Healthy older people who take regular aspirin to prevent stroke may actually be increasing their risk.
Some healthy older people take aspirin to ward off strokes
In the past 25 years the number of strokes associated with blood-thinning drugs such as aspirin or warfarin has risen seven-fold, a UK study found.
The risk is particularly high in the over 75s and aspirin may do more harm than good in healthy older people, The Lancet Neurology paper reported.
However, people advised to take daily aspirin by their GP should not stop.
Researchers at the University of Oxford compared figures on intracerebral haemorrhagic stroke - a type of stroke caused by bleeding in the brain - from 1981-85 and 2002-06.
They found that the number of strokes caused by high blood pressure had fallen by 65%, which in the under 75s meant the overall rate of strokes had halved.
But in the over 75s the stroke rate remained the same over the 25-year period.
A closer look at the data showed there had been an increase in the number of strokes in patients taking blood thinning drugs, known as antithrombotics.
In the first study the proportion of stroke patients on antithrombotic drugs was 4% but two decades later this had risen to 40%.
People with cardiovascular disease, who have a high risk of blood clot, are prescribed drugs like aspirin to thin the blood and reduce the risk of a heart attack or stroke.
But many healthy older people also take a regular aspirin in an attempt to ward off a stroke.
Study leader, Professor Peter Rothwell, said the increasing use of drugs such as aspirin may soon take over high blood pressure as the leading cause of intracerebral haemorrhagic stroke in the over 75s.
He warned than in healthy older adults the risks of taking aspirin may outweigh any benefits.
"GPs have been treating high blood pressure very aggressively and that is bringing dividends but there are other causes of stroke in the elderly which have become important.
"There are good reasons for taking aspirin or warfarin but there are elderly who take aspirin as a lifestyle choice and in that situation the trials have shown there's no benefit.
"And what our study suggests is that, particularly in the very elderly, the risks of aspirin outweigh the benefits," he said.
Dr Peter Coleman, deputy director of research and development for The Stroke Association said aspirin had gained a reputation of being part of a healthy lifestyle.
"However, this evidence indicates that if you are healthy and have a low risk of heart disease or stroke and unless advised by your GP to take aspirin on a daily basis then the increased risks from the side effects of aspirin are likely to outweigh the benefits of preventing a stroke."
He advised people to lower their risk of stroke by having regular blood pressure checks, eating a healthy diet, stopping smoking, only drinking alcohol in moderation, reducing salt intake and taking regular exercise.