Omeprazole (Prilosec^®) is a prescription medication that is approved to treat certain conditions of the esophagus, stomach, and intestines. Some of these conditions include gastroesophageal reflux disease (GERD), certain ulcers, and H. pylori infections. The drug works by decreasing the amount of acid the stomach produces. Side effects of omeprazole may include abdominal pain (stomach pain), upper respiratory infections, and headaches. Because omeprazole is not suitable for everyone, be sure to tell your healthcare provider (prior to taking omeprazole) about any medications you are taking, including prescription and non-prescription medicines.