Healthy people who take aspirin to prevent a heart attack or stroke could be doing more harm than good, warn researchers.
An analysis of more than 100,000 patients, published in Archives of Internal Medicine, concluded the risk of internal bleeding was too high.
The UK-led study said only people with a history of heart problems or stroke should take the tablets.
Experts said any decision should be made with a doctor.
Aspirin helps people who have had a heart attack or stroke. It prevents blood clots from forming by preventing cells, known as platelets, from sticking together. By reducing the number of clots formed, the tablets reduce the risk of another heart attack or stroke.
There have also been suggestions that the drug can prevent some cancers, however, the drug is known to increase the chance of internal bleeding, including bleeds on the brain.
The discussion has been whether at-risk or even healthy people should also take aspirin.
Official guidelines were issued in 2005 by the Joint British Societies, which includes the British Cardiac Society, British Hypertension Society and The Stroke Association. It recommended 75 mg of aspirin a day for high risk patients over the age of 50. The Drugs and Therapeutics Bulletin said in 2008 that preventative aspirin should be abandoned unless there was already evidence of cardiovascular disease.
Good or bad?
Researchers analysed data from nine trials, from a total of 102,621 patients.
They said that while there was a 20% reduction in non-fatal heart attacks in people taking aspirin, there was no reduction in deaths from heart attack, stroke or cancer.
Meanwhile the risk of potentially life threatening internal bleeding increased by 30%.
Lead researcher Prof Kausik Ray, from St George's, University of London, told the BBC: "If you treat 73 people for about six years you will get one of these non-trivial bleeds.
"If you treat about 160 people for the same period of time, you're preventing one heart attack that probably wouldn't have been fatal anyway.
"It suggests that the net benefit for aspirin is not there, it certainly doesn't prolong life. If you think about it the net benefit, actually there is net harm.
The study followed patients for an average of six years. An analysis led by Prof Peter Rothwell, from Oxford University, suggested that regularly taking aspirin reduced the risk of a series of cancers, when patients were followed for much longer.
Prof Rothwell said the new study was "very nicely done, but I don't think it develops [the argument] much further".
He added: "It really just emphasises the need for a more detailed analysis of how risks change over time."
Natasha Stewart, senior cardiac nurse at the British Heart Foundation, said: "Aspirin can help reduce the risk of heart attack or stroke among those with known heart disease, and this group of people should continue to take aspirin as prescribed by their doctor.
"Our advice is that people who don't have symptomatic or diagnosed heart disease shouldn't take aspirin because the risk of internal bleeding may outweigh the benefits.
"If you're taking prescribed aspirin and have any concerns, don't simply stop taking it. Always talk to your doctor first."
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