Drug Interaction Report

MONITOR: Carbamazepine may decrease the plasma concentrations and systemic effects of both endogenous and exogenous corticosteroids. The mechanism is accelerated corticosteroid metabolism due to induction of the CYP450 3A4 enzymatic pathway by carbamazepine.

MANAGEMENT: Patients treated concomitantly with carbamazepine may require higher dosages of corticosteroids or adrenocorticotropic agents. Pharmacologic response to these agents should be monitored more closely whenever carbamazepine is added to or withdrawn from therapy in patients stabilized on their existing corticosteroid or adrenocorticotropic regimen, and the dosage(s) adjusted as necessary.

GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of carbamazepine. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

In a small, randomized, crossover study, the administration of carbamazepine with grapefruit juice (compared to water) increased plasma drug concentrations by approximately 40%. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits.

MANAGEMENT: Patients receiving carbamazepine should be advised to avoid or limit consumption of alcohol. Given the drug's narrow therapeutic index, patients receiving carbamazepine therapy should preferably avoid the regular consumption of grapefruits and grapefruit juice to prevent any undue fluctuations in plasma drug levels. Patients should be advised to report signs of carbamazepine toxicity (nausea, visual disturbances, dizziness, or ataxia) to their physicians.