A popular type of medication relied upon by millions to fight heartburn may raise the risk of dementia in older adults, yet another study has found.

The drugs are known as proton pump inhibitors and biologically serve as a tap to turn off stomach acid. As a class they are the most widely sold medications in the world and include some of the most famous names on pharmacy shelves: Prilosec, Prevacid and Nexium, the so-called “purple pill.”

But a team of German researchers reporting Tuesday in the American Medical Association’s publication, JAMA Neurology, found that among people 75 and older who regularly took the drugs, the likelihood of dementia was 44 percent higher than people the same age who never took them.

The finding is not the first to link the drugs to dementia and the second blow in a month against a class of drugs ranked by the World Health Organization as among the most important.

In January, a team of physicians from Johns Hopkins University declared in JAMA Internal Medicine that proton pump inhibitors not only increased the risk of chronic kidney disease but 70 percent of the prescriptions written in the United States were probably unnecessary.

Writing in a separate editorial last month, Drs. Adam Jacob Schoenfeld and Deborah Grady of the University of California, San Francisco, further lambasted the drugs. “A large number of patients are taking PPIs for no clear reason,” they said, noting that some people are on the medications for heartburn that had long since resolved.

The dementia issue reported Tuesday raised a new round of questions.

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Dr. Britta Haenisch of the German Center for Neurodegenerative Diseases in Bonn found in a statistical analysis involving nearly 74,000 members of a large German pharmaceutical database that nearly 40 percent of those who regularly took the medications developed dementia. Haenisch analyzed patients’ information between 2004 and 2011.

Dr. Gisele Wolf-Klein, director of geriatric education for Northwell Health and professor of medicine at Hofstra-Northwell School of Medicine, said she does not see a connection between the drugs and dementia.

“The truth is we don’t know the etiology of the disease,” she said of dementia’s cause, “so it is impossible to say that these drugs are responsible.

“We should be very, very careful before we say that the reason for dementia is a protein pump inhibitor. These drugs are very commonly used and dementia is a very common condition,” Wolf-Klein said.

But Wolf-Klein’s colleague, Dr. Arun Swaminath, also a professor of medicine at Hofstra-Northwell School of Medicine and director of the inflammatory bowel disease program at Northwell-Lenox Hill Hospital in Manhattan, said there are reasons for additional research.

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“This study actually duplicated results of a previous one that also saw an increase in dementia,” Swaminath said. “So when you start seeing more than one study with the same finding, you tend to put more stock in it.”

Nevertheless, he is not yet convinced of an association between the drugs and memory decline. He said the new German research has limitations because it is a statistical study and looked at patients’ use of proton pump inhibitors retrospectively.

Swaminath insists the jury is still out because even though the German research has underlined a potent correlation, it has not proved causality.

More important, Swaminath said, even if the drugs are involved in dementia, no one knows what the mechanism of action is — how the drugs might set in motion the biological cascade of events that trigger memory decline.

Haenisch, meanwhile, did not tell doctors to stop prescribing heartburn drugs. She and her colleagues said physicians should follow guidelines for appropriate use.

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Still, proton pump inhibitors are not the only antacids linked to cognitive decline. In 2007, researchers reported that drugs belonging to a family called H2 blockers were also associated with an increased risk of dementia. H2 blockers include such commonly used medications as Tagamet, Axid and Pepsid.