Doctors should be cautious when prescribing some anti-depressants and bladder medications which may heighten the risk of developing dementia, researchers have said.
A "robust" link was found between certain anticholinergic drugs, used to treat a variety of conditions, and future incidence of the neurological disorder, according to a study published in the BMJ.
Patients who took anticholinergics - which work by blocking a key neurotransmitter in the body called acetylcholine - for more than a year had around a 30% increased risk of dementia, researchers said.
The clearest effect was associated with anticholinergics used to treat depression - for example, amitriptyline, dosulepin and paroxetine - and bladder conditions, such as tolterodine, oxybutynin and solifenacin.
However no risk was associated with the use of other anticholinergics, including antihistamines and drugs for stomach cramps, despite previous research suggesting they could lead to long-term cognitive decline.
Up to two million people in England are likely to be taking one of the anticholinergics identified by the study as linked to dementia at any one time, the authors said.
The study, the largest of its kind, compared the records of 40,770 patients over the age of 65 and diagnosed with dementia with 283,933 people without the condition.
Patients with dementia had a higher exposure to anticholinergics in the previous four to 20 years before diagnosis, the researchers from the University of East Anglia (UEA) found.
A typical patient aged 65 to 70 years old has around a 10% chance of developing dementia over the next 15 years.
This risk increased by three percentage points among those who had used anticholinergics for more than one year, they said.
A dementia risk was also linked with medications to treat Parkinson's disease, including procyclidine, but this was less certain.
The research, funded by Alzheimer's Society, does not prove that some anticholinergics cause dementia but the authors said doctors should consider the long-term effects when prescribing.
The drugs had already been shown to affect short-term cognition.
"There are robust associations between levels of anticholinergic antidepressants, antiparkinsons, and urologicals and the risk of a diagnosis of dementia up to 20 years after exposure," they wrote.
"Clinicians should continue to be vigilant with respect to the use of anticholinergic drugs, and should consider the risk of long-term cognitive effects, as well as short-term effects, associated with specific drug classes when performing their risk-benefit analysis."
Lead researcher Dr George Savva, from UEA's school of health sciences, said: "What we don't know for sure is whether the medication is the cause. It could be that these medications are being prescribed for very early symptoms indicating the onset of dementia.
"But, because our research shows that the link goes back up to 15 or 20 years before someone is eventually diagnosed with dementia, it suggests that reverse causation, or confounding with early dementia symptoms, probably isn't the case."
Dr Ian Maidment, of Aston University, lead pharmacist on the study, said: "The key thing is don't panic. It's a long-term effect so don't suddenly stop taking medication."
He said people "probably need to be more conservative in using these drugs" and doctors should be looking to limit "the overall anticholinergic burden" among the old and middle-aged.
Dr Doug Brown, chief policy and research officer at the Alzheimer's Society, said: "Dementia is the UK's biggest killer, and with no new treatments in 15 years, prevention is key.
"As well as our ongoing quest to find new treatments, we urgently need to understand what raises the risk of dementia so we can find ways to prevent it."
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