Which has superior acid-suppressive effect, 10 mg omeprazole once daily or 20 mg famotidine twice daily? Effects of single or repeated administration in Japanese Helicobacter pylori-negative CYP2C19 extensive metabolizers.

Low-dose omeprazole is superior to full-dose famotidine in maintenance therapy for gastroesophageal reflux disease, whereas "on-demand" famotidine is more effective for relief of episodes of heartburn. To explain this apparent discrepancy, intragastric pH was measured for 24-hr seven times in eight Japanese Helicobacter pylori-negative cytochrome P450 2C19 extensive metabolizers; on Days 1, 8, and 15 of repeated administration of 10 mg of omeprazole once daily and of 20 mg of famotidine twice daily and before medication. During repeated administration of omeprazole, mean intragastric pH and % time that intragastric pH > 4.0 were significantly higher and became greater. With famotidine, although these parameters were significantly higher, the degrees became smaller. Consequently, acid-suppressive effect was in the order; omeprazole < famotidine on Day 1, omeprazole approximately famotidine on Day 8, and omeprazole >famotidine on Day 15. This discrepancy possibly results from the "potentiation" of acid-suppressive effect of omeprazole and the "tolerance" phenomenon in respect to famotidine.