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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

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

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001429381</rptOwnerCik>
            <rptOwnerName>Reed Robert D.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2200 RIVER PLAZA DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SACRAMENTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>95833</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>436.453</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>57.28</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11894.758</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2269</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14163.758</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Common stock in the form of units granted under the Ventas Nonemployee Directors' Deferred Stock Compensation Plan (the &quot;Plan&quot;) in lieu of director fees pursuant to the director's deferral election.  Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Plan.</footnote>
        <footnote id="F2">Restricted stock units granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan (the &quot;Incentive Plan&quot;). Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Incentive Plan. These restricted stock units vest in two equal annual installments beginning on the first anniversary of the date of the grant.</footnote>
        <footnote id="F3">As of January 1, 2014, Reporting Person also owns options to purchase 20,000 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert D. Reed, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2014-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wf-form4_138879279520652.xml", "description": "FORM 4", "secsgml_size_bytes": 5496}]
X0306
4
2014-01-01
0
0000740260
VENTAS INC
VTR
0001429381
Reed Robert D.
2200 RIVER PLAZA DRIVE
SACRAMENTO
CA
95833
1
0
0
0
Common Stock
2014-01-01
4
A
0
436.453
57.28
A
11894.758
D
Common Stock
2014-01-01
4
A
0
2269
0
A
14163.758
D
Common stock in the form of units granted under the Ventas Nonemployee Directors' Deferred Stock Compensation Plan (the "Plan") in lieu of director fees pursuant to the director's deferral election.  Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Plan.
Restricted stock units granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan (the "Incentive Plan"). Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Incentive Plan. These restricted stock units vest in two equal annual installments beginning on the first anniversary of the date of the grant.
As of January 1, 2014, Reporting Person also owns options to purchase 20,000 shares of Issuer's common stock.
Robert D. Reed, By: T. Richard Riney, Attorney-In-Fact
2014-01-03