Over-the-counter sleeping aids and hay-fever treatments can increase the risk of Alzheimer's, a study says.
The sleeping aid Nytol and anti-allergy pills Benadryl and Piriton all belong to a class of medication highlighted in a warning from researchers.
Each of these drugs has "anticholinergic" blocking effects on the nervous system that are said to raise the likelihood of developing Alzheimer's and other forms of dementia significantly at higher doses over several years.
Other drugs on the risk-list include older "tricyclic" antidepressants such as doxepin, and the bladder control treatment Ditropan.
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Many of these medicines are taken by vulnerable older people, according to the scientists, who say their findings have public health implications.
Anticholinergic drugs block a nervous system chemical transmitter called acetylcholine, leading to side effects that may include drowsiness, blurred vision and poor memory.
People with Alzheimer's disease are known to lack acetylcholine.
Previous research has raised concerns about the use of anticholinergic drugs and mental impairment in the elderly but the new study, published in the journal Jama Internal Medicine, is the first to show a dose response linking greater use of the medicines with an increasing risk of dementia.
The scientists tracked the health of 3434 men and women, aged 65 and over, for around seven years while monitoring their use of anticholinergic drugs.