Risperidone and Lamotrigine: No Evidence of a Drug Interaction

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Sir: It has been suggested that adding lamotrigine to an ongoing treatment with risperidone and clozapine might significantly increase the serum concentration of risperidone.¹ However, lamotrigine is not known to inhibit the cytochrome P450 (CYP) enzymes CYP2D6 or CYP3A4, which are the major enzymes involved in the metabolism of risperidone.²

To extend the knowledge about this possible drug interaction,we performed a retrospective analysis on data obtainedfrom our routine therapeutic drug monitoring database for risperidoneand its active metabolite 9-hydroxyrisperidone.' ‹' ‹