Long term use of antidepressants and bladder medicines could increase the risk of dementia.
A new study by the University of East Anglia found that some classes of anticholinergic drugs, particularly those used to treat depression, Parkinson's and urinary incontinence, carry a higher risk than others.
Researchers said patients who take them for more than a year, were 30% more likely to develop dementia.
Researchers also said up to two million people in England are likely to be taking one of the anticholinergics identified by the study.
They said this does not prove that some anticholinergics cause dementia, but doctors should consider the long-term effects when prescribing them.
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."