Those who thought they had taken a weight-loss supplement ate more than the others.

London - Allergy pills taken by thousands of Britons could be causing memory problems, according to scientists.

They say common over-the-counter treatments, such as Benadryl for hayfever, have been found to increase forgetfulness and cause concentration difficulties in pensioners.

And they have warned there could be a link between memory problems and popular sleeping pills and anti-depressants.

The researchers are urging doctors to consider whether patients should be taking these medications in light of the possible side-effects.

Although they did not look at the risk of dementia, previous evidence suggests that having mild memory problems makes you far more susceptible to the illness. Academics from Montreal University in Canada looked at 68 studies which had examined the potential health risks of certain medications.

They found treatments for allergies, insomnia, depression and for overactive bladders were all linked to memory and concentration problems.

These drugs included Benadryl, for hayfever, benzodiazepines such as the sleeping pills diazepam and temazepam, and the anti-depressant amitriptyline.

Lead researcher Dr Cara Tannenbaum said that the study, published in the journal Drugs & Aging, showed the potential effects of such drugs on memory problems, or cognitive decline, was “undervalued”.

“Patients need this information so that they are more comfortable talking to their doctors and pharmacists about safer pharmacological or non-pharmacological treatment options,” she added. “Each individual has a right to make an informed choice based on preference and a thorough understanding of the effects the medications may have.”

The study added: “There is evidence suggesting that drug-induced mild cognitive impairment can occur with episodic use of medications for insomnia, anxiety or allergy symptoms.”

Experts believe that the drugs interfere with chemicals in the brain called neurotransmitters, which are responsible for passing on messages, resulting in forgetfulness and concentration problems, as well as dementia.

Earlier this year Harvard researchers found benzodiazepines increased the risk of developing the devastating illness by up to 50 percent.

As many as 15 million Britons suffer from some form of allergy, including hayfever, although not all seek treatment. Most antihistamine tablets are sold over the counter. Figures also show around 1.5 million Britons are on sleeping pills and six million on anti-depressants. - Daily Mail