Phenytoin Toxicity Secondary to an Oxcarbazepine–Phenytoin 2C19 Interaction

Background Polytherapy is common in the management of bipolar disorder, as are the side effects associated with this treatment strategy. Objective The authors review the literature on drug– drug interactions involving oxcarbazepine and identify specific mechanisms that may have clinical importance. Method The authors provide a case report of a patient who developed phenytoin toxicity associated with an oxcarbazepine–phenytoin interaction. Results Co-administration of phenytoin and oxcarbazepine resulted in toxic levels of phenytoin. Therefore, the patient's daily dosage of oxcarbazepine and phenytoin were reduced. Discussion Although oxcarbazepine is an inducer of the 3A4 isoenzyme, it acts as an inhibitor of the 2C19 isoenzyme, and it can raise levels of other agents, for example, phenytoin, that are also metabolized by this isoenzyme.