Esomeprazole is a 5-methoxy-2-{[(4-methoxy-3,5-dimethylpyridin-2-yl)methyl]sulfinyl}-1H-benzimidazole that has S configuration at the sulfur atom. An inhibitor of gastric acid secretion, it is used (generally as its sodium or magnesium salt) for the treatment of gastro-oesophageal reflux disease, dyspepsia, peptic ulcer disease, and Zollinger-Ellison syndrome. It has a role as a histamine antagonist, an EC 3.6.3.10 (H(+)/K(+)-exchanging ATPase) inhibitor, an anti-ulcer drug and an EC 1.4.3.4 (monoamine oxidase) inhibitor. It is a conjugate acid of an esomeprazole(1-). It is an enantiomer of a (R)-omeprazole.