This study explores the impact of clopidogrel on the pharmacokinetics of omeprazole related to CYP2C19 genetic polymorphisms. Twelve healthy volunteers (6 CYP2C19*1/*1, 5 CYP2C19*2/*2, and 1 CYP2C19*2/*3) are enrolled in a 2-phase randomized crossover trial. In each phase, the volunteers are administered a single oral dose of omeprazole 40 mg after pretreatment of either placebo or clopidogrel (300 mg on the first day and then 75 mg once daily for 3 consecutive days). Plasma concentrations of omeprazole and its metabolites are quantified by high-performance liquid chromatography with UV detection. After clopidogrel treatment, the AUC(0-infinity) of omeprazole increases by 30.02% +/- 18.03% (P = .004) and that of 5-hydroxyomeprazole decreases by 24.30% +/- 11.66% (P = .032) in CYP2C19*1/*1. The AUC(0-infinity) ratios of omeprazole to 5-hydroxyomeprazole increase by 74.98% +/- 35.48% (P = .001) and those of omeprazole to omeprazole sulfone do not change significantly (P = .832) in CYP2C19*1/*1. No significant alteration is observed in CYP2C19*2/*2 or *3. Clopidogrel inhibits CYP2C19-dependent hydroxylation of omeprazole in CYP2C19*1/*1 and has no impact on CYP3A4-catalyzed sulfoxidation of omeprazole.