSummaryofFairandCarryingValueofDebtInstrumentsDetails" roleURI="http://www.biogenidec.com/role/FairValueMeasurementsSummaryofFairandCarryingValueofDebtInstrumentsDetails">
        <link:definition>9954516 - Disclosure - Fair Value Measurements - Summary of Fair and Carrying Value of Debt Instruments (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsSummaryofFinancialAssetswithMaturitiesDetails" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsSummaryofFinancialAssetswithMaturitiesDetails">
        <link:definition>9954517 - Disclosure - Financial Instruments - Summary of Financial Assets with Maturities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsSummaryofMarketableSecuritiesDetails" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsSummaryofMarketableSecuritiesDetails">
        <link:definition>9954518 - Disclosure - Financial Instruments - Summary of Marketable Securities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsSummaryofContractualMaturitiesDetails" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsSummaryofContractualMaturitiesDetails">
        <link:definition>9954519 - Disclosure - Financial Instruments - Summary of Contractual Maturities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsNarrativeDetails" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsNarrativeDetails">
        <link:definition>9954520 - Disclosure - Financial Instruments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInstrumentsProceedsfromMarketableDebtSecuritiesDetails" roleURI="http://www.biogenidec.com/role/FinancialInstrumentsProceedsfromMarketableDebtSecuritiesDetails">
        <link:definition>9954521 - Disclosure - Financial Instruments - Proceeds from Marketable Debt Securities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsNarrativeDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsNarrativeDetails">
        <link:definition>9954522 - Disclosure - Derivative Instruments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsForeignCurrencyForwardContractsEnteredintoHedgeForecastedRevenuesDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsForeignCurrencyForwardContractsEnteredintoHedgeForecastedRevenuesDetails">
        <link:definition>9954523 - Disclosure - Derivative Instruments - Foreign Currency Forward Contracts Entered into Hedge Forecasted Revenues (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsSummaryofUnrealizedGainLossonDerivativeInstrumentsIncludedinAOCIDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsSummaryofUnrealizedGainLossonDerivativeInstrumentsIncludedinAOCIDetails">
        <link:definition>9954524 - Disclosure - Derivative Instruments - Summary of Unrealized Gain (Loss) on Derivative Instruments Included in AOCI (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsSummaryoftheEffectofDerivativesDesignatedasCashFlowHedgingInstrumentsDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsSummaryoftheEffectofDerivativesDesignatedasCashFlowHedgingInstrumentsDetails">
        <link:definition>9954525 - Disclosure - Derivative Instruments - Summary of the Effect of Derivatives Designated as Cash Flow Hedging Instruments (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsSummaryoftheEffectofDerivativesDesignatedasNetInvestmentHedgingInstrumentsDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsSummaryoftheEffectofDerivativesDesignatedasNetInvestmentHedgingInstrumentsDetails">
        <link:definition>9954526 - Disclosure - Derivative Instruments - Summary of the Effect of Derivatives Designated as Net Investment Hedging Instruments (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DerivativeInstrumentsSummaryoftheFairValueforourOutstandingDerivativesDetails" roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsSummaryoftheFairValueforourOutstandingDerivativesDetails">
        <link:definition>9954527 - Disclosure - Derivative Instruments - Summary of the Fair Value for our Outstanding Derivatives (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantandEquipmentComponentsofpropertyplantandequipmentnetDetails" roleURI="http://www.biogenidec.com/role/PropertyPlantandEquipmentComponentsofpropertyplantandequipmentnetDetails">
        <link:definition>9954528 - Disclosure - Property, Plant and Equipment - Components of property, plant, and equipment, net (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantandEquipmentNarrativeDetails" roleURI="http://www.biogenidec.com/role/PropertyPlantandEquipmentNarrativeDetails">
        <link:definition>9954529 - Disclosure - Property, Plant and Equipment - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesNarrativeDetails" roleURI="http://www.biogenidec.com/role/LeasesNarrativeDetails">
        <link:definition>9954530 - Disclosure - Leases - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesScheduleofOperatingLeasesDetails" roleURI="http://www.biogenidec.com/role/LeasesScheduleofOperatingLeasesDetails">
        <link:definition>9954531 - Disclosure - Leases - Schedule of Operating Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseCostsDetails" roleURI="http://www.biogenidec.com/role/LeasesOperatingLeaseCostsDetails">
        <link:definition>9954532 - Disclosure - Leases - Operating Lease Costs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseLiabilityMaturityDetails" roleURI="http://www.biogenidec.com/role/LeasesOperatingLeaseLiabilityMaturityDetails">
        <link:definition>9954533 - Disclosure - Leases - Operating Lease Liability Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseLiabilityMaturityDetails_1" roleURI="http://www.biogenidec.com/role/LeasesOperatingLeaseLiabilityMaturityDetails_1">
        <link:definition>9954533 - Disclosure - Leases - Operating Lease Liability Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseWeightedAverageRemainingTermandDiscountRateDetails" roleURI="http://www.biogenidec.com/role/LeasesOperatingLeaseWeightedAverageRemainingTermandDiscountRateDetails">
        <link:definition>9954534 - Disclosure - Leases - Operating Lease Weighted Average Remaining Term and Discount Rate (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesOperatingLeaseSupplementalCashFlowDisclosureDetails" roleURI="http://www.biogenidec.com/role/LeasesOperatingLeaseSupplementalCashFlowDisclosureDetails">
        <link:definition>9954535 - Disclosure - Leases - Operating Lease Supplemental Cash Flow Disclosure (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IndebtednessSummaryofIndebtednessDetails" roleURI="http://www.biogenidec.com/role/IndebtednessSummaryofIndebtednessDetails">
        <link:definition>9954536 - Disclosure - Indebtedness - Summary of Indebtedness (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IndebtednessNarrativeDetails" roleURI="http://www.biogenidec.com/role/IndebtednessNarrativeDetails">
        <link:definition>9954537 - Disclosure - Indebtedness - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IndebtednessTotalDebtMaturitiesDetails" roleURI="http://www.biogenidec.com/role/IndebtednessTotalDebtMaturitiesDetails">
        <link:definition>9954538 - Disclosure - Indebtedness - Total Debt Maturities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityNarrativeDetails" roleURI="http://www.biogenidec.com/role/EquityNarrativeDetails">
        <link:definition>9954539 - Disclosure - Equity - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquitySummaryofCommonStockDetails" roleURI="http://www.biogenidec.com/role/EquitySummaryofCommonStockDetails">
        <link:definition>9954540 - Disclosure - Equity - Summary of Common Stock (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityAccumulatedOtherComprehensiveIncomeLossDetails" roleURI="http://www.biogenidec.com/role/EquityAccumulatedOtherComprehensiveIncomeLossDetails">
        <link:definition>9954541 - Disclosure - Equity - Accumulated Other Comprehensive Income (Loss) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityReclassificationoutofAccumulatedOtherComprehensiveIncomeDetails" roleURI="http://www.biogenidec.com/role/EquityReclassificationoutofAccumulatedOtherComprehensiveIncomeDetails">
        <link:definition>9954542 - Disclosure - Equity - Reclassification out of Accumulated Other Comprehensive Income (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EarningsperShareBasicandDilutedEarningsPerShareDetails" roleURI="http://www.biogenidec.com/role/EarningsperShareBasicandDilutedEarningsPerShareDetails">
