Some commonly-prescribed drugs for depression, epilepsy and a few other conditions may increase a person's risk of dementia, according to researchers.
The medicines, which belong to a family of drugs called anticholinergics, have already been linked to short-term problems with thinking.
Now a large study of UK patients raises concerns about possible long-term brain side effects.
Experts stress the findings, in JAMA Internal Medicine, external, do not prove there is a direct risk or mean that patients should come off the drugs.
What are anticholinergics?
Anticholinergic drugs block the action of a chemical messenger used by the brain to control signals around the body.
Doctors prescribe them for a wide range of conditions and millions of people in the UK are on them.
They can be used to treat overactive bladder, chronic obstructive pulmonary disease, depression, epilepsy, psychosis, Parkinson's disease and some allergies. These medications include some antidepressants, antipsychotics and muscle relaxants.
What are the concerns?
The study, led by Prof Carol Coupland at the University of Nottingham, included more than 58,000 people with dementia and 225,000 without the condition.
The researchers looked at medication use going back over 20 years, before any dementia was diagnosed.
This revealed the link between strong anticholinergic medications and increased risk of dementia in the people aged 55 and older. Only certain drugs in this class of medicine - antidepressants, anti-Parkinson drugs, antipsychotics, bladder drugs and epilepsy drugs - were implicated.
There was no risk with other anticholinergic medicines, including asthma medication, muscle relaxants, heart rhythm drugs and ones given for gastrointestinal problems.
The researchers say their work suggests the link may be real, but there is not enough proof to be sure. It might be that other factors, not captured in the study, are involved instead.
It is also possible that some of the patients may have been put on these treatments for symptoms that were actually because of early, undiagnosed dementia.
But even if the link is true, the risk posed will be smaller than that of some other dementia risk factors, such as smoking, say the researchers.
What do experts advise?
Guidelines already recommend caution about prescribing anticholinergics, because they can cause adverse effects such as constipation, dry mouth, confusion and problems with thinking or memory.
Alternative treatments should be considered when possible, say the investigators.
Researcher Prof Tom Dening said: “This study provides further evidence that doctors should be careful when prescribing certain drugs that have anticholinergic properties.
"However, it’s important that patients taking medications of this kind don’t just stop them abruptly as this may be much more harmful. If patients have concerns, then they should discuss them with their doctor to consider the pros and cons of the treatment they are receiving.”
Dr Jana Voigt, from Alzheimer's Research UK, said: "Anticholinergics can have beneficial effects that doctors need to carefully weigh against any potential side effects. "There is a growing body of evidence that suggests certain anticholinergic drugs are linked to an increased dementia risk.
"In this well-conducted study, researchers looked at the information stored in thousands of people's health records. While finding a link between certain strong anticholinergic drugs and an increased risk of dementia, it doesn't tell us if these drugs cause the condition."