Lansoprazole is a proton pump inhibitor (PPI) that inhibits gastric acid production in the stomach. The drug is available over-the-counter in the United States, and its uses include the prevention and treatment of ulcers in the stomach and duodenum; the treatment of gastroesophageal reflux disease (GERD); and the treatment of Helicobacter pylori infections in which it is used as an adjunctive therapy. The most common side effects are diarrhea and skin rashes or itching.