The company's most commercially successful medication is esomeprazole (Nexium). The primary uses are treatment of gastroesophageal reflux disease, treatment and maintenance of erosive esophagitis, treatment of duodenal ulcers caused by "Helicobacter pylori", prevention of gastric ulcers in those on chronic NSAID therapy, and treatment of gastrointestinal ulcers associated with Crohn's disease. When it is manufactured the result is a mixture of two mirror-imaged molecules, R and S. Two years before the omeprazole patent expired, AstraZeneca patented S-omeprazole in pure form, pointing out that since some people metabolise R-omeprazole slowly, pure S-omeprazole treatment would give higher dose efficiency and less variation between individuals. In March 2001, the company began to market Nexium, as it would a brand new drug.