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We compared gastric pH values after therapeutic doses of lansoprazole and omeprazole in 17 healthy adult men. The pharmacokinetics of the two drugs were studied. A three-way crossover design compared the effects on gastric pH of 15 and 30 mg lansoprazole and 20 mg omeprazole-each given once daily for 5 days. Ambulatory 24-h intragastric pH levels were measured before dosing, after the first and fifth doses in each period, and 15 days after each dosing period. A positive relationship between the lansoprazole or omeprazole area under the curve (AUCs) and the 24-h mean pH values was found for each regimen. No differences in maximum concentration (Cmax) and AUC were noted from day 1 to day 5 for the two lansoprazole doses. With omeprazole, both Cmax and AUC levels were greater on day 5 than on day 1. All three regimens increased 24-h mean gastric pH, although 30 mg lansoprazole had the most significant effect. The percentage of time that gastric pH was >3, >4, and >5 was also significantly higher with 30 mg lansoprazole. All three regimens were associated with reversible elevations of serum gastrin, which more than doubled at some points. No clinically significant adverse events were documented.