        <link:definition>9954543 - Disclosure - Earnings per Share - Basic and Diluted Earnings Per Share (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EarningsperShareNarrativeDetails" roleURI="http://www.biogenidec.com/role/EarningsperShareNarrativeDetails">
        <link:definition>9954544 - Disclosure - Earnings per Share - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsSharebasedCompensationExpenseIncludedinConsolidationStatementsofIncomeDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsSharebasedCompensationExpenseIncludedinConsolidationStatementsofIncomeDetails">
        <link:definition>9954545 - Disclosure - Share-Based Payments - Share-based Compensation Expense Included in Consolidation Statements of Income (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsSummaryofSharebasedCompensationExpenseAssociatedwithEachSharebasedCompensatingProgramsDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsSummaryofSharebasedCompensationExpenseAssociatedwithEachSharebasedCompensatingProgramsDetails">
        <link:definition>9954546 - Disclosure - Share-Based Payments - Summary of Share-based Compensation Expense Associated with Each Share-based Compensating Programs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsNarrativeDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsNarrativeDetails">
        <link:definition>9954547 - Disclosure - Share-Based Payments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsEmployeeStockOptionsDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsEmployeeStockOptionsDetails">
        <link:definition>9954548 - Disclosure - Share-Based Payments - Employee Stock Options (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsMarketStockUnitsActivityDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsMarketStockUnitsActivityDetails">
        <link:definition>9954549 - Disclosure - Share-Based Payments - Market Stock Units Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsAssumptionsUsedinValuationofMarketBasedStockUnitsDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsAssumptionsUsedinValuationofMarketBasedStockUnitsDetails">
        <link:definition>9954550 - Disclosure - Share-Based Payments - Assumptions Used in Valuation of Market Based Stock Units (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsPerformanceStockUnitsSettledinStockActivityDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsPerformanceStockUnitsSettledinStockActivityDetails">
        <link:definition>9954551 - Disclosure - Share-Based Payments - Performance Stock Units Settled in Stock Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsAssumptionsUsedinValuationofPerformanceSharesDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsAssumptionsUsedinValuationofPerformanceSharesDetails">
        <link:definition>9954552 - Disclosure - Share-Based Payments - Assumptions Used in Valuation of Performance Shares (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsPerformanceStockUnitsSettledinCashActivityDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsPerformanceStockUnitsSettledinCashActivityDetails">
        <link:definition>9954553 - Disclosure - Share-Based Payments - Performance Stock Units Settled in Cash Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsTimeVestedRestrictedStockUnitsActivityDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsTimeVestedRestrictedStockUnitsActivityDetails">
        <link:definition>9954554 - Disclosure - Share-Based Payments - Time-Vested Restricted Stock Units Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ShareBasedPaymentsSharesIssuedUnderEmployeeStockPurchasePlanDetails" roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsSharesIssuedUnderEmployeeStockPurchasePlanDetails">
        <link:definition>9954555 - Disclosure - Share-Based Payments - Shares Issued Under Employee Stock Purchase Plan (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesIncomeBeforeIncomeTaxProvisionandtheIncomeTaxExpenseDetails" roleURI="http://www.biogenidec.com/role/IncomeTaxesIncomeBeforeIncomeTaxProvisionandtheIncomeTaxExpenseDetails">
        <link:definition>9954556 - Disclosure - Income Taxes - Income Before Income Tax Provision and the Income Tax Expense (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesNarrativeDetails" roleURI="http://www.biogenidec.com/role/IncomeTaxesNarrativeDetails">
        <link:definition>9954557 - Disclosure - Income Taxes - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesComponentsofDeferredTaxAssetsandLiabilitiesDetails" roleURI="http://www.biogenidec.com/role/IncomeTaxesComponentsofDeferredTaxAssetsandLiabilitiesDetails">
        <link:definition>9954558 - Disclosure - Income Taxes - Components of Deferred Tax Assets and Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesTaxRateDetails" roleURI="http://www.biogenidec.com/role/IncomeTaxesTaxRateDetails">
        <link:definition>9954559 - Disclosure - Income Taxes - Tax Rate (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesAccountingforUncertaintyinIncomeTaxesDetails" roleURI="http://www.biogenidec.com/role/IncomeTaxesAccountingforUncertaintyinIncomeTaxesDetails">
        <link:definition>9954560 - Disclosure - Income Taxes - Accounting for Uncertainty in Income Taxes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailSupplementalCashFlowInformationDetails" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailSupplementalCashFlowInformationDetails">
        <link:definition>9954561 - Disclosure - Other Consolidated Financial Statement Detail - Supplemental Cash Flow Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailOtherIncomeExpenseNetDetails" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailOtherIncomeExpenseNetDetails">
        <link:definition>9954562 - Disclosure - Other Consolidated Financial Statement Detail - Other Income (Expense), Net (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailNarrativeDetails" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailNarrativeDetails">
        <link:definition>9954563 - Disclosure - Other Consolidated Financial Statement Detail - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailGainLossonInvestmentsDetails" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailGainLossonInvestmentsDetails">
        <link:definition>9954564 - Disclosure - Other Consolidated Financial Statement Detail - Gain (Loss) on Investments (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OtherConsolidatedFinancialStatementDetailAccruedExpenseandOtherDetails" roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailAccruedExpenseandOtherDetails">
        <link:definition>9954565 - Disclosure - Other Consolidated Financial Statement Detail - Accrued Expense and Other (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsOCREVUSDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsOCREVUSDetails">
        <link:definition>9954566 - Disclosure - Collaborative and Other Relationships - OCREVUS (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsMosunetuzumabDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsMosunetuzumabDetails">
        <link:definition>9954567 - Disclosure - Collaborative and Other Relationships - Mosunetuzumab (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsCopromotionProfitSharingFormulaDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsCopromotionProfitSharingFormulaDetails">
        <link:definition>9954568 - Disclosure - Collaborative and Other Relationships - Co-promotion Profit Sharing Formula (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsProfitSharingDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsProfitSharingDetails">
        <link:definition>9954569 - Disclosure - Collaborative and Other Relationships - Profit Sharing (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsPretaxProfitSharingFormulaDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsPretaxProfitSharingFormulaDetails">
        <link:definition>9954570 - Disclosure - Collaborative and Other Relationships - Pretax Profit Sharing Formula (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsRevenuesfromAntiCD20TherapeuticProgramsDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsRevenuesfromAntiCD20TherapeuticProgramsDetails">
        <link:definition>9954571 - Disclosure - Collaborative and Other Relationships - Revenues from Anti-CD20 Therapeutic Programs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsIonisDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsIonisDetails">
