Elderly people with good health should not take aspirin daily, according to a larger study in the US and Australia.
There are proven benefits of the drug for people after a heart attack or stroke.
But the study found no benefit for healthy people over 70, and the pills increased the risk of potentially fatal internal bleeding.
Experts described the results as very important and warned against self-medication with aspirin.
People are prescribed aspirin after a heart attack or stroke because the drug dilutes the blood and reduces the likelihood of a seizure.
Some completely healthy people also choose aspirin to reduce their risk; and it is further explored whether the drug can be used to reduce the risk of cancer.
However, most research on the benefits of aspirin is being conducted in middle-aged people, and there is increasing evidence that the risks increase with age.
# No advantage & # 39;
The study was of 19,114 people in the US and Australia in good health, with no history of heart problems and at the age of 70 years.
Half received a daily, low-dose aspirin for five years.
Three reports in the New England Journal of Medicine showed that the tablets did not reduce their risk of heart problems or had other benefits.
They have increased the number of larger gastric bleeding.
Professor John McNeil of Monash University said, "It means millions of healthy elderly people around the world who are taking low-dose aspirin for no medical reason, possibly unnecessarily, because the study showed no overall benefit to compensate for the risk of bleeding.
"These findings will help inform prescribers who have long been unsure whether to recommend aspirin to healthy patients."
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The study also found an increase in deaths from cancer, although researchers believe that this requires further investigation, as this contradicts current findings in this area.
Professor Peter Rothwell of Oxford University, a leading expert on the drug, said the results were definitive: "Taking aspirin, if you are otherwise healthy, over 70 years old, if you have not had a heart attack or stroke, really low Use.
"And so self-medication with aspirin in the absence of a specific medical indication is not advisable."
The results do not apply to people who take aspirin because of a heart attack or stroke – they should continue to follow their doctor's advice.
And anyone who has been taking low-dose aspirin for a long time should not stop overnight, as it can also cause problems. Instead, they should discuss concerns with their family doctor, says Prof. Rothwell.
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