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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2005-06-02</periodOfReport>

    <dateOfOriginalSubmission>2005-06-06</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0000821616</issuerCik>
        <issuerName>BENTLEY PHARMACEUTICALS INC</issuerName>
        <issuerTradingSymbol>BNT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001218893</rptOwnerCik>
            <rptOwnerName>MURPHY JAMES R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>BENTLEY PARK</rptOwnerStreet1>
            <rptOwnerStreet2>2 HOLLAND WAY</rptOwnerStreet2>
            <rptOwnerCity>EXETER</rptOwnerCity>
            <rptOwnerState>NH</rptOwnerState>
            <rptOwnerZipCode>03833</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman, President &amp; CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $.02 par value</value>
            </securityTitle>
            <transactionDate>
                <value>2005-06-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>105770</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.985</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>245217</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Due to a typographical error, the original Form 4 filed on June 6, 2005 incorrectly reported this line item under Transaction Code &quot;E&quot;.  The sole purpose of this amended Form 4 is to properly report this line item under Transaction Code &quot;F&quot;.</footnote>
        <footnote id="F2">Represents previously-owned shares tendered to the Company to pay the option exercise price for shares previously disclosed on the original Form 4, and shares withheld by the Company (that were otherwise issuable upon exercise) to satisfy minimum federal tax withholding requirements in respect of the option exercises.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>James R. Murphy</signatureName>
        <signatureDate>2005-06-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
