A substituted benzimidazole which is a proton pump inhibitor and inhibits secretion of gastric acid, being given orally in the treatment of ulcers and gastro-oesophageal reflux; 5-methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridylmethylsulfinyl)benzimidazole, C₁₇H₁₉N₃O₃S. [Trademarks for this drug include Losec and Prilosec.]