Increased carbamazepine plasma concentrations after fluoxetine coadministration

The interaction between fluoxetine and carbamazepine was investigated in six normal, healthy male volunteers (aged 23 to 40 years). Subjects were given carbamazepine, 400 mg every morning, for 3 weeks. Venous carbamazepine blood samples were obtained at baseline and 1, 2, 4, 6, 8, 10, 12, and 24 hours after the morning dose. Fluoxetine, 20 mg every morning, was then coadministered with carbamazepine for 7 days. Venous carbamazepine blood samples were again obtained as described. Carbamazepine and carbamazepine-10,11-epoxide (CBZE) were assayed by HPLC. Addition of fluoxetine resulted in a significant increase in the area under the concentration-time curve of carbamazepine (105.93 +/- 18.05 micrograms/ml.hr versus 134.97 +/- 12.15 micrograms/ml.hr; t = 3.284; df = 5; p = 0.022) and CBZE (11.6 +/- 1.93 micrograms/ml.hr versus 15.2 +/- 2.4 micrograms/ml.hr; t = 2.805; df = 5; p = 0.038). Both oral and intrinsic clearance of carbamazepine was decreased significantly on fluoxetine addition (3.87 +/- 0.68 L/hr versus 2.98 +/- 0.26 L/hr; t = 3.025; df = 5; p = 0.029 and 17.90 +/- 4.9 L/hr versus 11.92 +/- 1.4 L/hr; t = 3.037; df = 5; p = 0.029, respectively). No significant changes were determined for fraction of absorbed dose, volume of distribution, absorption rate constant, and elimination rate constant. These findings suggest that fluoxetine can inhibit the metabolism of carbamazepine. Careful monitoring of patients is recommended when these two drugs are coadministered.