Omeprazole, lansoprazole and pantoprazole are metabolized by several human cytochromes P450, most prominently by CYP2C19 and CYP3A4. Only pantoprazole is also metabolized by a sulfotransferase. Differences in the quantitative contribution of these enzymes and in the relative affinities of the substrates explain some of the observed interactions with carbamazepin, diazepam, phenytoin and theophylline and of the impact of the CYP2C19 (mephenytoin) genetic polymorphism. Of these drugs, pantoprazole has the lowest potential for interactions, both in vitro and in human volunteer studies.