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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 obtained from our routine therapeutic drug monitoring database for risperidone and its active metabolite 9-hydroxyrisperidone.’ ‹’ ‹