People who carry the stomach bacteria known as Helicobacter pylori are at increased risk for ulcers and stomach cancer. But even when antibiotic treatment has eliminated the bacterium, stomach cancer may still arise. A new study suggests that one reason may be the long-term use of proton pump inhibitors, or P.P.I.s, acid-reducing medicines sold under brand names such as Prilosec and Prevacid.

Researchers studied 63,397 people in Hong Kong successfully treated for H. pylori infection, of whom 3,271 used P.P.I.s and 21,179 took H2-receptor antagonists, another type of acid-controller (Tagamet, Pepcid and other brands). Over an average 7.6 years, 153 of them developed gastric cancer.

Compared with those who used H2 blockers, those who took P.P.I.s had more than twice the risk for cancer, and the risk increased over time. The study, published in Gut, controlled for smoking, alcohol use, obesity, statin use, hypertension and many other factors.

“Even after the eradication of H. pylori, the risk of cancer persists with P.P.Is,” said the lead author, Dr. Wai Keung Leung, a professor of medicine at the University of Hong Kong. “But the absolute risk is not high, and I don’t want to discourage people from taking these drugs when necessary. There are people who benefit tremendously from them.”