Omeprazole is a racemate, from which the R- and S-isomers can be isolated. At the cellular level, both of these isomers convert to the same inhibitor of the H+,K+-ATPase and produce the same reduction in gastric acid secretion. However, the S-isomer, esomeprazole, is metabolized more slowly and reproducibly than the R-isomer and omeprazole, and therefore produces higher plasma concentrations for longer and, as a result, inhibits gastric acid production more effectively and for longer. Thus, esomeprazole has the pharmacological properties of a more effective form of treatment for disorders related to gastric acid secretion. Clinical studies have confirmed the anticipated increased efficacy, but have shown no evidence of impaired tolerability or increased toxicity when compared with omeprazole.