Common pills may cause one in ten cases of dementia
Common drugs including antidepressants could increase the risk of dementia by up to 50pc, a major study has found.
Experts said the findings had "enormous implications" for millions of people around the world, with half of middle-aged people taking one of the medications in many western countries.
The specific class of drugs - which are also prescribed to treat bladder conditions, Parkinson's disease and epilepsy - is known as anticholinergic medication.
Scientists said they could be responsible for as many as one in 10 cases of dementia.
The study by Nottingham University, published in 'JAMA Internal Medicine', involved more than 280,000 British patients over the age of 55 - including about 59,000 who already had a diagnosis of dementia.
Among both groups, more than half were taking some kind of anticholinergic drugs, which help to relax muscles, and work by blocking acetylcholine, a chemical that transmits messages in the nervous system.
Their findings showed an almost 50pc increased risk of dementia among patients aged 55 and over who had used strong anticholinergic meds daily for three years or more.
Britain's National Health Service advises that these drugs should be avoided by frail older people because of their impact on memory and thinking.
But experts said the new findings suggested caution should be taken in prescribing them to anyone who is middle-aged or older.
The study was observational, so could not prove that the drugs cause dementia.
Experts said that many of the conditions the drugs are prescribed for - such as depression and Parkinson's disease - are risk factors for dementia, which could also explain the link.
But researchers said that the findings could mean about 10pc of dementia diagnoses are attributable to the use of the drugs.
This would equate to about 250 cases of dementia in Ireland annually, and about a million new cases of the condition around the world every year.
Prof Clive Ballard, executive dean of medicine at the University of Exeter Medical School, said: "This is a very important finding with enormous and very practical implications that could improve brain health."
But he said it was important to note that some of the drugs might be more likely to be prescribed to those with problems such as psychiatric symptoms and urinary incontinence, and that those could be clues to increased risks of cognitive decline anyway.