The magnesium salt of esomeprazole, the S-isomer of omeprazole, with gastric proton pump inhibitor activity. In the acidic compartment of parietal cells, esomeprazole is protonated and converted into the active achiral sulphenamide; the active sulphenamide forms one or more covalent disulfide bonds with the proton pump hydrogen-potassium adenosine triphosphatase (H+/K+ ATPase), thereby inhibiting its activity and the parietal cell secretion of H+ ions into the gastric lumen, the final step in gastric acid production. H+/K+ ATPase is an integral membrane protein of the gastric parietal cell. OR A drug that blocks acid from being made in the stomach. It is used to treat acid reflux disease and to prevent certain types of gastrointestinal ulcers. Esomeprazole magnesium is being studied in the prevention of esophageal cancer and in the treatment of other conditions, including side effects of chemotherapy. It is a type of anti-ulcer agent. Also called esomeprazole and Nexium.