A study links the long-term use of some drugs with a higher risk of dementia.
In England, 1.5 to two million people are likely to be taking anticholinergics for depression, Parkinson's and bladder problems.
University of East Anglia researchers found more cases of dementia in patients prescribed larger quantities of particular anticholinergics.
But experts said patients should not stop taking them, as their benefits may outweigh any risk.
The study found no risk with other anticholinergic medicines used to treat common conditions such as hay fever, travel sickness and stomach cramps.
What did the study look at?
The research, funded by Alzheimer's Society and published in the British Medical Journal, looked at the medical records of 40,770 patients aged from 65 to 99 with a diagnosis of dementia between April 2006 and July 2015 and compared them with those of 283,933 people without dementia.
It also analysed more than 27 million prescriptions – making it the biggest study of its kind into the long-term impact of anticholinergic drugs in relation to dementia.
What are anticholinergic drugs?
They block acetylcholine, a chemical messenger that carries signals across the nervous system.
Some are available on prescription only.
What should patients do if they take any of these drugs?
The most important thing is "not to panic", according to Dr Ian Maidment, from Aston University.
"Don't do anything suddenly. Don't stop taking your medication," he told the BBC.
"As a patient, if you are concerned about it, go and speak to your doctor or your pharmacist. You don't have to see them urgently."
Not taking prescribed drugs could have serious consequences, Dr Maidment said.
"Having untreated depression is also a risk as people can die from that, so it is a question of balancing risks," he added.
How have experts reacted?
Alzheimer's Society head of research Dr James Prickett told the BBC that compared with the risk of dementia being caused by an unhealthy lifestyle, the potential risk of an anticholinergic drug contributing to the onset of the disease was "quite small".
"We don't exactly know within those that are taking these drugs, who is at the increased risk and who isn't," he said.
Dementia Research national director Prof Martin Rossor said: "It is important to be cautious about associations as they do not prove causation."
Alzheimer's Research UK research director Dr Carol Routledge said: "The study didn't investigate what might cause this link between anticholinergics and dementia risk, and researchers will need to build on these findings in future studies."
Rob Howard, professor of old age psychiatry at University College London, said: "It is possible that use of some of these drugs may have actually been to treat the very earliest symptoms of Alzheimer's disease, which can be associated with low mood and lower urinary tract infections, many years before the development of dementia."
Dr Parastou Donyai, associate professor of social and cognitive pharmacy at the University of Reading, said: "This type of study imagines that patients actually take their drugs as they were prescribed for them.
"But we know from other research that people with long-term health conditions really only take their medication as prescribed around half of the time – the other half, people either take more or less of their medication or not at all."