<?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>0001215187</rptOwnerCik>
            <rptOwnerName>LEWIS RAYMOND J</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>President</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>3389</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>60.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>167245</value>
                    <footnoteId id="F2"/>
                </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">As of January 18, 2014, Reporting Person also owns options to purchase 147,304 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Raymond J. Lewis, 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
0001215187
LEWIS RAYMOND J
353 N. CLARK STREET
SUITE 3300
CHICAGO,
IL
60654
0
1
0
0
President
Common Stock
2014-01-18
4
F
0
3389
60.96
D
167245
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.
As of January 18, 2014, Reporting Person also owns options to purchase 147,304 shares of Issuer's common stock.
Raymond J. Lewis, By: T. Richard Riney, Attorney-In-Fact
2014-01-21