Omeprazole is both an overthecounter and a prescription medication. The prescription form is used to treat gastroesophageal reflux disease, certain ulcers, ZollingerEllison Syndrome, erosive esophagitis, and infections cause by the bacteria H. pylori. This medication page refers to the prescription form of omeprazole. 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 one hour before a meal. Common side effects of omeprazole include headache, stomach pain, and nausea. It’s important not to take Omeprazole unless prescribed by a doctor. For RxWiki, I’m Lindsay.