Elderly people in good health should not take an aspirin a day, according to a major study in the US and Australia.
There are proven benefits of the drug for people after a heart attack or stroke.
But the trial found no benefit for healthy people over the age of 70, and the pills increased the risk of potentially fatal internal bleeding.
Experts described the results as very important and cautioned against self-medicating with aspirin.
People are prescribed aspirin after a heart attack or stroke because the drug thins the blood and reduces the chances of a repeat attack.
Some completely healthy people also choose to take aspirin to reduce their risk and there is continuing research into whether the drug can be used to cut the risk of cancer.
However, most research on the benefits of aspirin is performed on people in middle age and there is mounting evidence the dangers increase as we get older.