This study compares the efficacy and side effects of omeprazole and regular- and double-dose nizatidine in the treatment of duodenal ulcers. Duodenal ulcer healing rates in these three groups (omeprazole, 20 mg qd; nizatidine, 300 mg hs and 600 mg hs) were 81.8%, 19% and 30%, respectively, after two weeks of therapy; and 90.5%, 70% and 84.2%, respectively, after four weeks of treatment. Omeprazole had a significantly better healing rate than nizatidine, 300 mg or 600 mg, after two weeks of treatment (p < 0.01), but not after four weeks of treatment. Omeprazole relieved the ulcer pain sooner than nizatidine (p < 0.05). Smoking decreased the duodenal ulcer healing rate in the omeprazole group, but not in the nizatidine groups. Clinical features, such as sex, age, alcohol consumption, ulcer size, past history of upper gastrointestinal bleeding and duration of peptic ulcer history, did not collate with the healing rate. Patients with double-dose nizatidine did not show any benefits over those with a regular dose in this study. Adverse effects were minor, and there were no significant changes in biochemistry after therapy in these three groups of patients.