Four large clinical studies have shown that omeprazole, 20 mg once daily, is effective in preventing the significant gastroduodenal consequences of nonsteroidal anti-inflammatory drugs (NSAIDs). In the Scandinavian Collaborative Ulcer Recurrence (SCUR) study, patients were randomized (without initial endoscopy) to receive omeprazole or placebo for up to 3 months. Of the patients treated with omeprazole, 24.7% experienced treatment failure, compared with 50.0% of those on placebo. Fewer patients in the omeprazole group than in the placebo group developed a peptic ulcer (4.7% versus 16.7%, respectively) or dyspeptic symptoms (8.2% versus 20.0%, respectively). In the Omeprazole versus Placebo as Prophylaxis of Ulcers and Erosions from NSAID Treatment (OPPULENT) study, patients were also randomized to receive omeprazole or placebo. The estimated probability of remaining in remission for 6 months while receiving omeprazole was 0.78, compared with 0.53 for placebo. Fourteen (16.5%) patients on placebo developed peptic ulcer(s), compared with three (3.6%) patients on omeprazole. The Omeprazole versus Misoprostol for NSAID-Induced Ulcer Management (OMNIUM) study consisted of a healing and a prophylactic phase. Patients who successfully completed the healing phase were re-randomized to receive omeprazole, misoprostol, or placebo for up to 6 months. In patients receiving omeprazole, 36.5% experienced treatment failure, compared with 48.6% of those on misoprostol, and 67.7% of those on placebo. Omeprazole and misoprostol appeared to be equally effective in preventing gastric ulcer; by contrast, omeprazole was highly effective in preventing duodenal ulcer, when compared with misoprostol and placebo. The Acid Suppression Trial: Ranitidine versus Omeprazole for NSAID-Associated Ulcer Treatment (ASTRONAUT) study also consisted of a healing and a prophylactic phase. Patients who successfully completed the healing phase were re-randomized to receive omeprazole or ranitidine for up to 6 months. In patients receiving omeprazole, 26.2% experienced treatment failure, compared with 37.7% of those on ranitidine. The percentages of patients with a peptic ulcer relapse were 5.7% for omeprazole and 19.5% for ranitidine. The data show that omeprazole is an effective and well-tolerated agent for both primary and secondary (maintenance) prophylaxis in patients receiving NSAIDs.