Co-administration of efavirenz (600 mg once daily) with carbamazepine (400 mg once daily) decreased carbamazepine AUC, Cmax and Cmin by 27%, 20% and 35%, respectively, due to CYP3A4 induction.  Efavirenz AUC, Cmax and Cmin decreased by 36%, 21%, and 47%, respectively due to CYP3A4 and CYP2B6 induction. The steady-state AUC, Cmax and Cmin of the active carbamazepine epoxide metabolite remained unchanged. Coadministration of higher doses of either efavirenz or carbamazepine has not been studied. No dosage recommendation can be made. An alternative anticonvulsant should be considered. Carbamazepine plasma levels should be monitored periodically. 

Coadministration of carbamazepine (6 day dose escalation, then 400 mg once daily for 29 days) and efavirenz (600 mg for 14 days) to 12 subjects resulted in decreases in carbamazepine Cmax, AUC and Cmin of 20%, 27% and 35%, respectively. There was no change in Cmax or AUC of carbamazepine epoxide, but Cmin decreased by 13%. Data from 14 subjects receiving carbamazepine (6 day dose escalation, then 400 mg once daily for 15 days) and efavirenz (600 mg for 35 days) resulted in decreases in efavirenz Cmax, AUC and Cmin of 21%, 36% and 47%, respectively. There are insufficient data to make a dosing recommendation for efavirenz, Alternative anticonvulsant treatment should be used.

Coadministration of efavirenz (600 mg once daily) and carbamazepine (1 week dose escalation from 200 mg once daily to 400 mg once daily) was studied in healthy subjects. Pharmacokinetic kinetic profiles were determined at steady state (full dose for 2 weeks). Data from 14 subjects showed that carbamazepine decreased efavirenz AUC (36%), Cmax (21%) and Cmin (47%). Similarly, data from 12 subjects showed that efavirenz decreased carbamazepine AUC (27%), Cmax (20%) and Cmin (35%). There are no data for this combination using higher doses of either drug; therefore, no dose recommendation can be made. Use of alternate anticonvulsants may be necessary for optimal antiretroviral / anticonvulsant therapy. The drugs were generally safe and well-tolerated when administered alone or in combination.