An antidepressant for use in the patient receiving concomitant drug treatment, over-the-counter medications, or herbal products should lack cytochrome P-450 (CYP) 3A4 inductive or inhibitory activity to provide the least likelihood of a drug-drug interaction. This study addresses the potential of 4 diverse antidepressants (venlafaxine, nefazodone, sertraline, and fluoxetine) to inhibit or induce CYP3A4. In a 4-way crossover design, 16 subjects received clinically relevant doses of venlafaxine, nefazodone, or sertraline for 8 days or fluoxetine for 11 days. Treatments were separated by a 7- to 14-day washout period and fluoxetine was always the last antidepressant taken. CYP3A4 activity was evaluated for each subject at baseline and following each antidepressant using the erythromycin breath test (EBT) and by the pharmacokinetics of alprazolam (ALPZ) after 2-mg dose of oral ALPZ. Compared to baseline, venlafaxine, sertraline, and fluoxetine caused no apparent inhibition or induction of erythromycin metabolism (P > 0.05). For nefazodone, a statistically significant inhibition was observed (P < 0.0005). Nefazodone was also the only antidepressant that caused a significant change in ALPZ disposition, decreasing its area under the concentration-versus-time curve (AUC; P < 0.01), and increasing its elimination half-life (16.4 vs. 12.3 hours; P < 0.05) compared with values at baseline. No significant differences were found in the pharmacokinetics of ALPZ with any of the other antidepressants tested. These results demonstrate in vivo that, unlike nefazodone, venlafaxine, sertraline, and fluoxetine do not possess significant metabolic inductive or inhibitory effects on CYP3A4.