If you're taking an aspirin a day to ward off a heart attack, make sure you're a guy.

A new study suggests that aspirin may really be a man's drug. The study of more than 100,000 people found that women are resistant to the protective effects of aspirin, and the drug may be of no use to them when it comes to cardiovascular problems.

Scientists have long been puzzled over why the protective effects of aspirin vary so widely between clinical trials. Some trials show no difference between aspirin and placebo, while others report that aspirin reduces the risk of a heart attack by more than 50 percent.

This latest study, from The James Hogg iCAPTURE Center for Cardiovascular and Pulmonary Research, highlights the influence of gender on aspirin's protective powers. Researchers examined the results of 23 previously published studies on the effectiveness of aspirin and discovered the clinical trials made up mostly of men showed a clear advantage.

"Trials that recruited predominantly men demonstrated the largest risk reduction in non-fatal heart attacks," said Dr. Don Sin, one of the study's authors. "The trials that contained predominately women failed to demonstrate a significant risk reduction in these non-fatal 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 mechanisms of this resistance are not yet understood, although recent studies have shown that men and women have major differences in the structure and physiology of the heart's blood vessels.

"From our findings we would caution clinicians on prescribing aspirin to women, especially for primary prevention of heart attacks," said 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 was published this week in the online, peer-reviewed journal BMC Medicine.