        <link:definition>9954572 - Disclosure - Collaborative and Other Relationships - Ionis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsEisaiDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsEisaiDetails">
        <link:definition>9954573 - Disclosure - Collaborative and Other Relationships - Eisai (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoBAN2401andElenbecestatCollaborationDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoBAN2401andElenbecestatCollaborationDetails">
        <link:definition>9954574 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to BAN2401 and Elenbecestat Collaboration (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsADUHELMCollaborationAgreementDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsADUHELMCollaborationAgreementDetails">
        <link:definition>9954575 - Disclosure - Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoAducanumabCollaborationDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoAducanumabCollaborationDetails">
        <link:definition>9954576 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to Aducanumab Collaboration (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedtotheUCBCollaborationDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtotheUCBCollaborationDetails">
        <link:definition>9954577 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsAlkermesDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsAlkermesDetails">
        <link:definition>9954578 - Disclosure - Collaborative and Other Relationships - Alkermes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsAcordaDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsAcordaDetails">
        <link:definition>9954579 - Disclosure - Collaborative and Other Relationships - Acorda (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsOtherArrangementsDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsOtherArrangementsDetails">
        <link:definition>9954580 - Disclosure - Collaborative and Other Relationships - Other Arrangements (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoSageTherapeuticsDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoSageTherapeuticsDetails">
        <link:definition>9954581 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to Sage Therapeutics (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedToDenaliTherapeuticsDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedToDenaliTherapeuticsDetails">
        <link:definition>9954582 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related To Denali Therapeutics (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoSangamoTherapeuticsDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoSangamoTherapeuticsDetails">
        <link:definition>9954583 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to Sangamo Therapeutics (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CollaborativeandOtherRelationshipsSamsungBioepisDetails" roleURI="http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSamsungBioepisDetails">
        <link:definition>9954584 - Disclosure - Collaborative and Other Relationships - Samsung Bioepis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinVariableInterestEntitiesDetails" roleURI="http://www.biogenidec.com/role/InvestmentsinVariableInterestEntitiesDetails">
        <link:definition>9954585 - Disclosure - Investments in Variable Interest Entities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LitigationDetails" roleURI="http://www.biogenidec.com/role/LitigationDetails">
        <link:definition>9954586 - Disclosure - Litigation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingenciesDetails" roleURI="http://www.biogenidec.com/role/CommitmentsandContingenciesDetails">
        <link:definition>9954587 - Disclosure - Commitments and Contingencies (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EmployeeBenefitPlansDetails" roleURI="http://www.biogenidec.com/role/EmployeeBenefitPlansDetails">
        <link:definition>9954588 - Disclosure - Employee Benefit Plans (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformationNarrativeDetails" roleURI="http://www.biogenidec.com/role/SegmentInformationNarrativeDetails">
        <link:definition>9954589 - Disclosure - Segment Information - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformationGeographicInformationDetails" roleURI="http://www.biogenidec.com/role/SegmentInformationGeographicInformationDetails">
        <link:definition>9954590 - Disclosure - Segment Information - Geographic Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately" abstract="false" name="PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AverageMaturityOfMarketableSecurities" abstract="false" name="AverageMaturityOfMarketableSecurities" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_DeferredtaxliabilitiesGILTItaxcalculation" abstract="false" name="DeferredtaxliabilitiesGILTItaxcalculation" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed2.0Billion" abstract="false" name="FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed2.0Billion" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_AdministrativeSpaceMember" abstract="true" name="AdministrativeSpaceMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BusinessAcquisitionNumberOfCommonStockSharesAcquired" abstract="false" name="BusinessAcquisitionNumberOfCommonStockSharesAcquired" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="biib_ContractualAdjustmentsMember" abstract="true" name="ContractualAdjustmentsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_DistributorTwoMember" abstract="true" name="DistributorTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AverageStockPriceOnGrantDateInDollarsMinimum" abstract="false" name="AverageStockPriceOnGrantDateInDollarsMinimum" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:perShareItemType"/>
  <xs:element id="biib_A225SeniorNotesDueMay12030Member" abstract="true" name="A225SeniorNotesDueMay12030Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_WeightedAveragePeriodToRecognizeCostOfNonvestedAwards" abstract="false" name="WeightedAveragePeriodToRecognizeCostOfNonvestedAwards" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_PretaxProfitSharingFormulaTableTextBlock" abstract="false" name="PretaxProfitSharingFormulaTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo2.0Billion" abstract="false" name="FutureContingentPaymentForAnnualWorldwideNetSalesUpTo2.0Billion" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_UCBPharmaSAMember" abstract="true" name="UCBPharmaSAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NoncontrollingInterestIncreaseDecreaseOther" abstract="false" name="NoncontrollingInterestIncreaseDecreaseOther" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_A315SeniorNotesDueMay12050Member" abstract="true" name="A315SeniorNotesDueMay12050Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BardoxoloneSecuritiesMember" abstract="true" name="BardoxoloneSecuritiesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_Purchaseofcommonstock" abstract="false" name="Purchaseofcommonstock" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_LEQEMBICollaborationMember" abstract="true" name="LEQEMBICollaborationMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_Collaborationexpensesaccrual" abstract="false" name="Collaborationexpensesaccrual" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DenaliAndSangamoMember" abstract="true" name="DenaliAndSangamoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_NumberOfSharesOfCommonStockForIssuance" abstract="false" name="NumberOfSharesOfCommonStockForIssuance" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="biib_NumberOfDaysForCalculationOfAverageClosingStockPrice" abstract="false" name="NumberOfDaysForCalculationOfAverageClosingStockPrice" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch" abstract="false" name="AccruedExpensesOnFundingCommitmentRelatedToClinicalResearch" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_Totaldeferredchargesandprepaidtaxes" abstract="false" name="Totaldeferredchargesandprepaidtaxes" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_NumberOfWholesalers" abstract="false" name="NumberOfWholesalers" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_TermOfCollaborationAgreement" abstract="false" name="TermOfCollaborationAgreement" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_SummaryOfActivityRelatedToTheUCBCollaborationTableTextBlock" abstract="false" name="SummaryOfActivityRelatedToTheUCBCollaborationTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_NumberOfSquareFeet" abstract="false" name="NumberOfSquareFeet" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:areaItemType"/>
