phenytoin and carbamazepine

Both drugs are enzyme inducers. Phenytoin induces CYP3A4, the main enzyme for carbamazepine metabolism, while carbamazepine induces CYP2C9 and CYP3A4, the enzymes responsible for phenytoin metabolism. This mutual induction increases the clearance of both drugs, lowering their plasma concentrations.

Plasma concentrations of both drugs decrease, potentially leading to loss of seizure control. Conversely, abrupt withdrawal of one drug can increase concentrations of the other, causing toxicity. The interaction is clinically significant and may require dosage adjustments.

Monitor plasma concentrations of both drugs when initiating, adjusting, or stopping either drug. Adjust doses based on therapeutic drug monitoring. Consider alternative antiepileptics with fewer interactions. Monitor for signs of seizure recurrence or toxicity.

Well-established interaction, widely documented in pharmacokinetic studies and clinical guidelines.