Healthy people who take aspirin to prevent heart attacks could be doing themselves more harm than good, experts have warned.
The drug, which reduces the risk of blood clots, can be taken by patients who have already suffered a heart attack or are at risk of one.
Millions of others are also believed to take a daily dose as an "insurance policy" with the hope of guarding against heart trouble.
But the routine use of aspirin by healthy people to prevent heart problems "cannot be supported," professors from the Aspirin for Asymptomatic Atherosclerosis (AAA) said.
Their study found that the risk of cardiovascular problems had to be set against the increased risk of internal bleeding.
Professor Peter Weissberg, of the British Heart Foundation which partially funded the research, said: "We know that patients with symptoms of artery disease, such as angina, heart attack or stroke, can reduce their risk of further problems by taking a small dose of aspirin each day.
"The findings of this study agree with our current advice that people who do not have symptomatic or diagnosed artery or heart disease should not take aspirin, because the risks of bleeding may outweigh the benefits."
The study recruited 28,980 men and women aged between 50 and 75 who were free of clinically evident cardiovascular disease in central Scotland.
They were given either a daily dose of 100 milligrams of aspirin or a placebo.
Major bleeding requiring admission to hospital occurred in 34 (2 percent) subjects in the aspirin group and 20 (1.2 percent) of the placebo group.