This means the net effect in this group of patients is uncertain because the benefits and risks may cancel each other out. However, the researchers found that aspirin's benefits generally outweigh its risks among people who have vascular disease.
Colin Baigent, of the University of Oxford, and colleagues looked at serious vascular events (heart attack, stroke or vascular death) among 95,000 low-risk patients in six primary prevention trials and among 17,000 high-risk patients in 16 secondary prevention trials.
Among patients in the primary prevention trials, aspirin was found to reduce the risk of serious vascular events by 12 percent but increased the risk of internal bleeding by about one-third. Among patients in the secondary prevention studies -- who were already at high risk because they had previously experienced a stroke or heart attack -- aspirin reduced the risk of serious vascular events by about one-fifth, a benefit that outweighed the small additional risk of bleeding, the researchers said.
The results in both sets of trials were similar for men and women.
"The currently available trial results do not seem to justify general guidelines advocating the routine use of aspirin in all healthy individuals above a moderate level of risk for," the study authors wrote in the May 30 issue of The Lancet.
"Drug safety really matters when making recommendations for tens of millions of healthy people," Baigent said in a news release. "We don't have good evidence that, for healthy people, the benefits of long-term aspirin exceed the risks by an appropriate margin. If effectiveness is uncertain, then cost-effectiveness calculations are irrelevant."