Omeprazole is the first of a new class of drugs (proton pump blockers) approved in the United States and in Europe for its high efficiency as an inhibitor of gastric acid secretion. Omeprazole is a drug for short term use in patients with acid-peptic disease. A limited prevalence of hepatotoxic effects is reported by some authors (transitory rise in serum aminotransferase level) and it may be prescribed in patients with chronic liver disease although slower metabolism and greater bioavailability are observed. Omeprazole interacts with the cytochrome P-450 system in the liver: inhibition of several liver mono-oxygenases activities (inhibitory effect on diazepam, phenytoin and R-warfarin metabolism with prolonged elimination); induction of P-450 (IA1 and IA2) enzymes that may potentiate the hepatotoxic effect of phenacetin and acetaminophen or increase the tumorigenic effect of chemical carcinogens (polycyclic aromatic hydrocarbons, arylamines, aflatoxin). This latter concern is unfounded as based on a false extrapolation from the results of in vitro studies to those of in vivo situations. However, although omeprazole has proved to be remarkably free of side effects, postmarketing surveillance is recommended for potential interaction with other drugs that are known to be metabolized by the same liver enzymes.