Certain types of over-the-counter hayfever and sleeping pills can raise the risk of developing Alzheimer's disease, a new study has found.
Nytol, Benadryl and Piriton are among those highlighted in a warning from researchers at the University of Washington. The antidepressant Doxepin and bladder control treatment Ditropan were also listed.
These medications have "anticholinergic" effects, blocking a chemical transmitter in the body called acetylcholine, which can lead to blurred vision, poor memory and drowsiness.
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The research, published in the journal Jama Internal Medicine, monitored 3,434 men and women aged over 65 for seven years while recording their use of the anticholinergic drugs.
Around 637, almost one fifth, developed a form of dementia, including Alzheimer's, by the end of the study.
Those taking the highest dose of anticholinergic drugs had a 54 per cent greater risk of developing dementia than those who did not use the drugs at all, while the risk of developing Alzheimer's was 63 per cent greater.
The researchers advised that older people taking these medications should inform their doctor.
People taking at least 10mg per day of doxepin, 4mg per day of diphenhydramine (Nytol, Benadryl) or 5mg per day of oxybutynin (Ditropan) for more than three years had an increased risk of developing dementia, according to the study.
The antidepressant Prozac and anti-histamine loratadine (Claritin) were named as substitutes that did not have anticholinergic effects.
Dr Simon Ridley, head of research at Alzheimer's Research UK, told the Daily Telegraph that people should seek advice from their doctors before stopping their medication.
People with Alzheimer's disease are known to lack acetylcholine and it is feared the pills might make the condition worse.
However, Ridley said that the results do not necessarily show that it is the drugs causing the condition: it could instead be a result of the underlying condition for which people were taking the medication.
"Continued research to shed light on these links will be important for helping understand the benefits and potential risks of these drugs," he said.
