Men benefit more from the protective effects of aspirin on the heart and arteries than women, according to new research.
Many people take aspirin daily in the belief that doing so gives protection against heart attacks, but scientists have been baffled over the way the drug's benefits vary between clinical trials.
Some show no difference between aspirin and placebos, while others suggest that the drug reduces the risk of a heart attack by 50%.
In the new study, doctors examined the results of 23 previously published clinical trials involving more than 113,000 patients.
They then analysed how much the ratio of men to women patients affected the trial outcomes.
Dr Don Sin, a member of the team from the University of British Columbia in Vancouver, Canada, said: "Trials that recruited predominantly men demonstrated the largest risk reduction in nonfatal heart attacks. The trials that contained predominantly women failed to demonstrate a significant risk reduction in these nonfatal events.
"We found that a lot of the variability in these trials seems to be due to the gender ratios, supporting the theory that women may be less responsive to aspirin than men for heart protection."
Recent studies have shown that men and women have major differences in the structure and physiology of the heart's blood vessels.
And the authors said they believed that these differences could be responsible for the greater protection men get from aspirin.