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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-01-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000740260</issuerCik>
        <issuerName>VENTAS INC</issuerName>
        <issuerTradingSymbol>VTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001494933</rptOwnerCik>
            <rptOwnerName>Lillibridge Todd W.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>353 N. CLARK STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3300</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60654</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Medical Prop. Operations</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-01-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1281</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>60.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>109652.067</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 18, 2012 under the Ventas, Inc. 2006 Incentive Plan.</footnote>
        <footnote id="F2">Includes 217.049 shares acquired between July 3, 2013 and January 20, 2014 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes .024 shares acquired between September 27, 2013 and December 31, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F4">As of January 18, 2014, Reporting Person also owns options to purchase 59,883 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2014-01-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2014-01-18
0
0000740260
VENTAS INC
VTR
0001494933
Lillibridge Todd W.
353 N. CLARK STREET
SUITE 3300
CHICAGO
IL
60654
0
1
0
0
EVP, Medical Prop. Operations
Common Stock
2014-01-18
4
F
0
1281
60.96
D
109652.067
D
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 18, 2012 under the Ventas, Inc. 2006 Incentive Plan.
Includes 217.049 shares acquired between July 3, 2013 and January 20, 2014 under the Ventas Employee and Director Stock Purchase Plan.
Includes .024 shares acquired between September 27, 2013 and December 31, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of January 18, 2014, Reporting Person also owns options to purchase 59,883 shares of Issuer's common stock.
Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact
2014-01-21