Researchers said the drugs had an effect for a month after treatment

Common over-the-counter medicines should be avoided by older people as they have been linked to memory loss and problems in thinking, scientists have discovered.

Treatments for colds and flu, hay fever, allergy and heartburn tablets containing anti-cholinergic drugs had the effect for one month after treatment, a study found.

Effects associated with taking the drugs included having slower brain processing times and smaller brains overall.

Well known treatments including the heartburn medicine Zantac, Night Nurse Liquid containing Promethazine and the sleeping tablet Nytol, containing diphenhydramine, are included among drugs that may result in the effects, the research said.

Over the counter treatments for cold, flu, heartburn and sleeping tablets were found to block the chemical acetylcholine, which is involved in the transmission of electrical impulses between nerve cells

The drugs block the chemical acetylcholine, which is involved in the transmission of electrical impulses between nerve cells.

The treatments are prescribed for a wide range of conditions, including Parkinson’s disease, overactive bladder, chronic obstructive pulmonary disease, nausea and vomiting, sleeping problems, high blood pressure, depression and psychosis.

But the authors warn: ‘Use of AC [anti-cholinergic] medication among older adults should likely be discouraged if alternative therapies are available.’

Previous studies have linked the drugs with cognitive impairment, increased risk of dementia and falls.

However, the new study by Indiana University School of Medicine, is the first to explore their impact on brain metabolism and atrophy through brain scans.

Dr Shannon Risacher, the university’s assistant professor of radiology and imaging sciences, said: ‘These findings provide us with a much better understanding of how this class of drugs may act upon the brain in ways that might raise the risk of cognitive impairment and dementia,’

‘Given all the research evidence, physicians might want to consider alternatives to anticholinergic medications if available when working with their older patients.

‘The impact of these drugs have been know about for over a decade, with a 2013 study finding drugs with a strong anticholinergic effect cause cognitive problems when taken continuously for as few as 60 days. Drugs with a weaker effect could cause impairment within 90 days.’

Many over the counter medicines have been linked to slower brain processing times and smaller brains

The new study involved 451 participants, 60 of whom were taking at least one medication with medium or high anticholinergic activity.

The participants were drawn from a national Alzheimer’s research project - the Alzheimer’s Disease Neuroimaging Initiative - and the Indiana Memory and Ageing Study.

To identify possible physical and physiological changes that could be associated with the reported effects, researchers assessed the results of memory and other cognitive tests, positron emission tests (PET) measuring brain metabolism, and magnetic resonance imaging (MRI) scans for brain structure.

Patients taking anticholinergic drugs performed worse than older adults not taking the drugs on short-term memory and some tests of executive function, which cover a range of activities such as verbal reasoning, planning, and problem solving.

Anticholinergic drug users also showed lower levels of glucose metabolism - a biomarker for brain activity - in both the overall brain and in the hippocampus, a region of the brain associated with memory and which has been identified as affected early by Alzheimer’s disease.

The researchers also found significant links between brain structure revealed by the MRI scans and anticholinergic drug use, with the participants using anticholinergic drugs having reduced brain volume and larger ventricles, the cavities inside the brain.

Professor Risacher added: ‘These findings might give us clues to the biological basis for the cognitive problems associated with anticholinergic drugs, but additional studies are needed if we are to truly understand the mechanisms involved.’

The study was published in the journal JAMA Neurology.

John Smith, Chief Executive of the Proprietary Association of Great Britain, which represents makers of over the counter medicines, said the medicines linked to the study were not intended to be used on a daily basis.

Anticholinergic medicines include some over-the-counter allergy and cold and flu products. However they are intended for short term relief of symptoms only

He said: ‘It is important to note that the JAMA study only involved people with a mean age of 73 in what the researchers conceded was a small sample.

The study followed people who took medicines that were low, medium or high in anticholinergic activity, and concluded that the use of medication with medium or high anticholinergic activity should be discouraged in older adults if alternative therapies are available.

'However, due to the study limitations, the researchers propose that further and more advanced studies are needed.

‘Anticholinergic medicines include some over-the-counter allergy and cold and flu products but these are intended for short term relief of symptoms and not for continuous use as in the research.

‘If anyone has any concerns about their medicine, we would advise them to talk to their pharmacist.

'There is a range of different allergy, cold and flu products on the market which contain different ingredients, many of which were not considered in this study, and a pharmacist will be able to recommend a suitable product.

‘All over-the-counter medicines in the UK have been approved by the Medicines and Healthcare Products Regulatory Agency (MHRA) and are rigorously assessed for safety and efficacy.

'Once on the market, their safety is continually monitored in light of any emerging evidence.’