  <xs:element id="biib_ReductionInRoyaltyRate" abstract="false" name="ReductionInRoyaltyRate" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_PaymentsToAcquireInProcessResearchAndDevelopmentOperatingActivities" abstract="false" name="PaymentsToAcquireInProcessResearchAndDevelopmentOperatingActivities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SummaryOfActivityRelatedToSageTherapeuticsTableTextBlock" abstract="false" name="SummaryOfActivityRelatedToSageTherapeuticsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_SubleaseMember" abstract="true" name="SubleaseMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AssumptionsUsedInValuationOfMarketBasedStockUnitsTableTextBlock" abstract="false" name="AssumptionsUsedInValuationOfMarketBasedStockUnitsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_RelatedPartyCostsChangeOfControlReductionInCostSharingPercentage" abstract="false" name="RelatedPartyCostsChangeOfControlReductionInCostSharingPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_DueFromUnconsolidatedJointBusiness" abstract="false" name="DueFromUnconsolidatedJointBusiness" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AccruedPaymentToTerminationAgreement" abstract="false" name="AccruedPaymentToTerminationAgreement" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_BENEPALIMember" abstract="true" name="BENEPALIMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_SummaryofActivityRelatedtoAducanumabCollaborationTableTextBlock" abstract="false" name="SummaryofActivityRelatedtoAducanumabCollaborationTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_WarehouseUtilitiesAndSupportSpaceMember" abstract="true" name="WarehouseUtilitiesAndSupportSpaceMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost" abstract="false" name="AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsAmortizedCost" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SharesIssuedUnderEmployeeStockPurchasePlanTableTextBlock" abstract="false" name="SharesIssuedUnderEmployeeStockPurchasePlanTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_AuditorInformationAbstract" abstract="true" name="AuditorInformationAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="biib_CompletedTechnologyMember" abstract="true" name="CompletedTechnologyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_FutureContingentPaymentThreshold" abstract="false" name="FutureContingentPaymentThreshold" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" abstract="false" name="MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_IonisAndSageMember" abstract="true" name="IonisAndSageMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_OtherrevenuesTableTextBlock" abstract="false" name="OtherrevenuesTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_FacilityTypeDomain" abstract="true" name="FacilityTypeDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BIIB054Member" abstract="true" name="BIIB054Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_Royaltyoperatingprofitthresholdforhighestroyaltyratepercentage" abstract="false" name="Royaltyoperatingprofitthresholdforhighestroyaltyratepercentage" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_InterestInSubsidiary" abstract="false" name="InterestInSubsidiary" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssetsAndLiabilitiesNet" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssetsAndLiabilitiesNet" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DeferredTaxLiabilitiesDepreciationAmortizationOther" abstract="false" name="DeferredTaxLiabilitiesDepreciationAmortizationOther" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FacilityLocationAxis" abstract="true" name="FacilityLocationAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_RareDiseaseProductRevenueMember" abstract="true" name="RareDiseaseProductRevenueMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_OtherMember" abstract="true" name="OtherMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_CollaborativeArrangementProfitLossSharing" abstract="false" name="CollaborativeArrangementProfitLossSharing" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AccountsReceivableFromCollaborator" abstract="false" name="AccountsReceivableFromCollaborator" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SangamoTherapeuticsIncAgreementMember" abstract="true" name="SangamoTherapeuticsIncAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NonCurrentPortionOfNotesPayableAndTermLoanMember" abstract="true" name="NonCurrentPortionOfNotesPayableAndTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_BIIB111Member" abstract="true" name="BIIB111Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AccountsPayableToCollaborator" abstract="false" name="AccountsPayableToCollaborator" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RocheGroupGenentechMember" abstract="true" name="RocheGroupGenentechMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EquitySecuritiesNonCurrentMember" abstract="true" name="EquitySecuritiesNonCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ExpectedLicenseFeeAndRegulatoryMilestonePayments" abstract="false" name="ExpectedLicenseFeeAndRegulatoryMilestonePayments" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PaymentsToAcquireBusinessesGrossOutstandingEquityAwards" abstract="false" name="PaymentsToAcquireBusinessesGrossOutstandingEquityAwards" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_TimeVestedRestrictedStockUnitsActivityTableTextBlock" abstract="false" name="TimeVestedRestrictedStockUnitsActivityTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_Expensereflectedwithinstatementsofincome" abstract="false" name="Expensereflectedwithinstatementsofincome" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FutureRoyaltiesPercentageToBeReceivedOnSaleOfProduct" abstract="false" name="FutureRoyaltiesPercentageToBeReceivedOnSaleOfProduct" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_DistributorOneMember" abstract="true" name="DistributorOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PaymentsMadeToTerminationAgreement" abstract="false" name="PaymentsMadeToTerminationAgreement" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RepaymentsOfDebtIncludingPremiumsPaid" abstract="false" name="RepaymentsOfDebtIncludingPremiumsPaid" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax" abstract="false" name="EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_CollaborationProfitLossSharing" abstract="false" name="CollaborationProfitLossSharing" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_TimeVestedRestrictedStockUnitsMember" abstract="true" name="TimeVestedRestrictedStockUnitsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_TermOfCollaborationAgreementAdditionalLength" abstract="false" name="TermOfCollaborationAgreementAdditionalLength" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_AlkermesMember" abstract="true" name="AlkermesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries" abstract="false" name="Deferredtaxliabilitiestemporarydifferencespurchaseaccountingforeignsubsidiaries" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue" abstract="false" name="AvailableForSaleSecuritiesDebtMaturitiesAfterFiveYearsFairValue" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion" abstract="false" name="PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_EquitySecuritiesCurrentMember" abstract="true" name="EquitySecuritiesCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_FiniteLivedIntangibleAssetWeightedAverageRemainingUsefulLife" abstract="false" name="FiniteLivedIntangibleAssetWeightedAverageRemainingUsefulLife" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_RatioOfNumberOfSharesReservedUnderPlan" abstract="false" name="RatioOfNumberOfSharesReservedUnderPlan" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="biib_RevenuesfromantiCD20therapeuticprogramsTableTextBlock" abstract="false" name="RevenuesfromantiCD20therapeuticprogramsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_ShareOfCoPromotionProfits" abstract="false" name="ShareOfCoPromotionProfits" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_LicenseFee" abstract="false" name="LicenseFee" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_CollaborationProfitSharingMember" abstract="true" name="CollaborationProfitSharingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AmortizationandImpairmentofAcquiredIntangibleAssets" abstract="false" name="AmortizationandImpairmentofAcquiredIntangibleAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventoryAmortizationPeriod" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventoryAmortizationPeriod" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_BiosimilarsMember" abstract="true" name="BiosimilarsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AmortizationOfInventoryStepUp" abstract="false" name="AmortizationOfInventoryStepUp" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DevelopmentMilestonesMember" abstract="true" name="DevelopmentMilestonesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ShareOfNetProfitFromSageTherapeuticsPercent" abstract="false" name="ShareOfNetProfitFromSageTherapeuticsPercent" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="biib_ExpenseIncurredByCollaboration" abstract="false" name="ExpenseIncurredByCollaboration" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne" abstract="false" name="ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_GlobalLicensingCollaborationAgreementPurchasePricePerShare" abstract="false" name="GlobalLicensingCollaborationAgreementPurchasePricePerShare" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:perShareItemType"/>
  <xs:element id="biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears" abstract="false" name="AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" abstract="true" name="SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_RestructuringAndRelatedActivitiesExpectedOperatingExpenseReduction" abstract="false" name="RestructuringAndRelatedActivitiesExpectedOperatingExpenseReduction" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_InvestmentsByThirdPartyInJointVentureAsPerAgreement" abstract="false" name="InvestmentsByThirdPartyInJointVentureAsPerAgreement" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SummaryOfActivityRelatedToSangamoTherapeuticsTableTextBlock" abstract="false" name="SummaryOfActivityRelatedToSangamoTherapeuticsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_ShareRepurchaseProgram5Member" abstract="true" name="ShareRepurchaseProgram5Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NewAntiCd20Member" abstract="true" name="NewAntiCd20Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_UndesignatedPreferredStockMember" abstract="false" name="UndesignatedPreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GainLossOnRestructuringCharges" abstract="false" name="GainLossOnRestructuringCharges" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade" abstract="false" name="NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_NightstarMember" abstract="true" name="NightstarMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PriorityReviewVoucherMember" abstract="true" name="PriorityReviewVoucherMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_EffectiveIncomeTaxRateReconciliationDeconsolidationPercent" abstract="false" name="EffectiveIncomeTaxRateReconciliationDeconsolidationPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="biib_OtherrevenuesfromantiCD20therapeuticprograms" abstract="false" name="OtherrevenuesfromantiCD20therapeuticprograms" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_OperatingleasesupplementalcashflowdisclosureTableTextBlock" abstract="false" name="OperatingleasesupplementalcashflowdisclosureTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_AdditionalMilestonePayment" abstract="false" name="AdditionalMilestonePayment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_TECFIDERAMember" abstract="true" name="TECFIDERAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PerformanceStockUnitsSettledinCashMember" abstract="true" name="PerformanceStockUnitsSettledinCashMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_StrategicInvestmentsMember" abstract="true" name="StrategicInvestmentsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_CoPromotionProfitSharingFormulaTableTextBlock" abstract="false" name="CoPromotionProfitSharingFormulaTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_SummaryOfActivityRelatedToCollaborationWithCompanyEightTableTextBlockTableTextBlock" abstract="false" name="SummaryOfActivityRelatedToCollaborationWithCompanyEightTableTextBlockTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_BYOOVIZMember" abstract="true" name="BYOOVIZMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_A2017OmnibusEquityPlanMember" abstract="true" name="A2017OmnibusEquityPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NumberOfOphthalmolgyBiosimilarProductsExclusiveRights" abstract="false" name="NumberOfOphthalmolgyBiosimilarProductsExclusiveRights" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_FloatingRate364DayTrancheMember" abstract="true" name="FloatingRate364DayTrancheMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_EmployeeStockPurchasePlanMember" abstract="true" name="EmployeeStockPurchasePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_TysabriProductMember" abstract="true" name="TysabriProductMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_RoyaltyCostofSalesPolicyTextBlock" abstract="false" name="RoyaltyCostofSalesPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_ResearchAndDevelopmentAssetDomain" abstract="true" name="ResearchAndDevelopmentAssetDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_FutureContingentMilestoneTypesDomain" abstract="true" name="FutureContingentMilestoneTypesDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo" abstract="false" name="PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage" abstract="false" name="DebtInstrumentMakeWholeProvisionRedemptionPricePercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_U.S.2017TaxActMember" abstract="true" name="U.S.2017TaxActMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_DenaliTherapeuticsIncMember" abstract="true" name="DenaliTherapeuticsIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PaymentTermsOfAccountsReceivable" abstract="false" name="PaymentTermsOfAccountsReceivable" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAssetsNet" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAssetsNet" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PayablesToDivestitureOfInterestInJointVentureContingentDiscountOnSharesPercent" abstract="false" name="PayablesToDivestitureOfInterestInJointVentureContingentDiscountOnSharesPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_RevenuesFromAntiCD20TherapeuticProgramsMember" abstract="true" name="RevenuesFromAntiCD20TherapeuticProgramsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear" abstract="false" name="PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SummaryOfActivityRelatedToDenaliTherapeuticsCollaborationTableTextBlock" abstract="false" name="SummaryOfActivityRelatedToDenaliTherapeuticsCollaborationTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_OperatingLeaseArea" abstract="false" name="OperatingLeaseArea" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:areaItemType"/>
