Prilosec, a brandname form of omeprazole, is both an overthecounter and a prescription medication. The prescription form is used to treat gastroesophageal reflux disease, ulcers, and other conditions such as ZollingerEllison syndrome. Prilosec belongs to a group of drugs called proton pump inhibitors, which work by reducing the amount of acid made in the stomach.This medication comes in capsule form or in packets to be mixed with water to form a liquid. It is taken once or twice daily at least 1 hour before a meal. Common side effects of Prilosec include headache, stomach pain, and nausea. As is always the case, take.