Proton pump inhibitors are used at different dosages for the treatment of acid-related gastrointestinal disorders, such as gastro-oesophaeal reflux and peptic ulcer disease. Comparisons of four different proton pump inhibitors: lansoprazole, omeprazole, pantoprazole, and rabeprazole show that they all have similar potency and efficacy. Rabeprazole, however, displays a slightly more rapid onset of acid inhibition than the others; the clinical advantage of this seems limited. The S-isomer of omeprazole, esomeprazole, exhibits a somewhat higher potency than the other proton pump inhibitors. Reports supporting a clinical advantage of this property are not convincing. To conclude, all inhibitors seem comparable as regards inhibition of gastric acid secretion.