  <xs:element id="biib_IncomeLossFromEquityMethodInvestmentsNetOfTax" abstract="false" name="IncomeLossFromEquityMethodInvestmentsNetOfTax" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AVONEXMember" abstract="true" name="AVONEXMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_DebtInstrumentRedemptionRate" abstract="false" name="DebtInstrumentRedemptionRate" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_MSProductRevenuesMember" abstract="true" name="MSProductRevenuesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ReataPharmaceuticalsIncMember" abstract="true" name="ReataPharmaceuticalsIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOperatingLeaseAssets" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOperatingLeaseAssets" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RoyaltyAttributedToOCREVUSMember" abstract="true" name="RoyaltyAttributedToOCREVUSMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_CoPromotionOperatingProfitThresholdToDetermineShareOfCoPromotionOperatingProfitPriorToAmendment" abstract="false" name="CoPromotionOperatingProfitThresholdToDetermineShareOfCoPromotionOperatingProfitPriorToAmendment" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_InvestmentsInVariableInterestEntitiesAbstract" abstract="true" name="InvestmentsInVariableInterestEntitiesAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="biib_SKYCLARYSMember" abstract="true" name="SKYCLARYSMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpenseAndOther" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpenseAndOther" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RevenuesInPercentageFromAntiCD20TherapeuticPrograms" abstract="false" name="RevenuesInPercentageFromAntiCD20TherapeuticPrograms" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_CorporateDebtSecuritiesNonCurrentMember" abstract="true" name="CorporateDebtSecuritiesNonCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GovernmentSecuritiesCurrentMember" abstract="true" name="GovernmentSecuritiesCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember" abstract="true" name="MortgageAndOtherAssetBackedSecuritiesCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_FacilityLocationDomain" abstract="true" name="FacilityLocationDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BusinessCombinationConsiderationTransferredEquityCompensationPreAcquisitionServices" abstract="false" name="BusinessCombinationConsiderationTransferredEquityCompensationPreAcquisitionServices" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_CollaborativeArrangementProfitLossSharingComponentsAxis" abstract="true" name="CollaborativeArrangementProfitLossSharingComponentsAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree" abstract="false" name="PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_SamsungBiosimilarAgreementMember" abstract="true" name="SamsungBiosimilarAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_FAMPYRAMember" abstract="true" name="FAMPYRAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_JointVentureOwnerShipPercentageByThirdParty" abstract="false" name="JointVentureOwnerShipPercentageByThirdParty" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" abstract="true" name="SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_SummaryOfCommonStockTableTextBlock" abstract="false" name="SummaryOfCommonStockTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_MinimumQualifyingAgeForEmployeeBenefitPlan" abstract="false" name="MinimumQualifyingAgeForEmployeeBenefitPlan" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember" abstract="true" name="MortgageAndOtherAssetBackedSecuritiesNonCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_SolothurnSwitzerlandMember" abstract="true" name="SolothurnSwitzerlandMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxLiability" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxLiability" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SalesTriggerGrossSalesThreshold" abstract="false" name="SalesTriggerGrossSalesThreshold" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AcceleratedDepreciationAndOtherCostsMember" abstract="true" name="AcceleratedDepreciationAndOtherCostsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_PercentageOfFutureDevelopmentCosts" abstract="false" name="PercentageOfFutureDevelopmentCosts" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherNoncurrentAssetsOperatingLeaseAssets" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherNoncurrentAssetsOperatingLeaseAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AcquiredInProcessResearchAndDevelopment" abstract="false" name="AcquiredInProcessResearchAndDevelopment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_BIIB080Member" abstract="true" name="BIIB080Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_RegulatoryMilestonesMember" abstract="true" name="RegulatoryMilestonesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_IntangibleAssetsExcludingGoodwillTableTextBlock" abstract="false" name="IntangibleAssetsExcludingGoodwillTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_PercentageOfMinimumInvestmentReturn" abstract="false" name="PercentageOfMinimumInvestmentReturn" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear" abstract="false" name="PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpenseAndOther" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpenseAndOther" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_GlobalLicensingCollaborationAgreementSharesPurchased" abstract="false" name="GlobalLicensingCollaborationAgreementSharesPurchased" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="biib_TGNMember" abstract="true" name="TGNMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GenentechMember" abstract="true" name="GenentechMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_IMRALDIMember" abstract="true" name="IMRALDIMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ADUHELMMember" abstract="true" name="ADUHELMMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones" abstract="false" name="EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RevenueRemainingPerformanceObligationNumberOfPayments" abstract="false" name="RevenueRemainingPerformanceObligationNumberOfPayments" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_AcquiredAndInLicensedRightsAndPatentsMember" abstract="true" name="AcquiredAndInLicensedRightsAndPatentsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_Totalpaymenttoentercollaborationagreement" abstract="false" name="Totalpaymenttoentercollaborationagreement" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FumarateMember" abstract="true" name="FumarateMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" abstract="false" name="MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_SPINRAZAMember" abstract="true" name="SPINRAZAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_MarketStockUnitsMember" abstract="true" name="MarketStockUnitsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses" abstract="false" name="ShareBasedCompensationExpenseIncludedInCostsAndExpenses" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_Collaborationprofitlosssharing" abstract="false" name="Collaborationprofitlosssharing" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DirectorsPlanMember" abstract="true" name="DirectorsPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ExpectedAdditionalMilestonePaymentsWhenMeetingCertainSalesThreshold" abstract="false" name="ExpectedAdditionalMilestonePaymentsWhenMeetingCertainSalesThreshold" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_UnrealizedLossOnDerivatives" abstract="false" name="UnrealizedLossOnDerivatives" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_EquityAwardsOfAcquiredCompanyMember" abstract="true" name="EquityAwardsOfAcquiredCompanyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_EquityMethodInvestmentOwnershipPercentageBeforeAdditionalPurchaseTransaction" abstract="false" name="EquityMethodInvestmentOwnershipPercentageBeforeAdditionalPurchaseTransaction" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_CollaborativeArrangementProfitLossSharingComponentsDomain" abstract="true" name="CollaborativeArrangementProfitLossSharingComponentsDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_StockOptionsMember" abstract="true" name="StockOptionsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_UnrealizedGainOnDerivatives" abstract="false" name="UnrealizedGainOnDerivatives" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_Expenseincurredbythecollaboration" abstract="false" name="Expenseincurredbythecollaboration" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_RobinKramerMember" abstract="true" name="RobinKramerMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_SeriesXPreferredStockMember" abstract="false" name="SeriesXPreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PerformanceStockUnitsSettledinStockMember" abstract="true" name="PerformanceStockUnitsSettledinStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_AducanumabMember" abstract="true" name="AducanumabMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_SummaryOfProductRevenueReservesIncludedInConsolidatedBalanceSheetsTableTextBlock" abstract="false" name="SummaryOfProductRevenueReservesIncludedInConsolidatedBalanceSheetsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_InterferonMember" abstract="true" name="InterferonMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