Long-term use of proton pump inhibitor (PPI) acid-suppressing agents, such as omeprazole, has been associated with an increased risk of stomach cancer (also called gastric cancer). This is based on the results of several observational studies and is thought to be because PPIs such as omeprazole are potent gastric acid suppressants that may increase the risk of gastric cancer by causing atrophy (thinning) of the stomach lining, elevating levels of a hormone called gastrin, and an overgrowth of bacteria in the stomach.