Garlic (Allium sativum L.) is a commonly used food and herbal supplement. The objective of this study was to assess in healthy volunteers (N = 14) the influence of a garlic extract on the activity of cytochrome P450 (CYP) 2D6 and 3A4. Probe substrates dextromethorphan (CYP2D6) and alprazolam (CYP3A4) were administered orally at baseline and again after treatment with garlic extract (3 x 600 mg twice daily) for 14 days. Urinary dextromethorphan/dextrorphan ratios and alprazolam plasma concentrations were determined by HPLC at baseline and after garlic extract treatment. The ratio of dextromethorphan to its metabolite was 0.044 +/- 0.48 at baseline and 0.052 +/- 0.095 after garlic supplementation. There were no significant differences between the baseline and garlic phases (P > or =.05). For alprazolam, there were no significant differences in pharmacokinetic parameters at baseline and after garlic extract treatment (all P values > or =.05; maximum concentration in plasma, 27.3 +/- 2.6 ng/mL versus 27.3 +/- 4.8 ng/mL; time to reach maximum concentration in plasma, 1.9 +/- 1.4 h versus 2.4 +/- 1.8 h; area under the time-versus-concentration curve, 537 +/- 94 h. ng. mL(-1) versus 548 +/- 159 h. ng. mL(-1); half-life of elimination, 13.7 +/- 4.4 h versus 14.5 +/- 4.3 h). Our results indicate that garlic extracts are unlikely to alter the disposition of coadministered medications primarily dependent on the CYP2D6 or CYP3A4 pathway of metabolism.