NumeratorAbstract" abstract="true" name="NumeratorAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities" abstract="false" name="InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_ThreePointTwoFivePercentSeniorNotesDueFebruaryTwentyFiftyOneMember" abstract="true" name="ThreePointTwoFivePercentSeniorNotesDueFebruaryTwentyFiftyOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EuropeExcludingGermanyMember" abstract="true" name="EuropeExcludingGermanyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EquityMethodInvestmentsExpectedProfitShare" abstract="false" name="EquityMethodInvestmentsExpectedProfitShare" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_DivestituresAbstract" abstract="true" name="DivestituresAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="biib_ShareOfNonControlingInterestRecognized" abstract="false" name="ShareOfNonControlingInterestRecognized" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="biib_ContractOptionExerciseFee" abstract="false" name="ContractOptionExerciseFee" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SOD1Member" abstract="true" name="SOD1Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GlobalLicensingCollaborationAgreementResearchDevelopmentRegulatoryAndCommercialMilestonePayments" abstract="false" name="GlobalLicensingCollaborationAgreementResearchDevelopmentRegulatoryAndCommercialMilestonePayments" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_UpfrontPaymentForCollaborationAgreement" abstract="false" name="UpfrontPaymentForCollaborationAgreement" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_EffectiveIncomeTaxRateReconciliationLitigationSettlementPercent" abstract="false" name="EffectiveIncomeTaxRateReconciliationLitigationSettlementPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_InventoryNetCurrentAndNoncurrent" abstract="false" name="InventoryNetCurrentAndNoncurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_CurrentPortionOfNotesPayableAndTermLoanMember" abstract="true" name="CurrentPortionOfNotesPayableAndTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_EisaiMember" abstract="true" name="EisaiMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization" abstract="false" name="EffectiveIncomeTaxRateReconciliationIntellectualPropertyRightsReorganization" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_FairValueIndefiniteLivedIntangibleAssetsDiscountRate" abstract="false" name="FairValueIndefiniteLivedIntangibleAssetsDiscountRate" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_ResearchanddevelopmentassetAxis" abstract="true" name="ResearchanddevelopmentassetAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_DiscreteIncomeTaxExpenseBenefit" abstract="false" name="DiscreteIncomeTaxExpenseBenefit" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesTableTextBlock" abstract="false" name="AnalysisOfAmountOfAndChangeInProductRevenueReservesTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_Equitymethodinvestmentbasisdifferenceamortizationperiod" abstract="false" name="Equitymethodinvestmentbasisdifferenceamortizationperiod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_ResearchAndDevelopmentExpenseDevelopmentOfProductCandidate" abstract="false" name="ResearchAndDevelopmentExpenseDevelopmentOfProductCandidate" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FuturePercentageOfCoPromotionOperatingProfits" abstract="false" name="FuturePercentageOfCoPromotionOperatingProfits" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_OperatingLeaseRightOfUseAssetDerecognized" abstract="false" name="OperatingLeaseRightOfUseAssetDerecognized" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_A2022CostSavingInitiativesMember" abstract="true" name="A2022CostSavingInitiativesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_ContractManufacturingAndOtherRevenueMember" abstract="true" name="ContractManufacturingAndOtherRevenueMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_TheCreditFacilityMember" abstract="true" name="TheCreditFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ReserveforCashDiscountsMember" abstract="true" name="ReserveforCashDiscountsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_RituxanMember" abstract="true" name="RituxanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_FLIXABIMember" abstract="true" name="FLIXABIMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_CorporateDebtSecuritiesCurrentMember" abstract="true" name="CorporateDebtSecuritiesCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_DenominatorAbstract" abstract="true" name="DenominatorAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="biib_BusinessOverviewPolicyTextBlock" abstract="false" name="BusinessOverviewPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_PaymentDueAtSecondAnniversaryMember" abstract="true" name="PaymentDueAtSecondAnniversaryMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_WorkforceReductionMember" abstract="true" name="WorkforceReductionMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_EntityWidePercentageOfRevenueFromMajorDistributors" abstract="false" name="EntityWidePercentageOfRevenueFromMajorDistributors" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_StrategicInvestmentPortfolio" abstract="false" name="StrategicInvestmentPortfolio" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_AverageStockPriceOnGrantDateInDollarsMaximum" abstract="false" name="AverageStockPriceOnGrantDateInDollarsMaximum" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:perShareItemType"/>
  <xs:element id="biib_CommercialMilestonesMember" abstract="true" name="CommercialMilestonesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BIIB115Member" abstract="true" name="BIIB115Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_IonisPharmaceuticalsMember" abstract="true" name="IonisPharmaceuticalsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ComputerSoftwareAndHardware" abstract="false" name="ComputerSoftwareAndHardware" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SegmentGeographicalGroupsOfCountriesGroupOtherMember" abstract="true" name="SegmentGeographicalGroupsOfCountriesGroupOtherMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_A125BroadwayBuildingMember" abstract="true" name="A125BroadwayBuildingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_IonisSangamoDenaliAndSageMember" abstract="true" name="IonisSangamoDenaliAndSageMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_A2015ESPPMember" abstract="true" name="A2015ESPPMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PayablesToDivestitureOfInterestInJointVentureNumberOfPayments" abstract="false" name="PayablesToDivestitureOfInterestInJointVentureNumberOfPayments" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_BIIB112Member" abstract="true" name="BIIB112Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_OCREVUSMember" abstract="true" name="OCREVUSMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_OtherClinicalProgramsMember" abstract="true" name="OtherClinicalProgramsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_MilestonePaymentsMember" abstract="true" name="MilestonePaymentsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PlanAssetsForDeferredCompensationFairValueDisclosure" abstract="false" name="PlanAssetsForDeferredCompensationFairValueDisclosure" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" abstract="true" name="SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_OtherresearchanddiscoveryMember" abstract="true" name="OtherresearchanddiscoveryMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionFour" abstract="false" name="PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionFour" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentGoodwill" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentGoodwill" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DeferredTaxAssetsCounterpartyTaxBasisInAsset" abstract="false" name="DeferredTaxAssetsCounterpartyTaxBasisInAsset" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_DerivativeInstrumentsUnrealizedGainLossTableTextBlock" abstract="false" name="DerivativeInstrumentsUnrealizedGainLossTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_AccruedMilestonePayments" abstract="false" name="AccruedMilestonePayments" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PerformanceStockUnitsSettledinCashTableTextBlock" abstract="false" name="PerformanceStockUnitsSettledinCashTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_MajorDistributorDomain" abstract="true" name="MajorDistributorDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_VUMERITYMember" abstract="true" name="VUMERITYMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GovernmentSecuritiesNonCurrentMember" abstract="true" name="GovernmentSecuritiesNonCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PaymentForDebtExtinguishmentOrDebtPrepaymentCostIncludingAccruedInterest" abstract="false" name="PaymentForDebtExtinguishmentOrDebtPrepaymentCostIncludingAccruedInterest" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsOtherAssetsAndLiabilitiesNet" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsOtherAssetsAndLiabilitiesNet" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_ConvergencePharmaceuticalsMember" abstract="true" name="ConvergencePharmaceuticalsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ContractManufacturingRoyaltyAndOtherRevenueMember" abstract="true" name="ContractManufacturingRoyaltyAndOtherRevenueMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_PerformanceStockUnitsSettledinStockTableTextBlock" abstract="false" name="PerformanceStockUnitsSettledinStockTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_OtherAssetsCurrentFairValueDisclosure" abstract="false" name="OtherAssetsCurrentFairValueDisclosure" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_FacilityTypeAxis" abstract="true" name="FacilityTypeAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_BIIB111AndBIIB112Member" abstract="true" name="BIIB111AndBIIB112Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_MosunetuzumabMember" abstract="true" name="MosunetuzumabMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ResidualU.S.taxliabilityonreversalofdeferredtaxliabilitiesonforeignsubsidiariespurchaseaccounting" abstract="false" name="ResidualU.S.taxliabilityonreversalofdeferredtaxliabilitiesonforeignsubsidiariespurchaseaccounting" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_OperatingLeaseLiabilityDerecognized" abstract="false" name="OperatingLeaseLiabilityDerecognized" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PLEGRIDYMember" abstract="true" name="PLEGRIDYMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_SageTherapeuticsIncMember" abstract="true" name="SageTherapeuticsIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_GlobalLicensingCollaborationAgreementAmountSharesPurchased" abstract="false" name="GlobalLicensingCollaborationAgreementAmountSharesPurchased" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_ERISAClassActionLitigaitonMember" abstract="true" name="ERISAClassActionLitigaitonMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_GAZYVAMember" abstract="true" name="GAZYVAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables" abstract="false" name="PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_FutureContingentMilestoneTypesAxis" abstract="true" name="FutureContingentMilestoneTypesAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_ThresholdOfGrossSalesToBeAchievedInAnyTwelveConsecutiveMonthsUnderOptionOne" abstract="false" name="ThresholdOfGrossSalesToBeAchievedInAnyTwelveConsecutiveMonthsUnderOptionOne" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_GainLossOnDispositionOfPropertyPlantEquipmentTenantAllowance" abstract="false" name="GainLossOnDispositionOfPropertyPlantEquipmentTenantAllowance" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_NeurimmuneMember" abstract="true" name="NeurimmuneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_BiologicsManufacturingMember" abstract="true" name="BiologicsManufacturingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_A2023CostSavingInitiativesMember" abstract="true" name="A2023CostSavingInitiativesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOperatingLeaseLiabilities" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOperatingLeaseLiabilities" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_OperatingleaseweightedaverageremainingtermanddiscountrateTableTextBlock" abstract="false" name="OperatingleaseweightedaverageremainingtermanddiscountrateTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_DeferredTaxLiabilitiesTaxCreditCarryforwards" abstract="false" name="DeferredTaxLiabilitiesTaxCreditCarryforwards" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_A2020SeniorNotesMember" abstract="true" name="A2020SeniorNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_ManufacturingAndOtherCosts" abstract="false" name="ManufacturingAndOtherCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_Derivativesusedinnetinvestmenthedgenetoftaxexcludedcomponentincreasedecrease" abstract="false" name="Derivativesusedinnetinvestmenthedgenetoftaxexcludedcomponentincreasedecrease" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_OtherAssetsMeasurementInput" abstract="false" name="OtherAssetsMeasurementInput" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="biib_AcordaMember" abstract="false" name="AcordaMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_InnoCarePharmaLimitedInnoCareAgreementMember" abstract="true" name="InnoCarePharmaLimitedInnoCareAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PercentageParValueOfSeniorNotes" abstract="false" name="PercentageParValueOfSeniorNotes" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_InventoryIdleCapacityChargesAndContractualCommitmentsMember" abstract="true" name="InventoryIdleCapacityChargesAndContractualCommitmentsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_UpfrontAndMilestonePaymentsMadeToCollaborativePartner" abstract="false" name="UpfrontAndMilestonePaymentsMadeToCollaborativePartner" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_LesseeLeaseTableTableTextBlock" abstract="false" name="LesseeLeaseTableTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="biib_PayablesToDivestitureOfInterestInJointVenture" abstract="false" name="PayablesToDivestitureOfInterestInJointVenture" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_CentersForMedicareAndMedicaidServiceMember" abstract="true" name="CentersForMedicareAndMedicaidServiceMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_PercentageOfRoyaltiesAsPerCollaboration" abstract="false" name="PercentageOfRoyaltiesAsPerCollaboration" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="biib_RoyaltiesPayablePeriodPostFDAApproval" abstract="false" name="RoyaltiesPayablePeriodPostFDAApproval" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_PeriodOfCollaborationAgreement" abstract="false" name="PeriodOfCollaborationAgreement" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="biib_ResearchAndDevelopmentExpenseOptionExerciseFee" abstract="false" name="ResearchAndDevelopmentExpenseOptionExerciseFee" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax" abstract="false" name="EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_InvestmentInCommonStockSharesPurchased" abstract="false" name="InvestmentInCommonStockSharesPurchased" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="biib_MajorDistributorAxis" abstract="true" name="MajorDistributorAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear" abstract="false" name="CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="biib_PaymentDueAtFirstAnniversaryMember" abstract="true" name="PaymentDueAtFirstAnniversaryMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_NumberOfEqualAnnualIncrements" abstract="false" name="NumberOfEqualAnnualIncrements" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="biib_A2023TermLoanThreeYearTrancheMember" abstract="true" name="A2023TermLoanThreeYearTrancheMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_A2023TermLoanMember" abstract="true" name="A2023TermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_A2015SeniorNotesMember" abstract="true" name="A2015SeniorNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_QALSODYMember" abstract="true" name="QALSODYMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_E2609andBAN2401Member" abstract="true" name="E2609andBAN2401Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="biib_LenderDisputeMember" abstract="true" name="LenderDisputeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="biib_ResearchAndDevelopmentCostsPercentage" abstract="false" name="ResearchAndDevelopmentCostsPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
</xs:schema>
