A study has found that painkillers such as ibuprofen cannot prevent Alzheimer's disease - but does not rule out that they may delay its onset.
Work had raised hope that non-steroidal anti-inflammatory drugs (NSAIDs) might have a preventative effect.
But the latest 12-year study found the risk of dementia was 66% higher in people with heavy NSAID use, compared with those with little or no use.
The University of Washington study features in the journal Neurology.
NSAIDs are useful for relieving pain from conditions including arthritis.
A US study published last year and based on data from almost 250,000 veterans showed those who used ibuprofen for more than five years were more than 40% less likely to develop Alzheimer's.
The latest research focused on 2,736 people with an average age of 75 at the start of the study.
Lead researcher Dr Eric Larson said: "Although we hoped to find a protective effect, there was none.
"Thus, for this age group, there's no basis for taking NSAIDs to prevent Alzheimer's disease."
His colleague Dr John Breitner said a key difference between the latest study and previous work was that the participants were older.
He said: "It has been argued for some time that NSAID use delays the onset of Alzheimer's disease.
"It would follow that studies looking at younger people who use NSAIDs would show fewer cases of Alzheimer's, while in groups of older people there might be more cases, including those that would have occurred earlier if they had not been delayed.
"We must not ignore the fundamental finding, which is an increase in the risk of dementia in the NSAID users.
"We need further research to understand that result more clearly."
Rebecca Wood, of the Alzheimer's Research Trust, said there were no clear evidence pinning down the effect of NSAIDs - if any - on dementia.
She said: "Work in this area is complicated; many factors can interfere with the findings.
"We know that inflammation is involved in Alzheimer's - so investigating the effects of anti-inflammatory drugs makes sense."
But she added: "We do not advise that people take aspirin or ibuprofen just to try to protect against dementia, as they can cause dangerous side-effects, including stomach ulcers and kidney problems.
"Always consult a medical professional if you are concerned about your health."