The effect of efavirenz on the pharmacokinetics of carbamazepine and vice versa was investigated in adult healthy subjects in a randomized, open-label, 2-period crossover, multiple-dose study. Subjects were randomized to receive either efavirenz 600 mg qd for 14 days or carbamazepine titrated to 400 mg qd for 21 days followed with both drugs for another 21 or 14 days. The pharmacokinetics was evaluated for efavirenz, carbamazepine, and the major metabolite of carbamazepine, carbamazepine-10,11-epoxide. Coadministration of carbamazepine with efavirenz significantly reduced the exposure of efavirenz (geometric mean ratios [90% confidence interval]: area of plasma concentration-time curve during the dosing interval of 24 hours [AUCtau], 0.64 [0.60-0.68]; maximum plasma concentration [C(max)], 0.79 [0.74, 0.85]) and carbamazepine (AUC(tau), 0.73 [0.67-0.80]; C(max), 0.80 [0.76, 0.85]) but had minimal impact on the exposure of carbamazepine-10,11-epoxide (AUC(tau), 0.99 [0.85-1.15]; C(max), 1.05 [0.91, 1.22]). In summary, a 2-way pharmacokinetic interaction between efavirenz and carbamazepine was demonstrated in this study.