Prilosec is a type of medication called an acid pump inhibitor. GERD symptoms occur when a small valve, called a sphincter, between the stomach and the esophagus, which is the tube the food travels through on its way to the stomach, leaks. When this happens, the digestive fluids and acids from the stomach can back up into the esophagus. Stomach acids can irritate and sometimes even damage the fragile lining of the esophagus.

Prilosec turns off some of the millions of tiny "acid pumps" that line the stomach. Therefore, the stomach does not produce as much acid, although there is still enough to properly digest food. Once stomach acid is under control, there is less to back up into the esophagus. This relieves the typical GERD symptoms such as heartburn.

In some cases, the lining of the esophagus may have been damaged before treatment began. Sometimes the acid wears away, or erodes, the esophageal lining, causing a condition called erosive esophagitis. Prilosec also heals this condition by decreasing the amount of acid backing up into the esophagus.