The protective effect of aspirin (acetylsalicylic acid) against myocardial infarction (heart attack) has been found to vary wildly from clinical trial to clinical trial, with the reduction in risk of heart attack ranging from 0 to 50 per cent. New research from the University of British Columbia's (UBC) James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research in Canada has found that a considerable proportion (27 per cent) of this dramatic variation can be accounted for by considering the genders of those participating in the studies. The study, published in the journal BMC Medicine, found that women were less likely to benefit from taking the drug to prevent heart attack. "Trials that recruited predominantly men demonstrated the largest risk reduction in non-fatal heart attacks," said Dr Don Sin, associate professor at UBC and one of the authors of the study. "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." Aspirin irreversibly inhibits the COX-1 enzyme while modifying the activity of COX-2 to produce anti-inflammatory lipoxins rather than the pro-inflammatory prostanoids. In addition, low-dose, long term aspirin use has been shown to irreversibly block the formation of thromboxane A2, inhibiting platelet aggregation and reducing the risk of heart attacks. The researchers note that no clear mechanism has yet been found to explain why women demonstrate higher levels of resistance to the heart attack reducing properties of the drug than men. However, they do suggest that major structural and physiological differences in the coronary vasculature of men and women could play a role in the differing responses. Women have smaller coronary vessels than men that are also stiffer due to increased deposition of fibrotic tissue of the vessel walls. In addition, women are more likely to demonstrate impaired responses to vascular dilators such as acetylcholine. "From our findings we would caution clinicians on the prescribing 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." Whether gender could play a role in the efficacy of other drugs is, as yet, unknown. However, these results raise the question as to whether gender specific drugs could play a part in the personalisation of medicine.