This review comprises data from more than 19,000 individuals who have taken part in clinical studies of omeprazole. Isolated, non-specific adverse events which might be attributable to omeprazole have included nausea, dizziness, headache and diarrhoea. These events have been generally mild and transient and have not usually required either a reduction of dose or cessation of therapy. The frequency and spectrum of adverse events have been the same in those over 65 years of age as in younger patients. No drug-related adverse events have been found in patients with renal insufficiency or severe liver failure. More than 1.2 million patient treatments of omeprazole have now been given. The overall incidence of adverse events with omeprazole is low, and in comparative studies has been in the same range as that found with H2-receptor antagonists. Importantly, no dose-related adverse events have been observed with omeprazole in the dose range 10-60 mg/day. Furthermore, none of the serious adverse events that have been reported have been attributable to omeprazole. No histological changes in oxyntic endocrine cells have been found after short-term periods of treatment with either omeprazole or H2-receptor antagonists in patients with peptic ulcer disease. Long-term continuous high-dose omeprazole treatment of patients with Zollinger-Ellison syndrome has not induced any significant increase in the oxyntic endocrine cell hyperplasia. Investigations of the gastric mucosa from patients in a compassionate use programme who have received omeprazole, usually 20 mg daily, for periods of up to 37 months, have been performed. Two hundred and forty-eight patients had their last biopsy taken after at least 11 months of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)