Abstract Lansoprazole (Prevacid) is an antiulcerative drug used for the treatment of duodenal and gastric ulcers, reflux oesophagitis, and Zollinger–Ellison syndrome. During the bulk synthesis of lansoprazole, we have observed five impurities: lansoprazole N-oxide, lansoprazole sulfone N-oxide, lansoprazole sulfide, lansoprazole sulfone and N-aralkyl lansoprazole. The present work describes the synthesis and characterization of these ...