Researchers from Harvard Medical School investigated the effects of aspirin in 39,876 women taking part in the ongoing Women's Health Study. The participants, all 45 years of age or above, were randomly assigned to take either 100 mg of aspirin or a placebo every other day. They were followed for 10 years, with the researchers recording first major cardiovascular disease (CVD), incorporating nonfatal heart attacks, nonfatal strokes, or deaths from cardiovascular causes. A total of 477 major cardiovascular events occurred in the aspirin group as compared to 522 events in the placebo group. This corresponds to a non-significant 9 per cent reduction in the overall risk of a major cardiovascular event. The reduction in risk of stroke was however significant, at 17 per cent, mainly due to a 24 per cent reduction in ischemic stroke (in which part of the brain does not get sufficient blood for a period of time). Detailed analysis showed that major CVD was reduced with aspirin use among women over 65, but not among those aged 65 or younger. The aspirin group experienced a 40 per cent higher risk of gastrointestinal bleeding requiring transfusion.

The researchers strengthened their observations with further analysis using an additional 55,580 participants from five other randomized trials. They conclude that aspirin lowers the risk of stroke but does not affect the risk of CVD. The findings to date, therefore, show opposite effects in men and women for long-term aspirin use to prevent cardiovascular events. The reasons for this difference are unclear, and require further exploration. Nevertheless, aspirin has well-established benefits for both men and women with known cardiovascular disease.
Ridker, P M et al. A randomized trial of low-dose aspirin in the primary prevention of cardiovascular disease in women. The New England Journal of Medicine, Vol. 352, March 2005, pp. 1293-1304