A popular group of heartburn drugs may be linked to dementia in seniors.

A German study published in the medical journal JAMA Neurology found that seniors who regularly took proton-pump inhibitors like Nexium, Prilosec, and Prevacid were 44% more likely to end up with dementia. Proton-pump inhibitors, commonly used to decrease acid in the stomach, are used by more than 15 million people, according to NPR.

The study followed more than 73,000 elderly participants to track their use of the drug class and incidence of dementia, so it did not establish a cause-and-effect relationship. Prilosec is made by Procter & Gamble (PG), AstraZeneca makes Nexium, and Takeda Pharmaceuticals makes Prevacid.

In an editorial accompanying the study’s publication this week, University of Pittsburgh’s Lewis Kuller wrote: “Whether PPIs actually cause an increase in the risk of dementia is therefore an important question requiring further evaluation … This is a very important issue given the very high prevalence of pharmacological drugs’ long-term use in elderly populations that have a very high risk of dementia.”

The study builds off previous findings that the drugs might increase the risk of heart disease and chronic kidney disease, NPR reported.

P&G said in a statement that Prilosec is a “safe and effective” treatment for heartburn, and that patients should discuss any concerns with their doctors. AstraZeneca spokeswoman Michele Meixell said in a statement that the pharmaceutical company was not consulted during the study and believes that its proton-pump inhibitor medications are all “generally safe and effective when used in accordance with the label.” Fortune has also reached out to Takeda for comment and will update this story if they respond.