<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2015-02-25</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283072</rptOwnerCik>
            <rptOwnerName>GUGGENHEIM CAPITAL LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>227 WEST MONROE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4900</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001425498</rptOwnerCik>
            <rptOwnerName>Guggenheim Partners, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>135 EAST 57TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001581086</rptOwnerCik>
            <rptOwnerName>Guggenheim Partners Investment Management Holdings, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>227 W MONROE ST</rptOwnerStreet1>
            <rptOwnerStreet2>STE 4800</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001425852</rptOwnerCik>
            <rptOwnerName>Guggenheim Partners Investment Management, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>100 WILSHIRE BOULEVARD, &amp;NBSP5TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90401</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2015-02-25</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>9744127</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2015-02-25</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2195690</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F4"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2015-02-25</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>109905</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F5"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2015-02-25</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>27778</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnotes</value>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each share of Series A Convertible Preferred Stock (&quot;Preferred Stock&quot;) is convertible at any time, at the holder's election, and has no expiration date.</footnote>
        <footnote id="F2">The Preferred Stock is convertible into a number of shares of common stock of the Issuer (&quot;Common Stock&quot;) equal to the quotient determined by dividing (i) the sum of $1,000 plus any unpaid dividends on the Preferred Stock at such time, by (ii) the Conversion Price then in effect, except that, without prior stockholder approval, the Preferred Stock shall not be convertible into more than 19.99% of the shares of Common Stock outstanding immediately prior to the date of issuance of the Preferred Stock.  The initial Conversion Price is $4.14, and may be adjusted from time to time pursuant to the Certificate of Designation of Series A Convertible Preferred Stock.  Based on this initial Conversion Price, each share of Preferred Stock is initially convertible into 241.55 shares of Common Stock.</footnote>
        <footnote id="F3">These shares of Preferred Stock are held directly by Guggenheim Private Debt Fund Note Issuer, LLC (&quot;GPDF&quot;). Guggenheim Partners Investment Management, LLC (&quot;GPIM&quot;) serves as manager to GPDF.</footnote>
        <footnote id="F4">These shares of Preferred Stock are held directly by NZC Guggenheim Fund LLC (&quot;NZCG&quot;). GPIM serves as manager to NZCG.</footnote>
        <footnote id="F5">These shares of Preferred Stock are held directly by Maverick Enterprises, Inc. (&quot;Maverick&quot;).  GPIM serves as investment manager to Maverick.</footnote>
        <footnote id="F6">These shares of Preferred Stock are held directly by Verger Capital Fund LLC (&quot;Verger&quot;).  GPIM serves as sub-advisor to Verger.</footnote>
        <footnote id="F7">Guggenheim Partners Investment Management Holdings, LLC (&quot;GPIMH&quot;) is the majority owner of GPIM.  Guggenheim Partners, LLC is the majority indirect owner of GPIMH.  Guggenheim Capital, LLC is the majority owner of Guggenheim Partners, LLC.  Each of the Reporting Persons disclaims beneficial ownership over the Preferred Stock, except to the extent of its pecuniary interest therein, and this statement shall not be construed as an admission that such Reporting Person is the beneficial owner of any Preferred Stock for purposes of Section 16 of the Exchange Act or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Guggenheim Capital, LLC By: Robert Saperstein, Managing Director /s/ Robert Saperstein</signatureName>
        <signatureDate>2015-03-09</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Guggenheim Partners, LLC By: Guggenheim Capital, LLC, parent company By: Robert Saperstein, Managing Director /s/ Robert Saperstein</signatureName>
        <signatureDate>2015-03-09</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Guggenheim Partners Investment Management Holdings LLC By: Guggenheim Capital, LLC, parent company By: Robert Saperstein, Managing Director /s/ Robert Saperstein</signatureName>
        <signatureDate>2015-03-09</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Guggenheim Partners Investment Management, LLC By: Guggenheim Capital LLC, parent company By: Robert Saperstein, Managing Director /s/ Robert Saperstein</signatureName>
        <signatureDate>2015-03-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
