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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3/A</documentType>

    <periodOfReport>2013-07-01</periodOfReport>

    <dateOfOriginalSubmission>2013-07-08</dateOfOriginalSubmission>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000353230</issuerCik>
        <issuerName>OMNICARE INC</issuerName>
        <issuerTradingSymbol>OCR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001024376</rptOwnerCik>
            <rptOwnerName>CARLSON JAMES G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>900 OMNICARE CENTER</rptOwnerStreet1>
            <rptOwnerStreet2>201 EAST FOURTH STREET</rptOwnerStreet2>
            <rptOwnerCity>CINCINNATI</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>45202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>3.25% Convertible Senior Debentures Due 2035</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>78.74</value>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2035-12-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">This amendment is filed to report convertible senior debentures held by the reporting person in a brokerage account that were inadvertently omitted from the reporting person's Form 3.</footnote>
        <footnote id="F2">Convertible (a) during a fiscal quarter if the closing price of the company's common stock exceeds 130% of the then current conversion price for at least 20 trading days in the 30 trading day period ending on the last trading day of the preceding quarter, (b) during a five trading day period following any five consecutive trading day period in which the average trading price of the debentures was less than 98% of the average closing sale price of the company's common stock multiplied by the then current conversion rate, (c) on or after December 15, 2033 and (d) upon certain other events as set forth in the related indenture.</footnote>
        <footnote id="F3">As of July 1, 2013, the reporting person held $25,000 aggregate principal amount of debentures, which were convertible, if and when applicable, into cash and shares of the company's common stock based on a conversion rate of 12.7008 shares of common stock per $1,000 aggregate principal amount of debentures (or $78.74 per share of common stock), subject to adjustment upon certain events as set forth in the related indenture.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert Burkhard, attorney-in-fact for James G. Carlson</signatureName>
        <signatureDate>2014-09-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
