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    <periodOfReport>2012-12-20</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000944765</issuerCik>
        <issuerName>MERGE HEALTHCARE INC</issuerName>
        <issuerTradingSymbol>MRGE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001163431</rptOwnerCik>
            <rptOwnerName>FERRO MICHAEL W JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>350 NORTH ORLEANS STREET, 10TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60654</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F6"/>
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                    <value>See Footnote</value>
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    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Distribution by Merrick RIS, LLC (&quot;RIS&quot;) for no consideration to its direct and indirect owners.</footnote>
        <footnote id="F2">Indirectly owned by Michael W. Ferro, Jr. (&quot;Mr. Ferro&quot;).  Mr. Ferro is the Chairman and Chief Executive Officer of RIS.  Mr. Ferro and trusts for the benefit of Mr. Ferro's family members own a majority of the equity interests in RIS.  As a result of the foregoing, Mr. Ferro indirectly controls all shares of Merge common stock owned by RIS.  Mr. Ferro disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F3">Directly owned by Merrick Ventures, LLC (&quot;Merrick Ventures&quot;) and indirectly owned by Mr. Ferro. Mr. Ferro and trusts for the benefit of Mr. Ferro's family members own a majority of the equity interests in Merrick Ventures. Mr. Ferro is the Chairman and Chief Executive Officer of Merrick Ventures. As a result of the foregoing, Mr. Ferro indirectly controls all shares of Merge common stock owned by Merrick Ventures. Mr. Ferro disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F4">Directly owned by Merrick Venture Management, LLC (&quot;MVM&quot;) and indirectly owned by Mr. Ferro. Mr. Ferro is the Chief Executive Officer and Managing Member of MVM. Mr. Ferro and trusts for the benefit of Mr. Ferro's family members own all of the equity interets in MVM. As a result of the foregoing, Mr. Ferro indirectly controls all shares of Merge common stock owned by MVM. Mr. Ferro disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F5">Distribution by MVM for no consideration to its direct and indirect owners.</footnote>
        <footnote id="F6">Indirectly owned by Mr. Ferro through a trust for the benefit of Mr. Ferro's family (&quot;Family Trust&quot;).  Mr. Ferro disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F7">Contribution by Mr. Ferro to Merrick Venture Management Holdings, LLC (&quot;MVMH&quot;) in consideration for 60% of the ownership interest in MVMH and a promissory note.</footnote>
        <footnote id="F8">Contribution by Family Trust in consideration for 40% of the ownership interest in MVMH and a promissory note.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Julie Ann B. Schumitsch, by Power of Attorney for Michael W. Ferro, Jr.</signatureName>
        <signatureDate>2012-12-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
