A daily aspirin may not be worth the risk for people with no previous heart condition.
A study of more than 100,000 people by St. George's, University of London, found that an aspirin a day could reduce heart risks by 10 percent.
But long-term there is a 30 percent increase in the risk of life-threatening or debilitating internal bleeding.
Aspirin, which thins the blood and helps prevent clotting, is a standard treatment for patients recovering from heart attacks or strokes.
Many doctors have also prescribed regular aspirin on a precautionary basis to healthy people who may be at increased risk due to their medical and family history.
But long-term use of aspirin can lead to stomach ulcers and internal bleeding.
For every 120 people treated with aspirin for around six years, one cardiovascular event was averted.
But over the same period, one in 73 people suffered from potentially significant bleeding.
Lead researcher Dr. Rao Seshasai said, "The beneficial effect of aspirin on preventing future cardiovascular disease events in people with established heart attacks or strokes is indisputable.
"We urge people with these conditions not to discontinue their medication unless advised to do so by their physicians for valid reasons.
"However, the benefits of aspirin in those individuals not known to have these conditions are far more modest than previously believed and, in fact, aspirin treatment may potentially result in considerable harm due to major bleeding."