Omeprazole, sold under the brand names Prilosec and Losec among others, is a medication used in the treatment of gastroesophageal reflux disease, peptic ulcer disease, and ZollingerâEllison syndrome.[1] It is also used to prevent upper gastrointestinal bleeding in people who are at high risk.[1] It can be taken by mouth or injected into a vein.[1][ ...