For years, doctors have been telling us that taking a low-dose aspirin once a day can help prevent heart attacks, but new research suggests that this may only be true for men.

Scientists have long wondered why the protective effects of aspirin seem to vary so widely in different clinical trials. Some trials show no difference between aspirin and placebo, while others find that aspirin reduces the risk of a heart attack by more than 50%.

‘We found that a lot of the variability in these trials seems to be due to the gender ratios...’

This latest study, from The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research at the University of British Columbia, may have found at least part of the answer. Researchers reviewed the results of 23 previously published studies involving more than 113,000 patients. They found that the ratio of men to women in these trials largely accounted for the differences in their outcomes.

"Trials that predominantly studied men demonstrated the largest risk reduction in nonfatal heart attacks," says Dr. Don Sin, one of the study's authors. "The trials that contained predominately 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."

The reason for this is not yet understood, although men and women have important differences in the structure and physiology of the heart's blood vessels.

Though relatively harmless, aspirin does have side effects, including irritation and even bleeding in the gastrointestinal tract.

What should women concerned about their hearts do?

"From our findings we would caution clinicians on the prescribing of aspirin to women, especially for primary prevention of heart attacks," says Dr Sin. "Whether or not other pharmaceutical products would be more effective for women is unclear; more sex-specific studies should now be conducted."

The study is published in the online open access journal BMC Medicine.