Commonly used over-the-counter drugs, used to treat conditions including insomnia and hay fever, have been linked to an increased risk of developing dementia or Alzheimer's, a study shows.

What all of the drugs have in common is what's known as an "anticholinergic" effect where they block acetylcholine, a neurotransmitter, in the brain and body.

Such drugs include antihistamines such as Benadryl, antidepressants, sleeping aids, cardiovascular meds, muscle relaxants and gastrointestinal drugs meant to treat such conditions as diarrhea and ulcers.

Some brand names in the category include Benadryl, Xanax, Sominex, Valium, Ativan, Luminal, Limbitrol and Tavist.

Taking anticholinergic medication at elevated doses or over a long period time can significantly increase a risk of developing Alzheimer's disease or other forms of dementia, say researchers from University of Washington's School of Pharmacy.

Many older people take such drugs, but older people are often more sensitive to the drugs and to their side effects, the researchers say.

"If providers need to prescribe a medication with anticholinergic effects because it is the best therapy for their patient, they should use the lowest effective dose, monitor the therapy regularly to ensure it's working, and stop the therapy if it's ineffective," says Dr. Shelly Gray, director of the university's geriatric pharmacy program.

For the study the UW researchers tracked around 3,500 seniors taking part in a long-running study known as the Adult Changes in Thought (ACT), a cooperative project of the university and the National Institute on Aging.

Previous studies had suggested a link between anticholinergic drugs and dementia, but the recent study was more rigorous, with a longer 7-year follow up period and an improvement analysis of medication intake based on pharmacy records, which included substantial records of nonprescription medicine use.

The more a person uses anticholinergic medications, the greater the risk of developing Alzheimer's, the study findings suggest, and that higher risk can continue, and may prove irreversible, even years after someone stop taking such drugs, the researchers say.

"Older adults should be aware that many medications -- including some available without a prescription, such as over-the-counter sleep aids -- have strong anticholinergic effects," Gray says.

"But of course, no one should stop taking any therapy without consulting their health care provider," she says. "Health care providers should regularly review their older patients' drug regimens -- including over-the-counter medications -- to look for chances to use fewer anticholinergic medications at lower doses."