<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>3</documentType><periodOfReport>2004-01-01</periodOfReport>
    <noSecuritiesOwned>1</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001080709</issuerCik>
        <issuerName>ARENA PHARMACEUTICALS INC</issuerName>
        <issuerTradingSymbol>ARNA</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001275813</rptOwnerCik>
            <rptOwnerName>AJIT-SIMH K A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ARENA PHARMACEUTICALS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>6166 NANCY RIDGE DRIVE</rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92121</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>VP, Quality Systems</officerTitle></reportingOwnerRelationship></reportingOwner><footnotes/><remarks/><ownerSignature><signatureName>Adam S. Chinnock, as Attorney in Fact</signatureName><signatureDate>2004-01-09</signatureDate></ownerSignature></ownershipDocument>

