Omeprazole (INN) (IPA: ) is a proton pump inhibitor used in the treatment of dyspepsia, peptic ulcer disease (PUD), gastroesophageal reflux disease (GORD/GERD) and Zollinger-Ellison syndrome. It was first marketed by AstraZeneca as the magnesium salt omeprazole magnesium under the trade names Losec and Prilosec, and is now also available from generic manufacturers under various trade names. Omeprazole is one of the most widely prescribed drugs internationally and is available over the counter in some countries.

Faced with the loss of patent protection and competition from generic manufacturers, AstraZeneca developed, launched, and heavily marketed esomeprazole (Nexium), a single enantiomer form of omeprazole.

In 1990, in the U.S., at the FDA's request, the brand name Losec was changed to Prilosec to avoid confusion with the diuretic Lasix (furosemide).Farley D. Making It Easier to Read Prescriptions. FDA Consumer magazine. July-August 1995. URL: http://www.fda.gov/fdac/features/695_prescrip.html. Accessed on: June 11, 2006. Unfortunately, the new name has lead to confusion between omeprazole (Prilosec) and fluoxetine (Prozac), an antidepressant.