 -- Common over-the-counter drugs such as Benadryl may be linked to dementia, according to a new study published this week in JAMA Internal Medicine.

Researchers at University of Washington and a nonprofit called Group Health followed 3,434 adults over age 65 for more than seven years. None of them had dementia or Alzheimer's disease when the study started, and they kept careful records about the drugs they took. The researchers found that people who took more anticholinergic drugs -- which affect the nervous system and are in antihistamines and some bladder control medications -- were more likely to develop dementia.

The link has been studied before but this is an especially good study, said Dr. James Leverenz, a neurologist who directs the Cleveland Center for Brain Health at the Cleveland Clinic and was not party of the new study.

"People want to be very careful about what kind of medicines they taking or if they already have some issues with their thinking skills," Leverenz said. "I knew when I first start taking [antihistamines] for allergies, I feel a little clouded and sleepy. Some of that is the anticholinergic effect in the brain."

Still, he stressed the researchers only found an association between the drugs and dementia and did not determine whether the drugs caused dementia. More research is needed, he said, adding that some antihistamine products have fewer effects on the cholinergic system of the brain, which is involved in thinking skills and memory. He said this link between dementia and these drugs has not been studied in younger adults.

"Consult with your primary clinician if you have questions," he said. "Carefully read the contents of medications you take over the counter."