Commonly used painkillers which had been thought to prevent Alzheimer's have no effect, research suggests.
Daily aspirin use is controversial
Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) did not protect memory or prevent dementia, two US studies found.
The first, in the British Medical Journal, looked at over 6,000 women. The other in Neurology looked at 2,000 with a family history of Alzheimer's.
UK experts cautioned the drugs can also carry serious side effects.
Past research on the possible benefit of aspirin and NSAIDs has been mixed, with some studies showing slight benefits.
In the studies, by teams at Brigham and Women's Hospital and Johns Hopkins School of Medicine, participants took either the drug (low dose aspirin, naproxen or celecoxib) or a dummy treatment, called a placebo, for up to 11 years.
Over the course of the investigation, they were asked to complete tasks designed to test memory and were checked for signs of dementia.
The researchers found no differences between the groups, suggesting the drugs offered no protection against Alzheimer's or cognitive decline.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said the results were disappointing.
"The number of people in the UK with dementia is expected to double in a generation unless we can find a way to prevent it.
"Unfortunately aspirin does not appear to give the protection that some scientists had hoped for given the potential links between dementia and heart disease, so we urgently need to invest in more research to find other treatments."
She said another NSAID, ibuprofen, had been shown to dissolve away protein clumps in the brain that are linked to Alzheimer's disease.
And in the aspirin study, there was some evidence that verbal fluency was better in women taking this drug compared with those on the placebo. Ms Wood said this warranted further investigation.
She cautioned: "It is not advisable to take aspirin or ibuprofen to try to protect against dementia as they can cause dangerous side-effects, including stomach ulcers and kidney problems."
There is evidence to suggest that people who have had a heart attack, or are considered at risk of heart disease benefit from being prescribed aspirin.
Dr Susanne Sorenesen, head of research at the Alzheimer's Society, said: "There is nothing to suggest painkillers reduce the onset of Alzheimer's or other types of dementia.
"The best evidence for reducing risk remains a healthy lifestyle, with regular exercise and a balanced diet. A healthy heart leads to a health mind."