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People who regularly take aspirin to ward off strokes in later life could be doing themselves more harm than good.
Researchers have found that the proportion of people over 75 in the UK who have suffered bleeding strokes has jumped ten-fold in the past 25 years, reflecting the increased use of antithrombotic drugs such as aspirin and warfarin.
There are two main types of stroke: one occurs when the blood flow to the brain is blocked by a clot; the other happens when a blood vessel bursts and bleeds into the brain.
Antithrombotic drugs can reduce the risk of strokes and heart attacks in people who are known to be at risk of the diseases, because they thin the blood and lessen the chance of developing blood clots. But thinner blood also raises the risk of bleeding.
Although there is no clear evidence to suggest an overall benefit for healthy people with a low risk of heart disease or stroke, many choose to take aspirin regularly as an insurance policy for later life.
In the study, Peter Rothwell of Oxford University's department of clinical neurology, found large numbers of older people suffered from bleeding in the brain when they took antithrombotic drugs.
"There's no doubt that aspirin and warfarin - when indicated - are beneficial, irrespective of age. But there is a tendency now for people to take aspirin just in case. They don't have a history of vascular disease, but they're keen to take it to prevent stroke. What (our) trials have shown is that there is very little evidence that, if you don't have vascular disease, aspirin does any good - the risks outweigh the benefits."
In the study, Professor Rothwell examined the causes of almost 1,000 strokes recorded as part of the Oxford Vascular Study, which ran for four years from 2002. He compared the results with the causes of a similar number of strokes recorded in the Oxford Community Stroke Project, from 1981 to 1986.
"In the earlier study, the majority of haemorrhages occurred in people with evidence of fairly severe high blood pressure before the event," said Prof Rothwell. "What we found now, in the last few years, was that while some of the younger people had hypertension as a cause, the vast majority over 75 didn't have hypertension, or it had been treated years ago."
One key difference was that in the later records large numbers of people were taking antithrombotic drugs regularly. "We had about 50 haemorrhages in each of the two studies - the use of antithrombotics in patients with haemorrhages was four per cent in the 1980s, versus just over 40 per cent now."
Working out exactly how the antithrombotic drugs affect users will need more research. Prof Rothwell said: "It's probably because of the changes that occur with ageing in the very small blood vessels in the brain - they tend to become more prone to bleeding. Aspirin makes you more prone to bleed."
