Gender differences in aspirin effectiveness
UTRECHT, THE NETHERLANDS. Aspirin has been found effective in the prevention of heart attack (myocardial infarction), ischemic stroke, and cardiovascular death in patients who have already experienced a heart attack or stroke. Its benefits in primary prevention, that is in the prevention of a first heart attack or stroke, are much less clear. Researchers at the University Medical Center in Utrecht have published a major study aimed at determining the benefits and risks of taking a daily aspirin for primary prevention of cardiovascular events. Taking into account all major studies on the subject as well as discharge statistics from Dutch hospitals, the researchers developed a computer model for predicting the risk of a first heart attack, ischemic stroke, hemorrhagic stroke, major gastrointestinal bleeding, and death in four specific age groups of men and women who were, or were not, taking aspirin on a daily basis.

Using a 55-year-old man with no cardiovascular risk factors as an example, they found an annual incidence of a first heart attack to be 0.40%/year with no aspirin and 0.28%/year with daily aspirin, or a relative risk decrease of 30%. There was no decrease in risk of a first ischemic stroke, but the relative risk increase of a first hemorrhagic stroke was 42%, and that of major gastrointestinal bleeding 42%. The daily aspirin did not prevent a first heart attack in 55-year-old women, but did reduce the risk of ischemic stroke from 0.07%/year to 0.05%/year, or a relative risk reduction of 24%. However, this benefit was offset by a relative risk increase of hemorrhagic stroke of 5% and of major gastrointestinal bleeding of 70%.

Overall, the researchers concluded that the risk involved in the daily aspirin ritual outweighs the benefit in healthy 55-year-old women. A healthy 55-year-old man may gain 3 days of "Quality Adjusted Life Years" (QALY) over a 10-year period by taking aspirin on a daily basis.

The net benefits of daily aspirin usage increased with increasing age and the presence of cardiovascular risk factors. For healthy men, the gain in QALY over a 10-year period was 9 days at age 65 years and 15 days at age 75 years. Corresponding numbers for women were a loss of one day at age 65 years and a gain of 6 days at age 75. However, for men with 5 times normal cardiovascular risk the net gain in QALY over a 10-year period was 34 days at age 55, 68 days at age 65, and 108 days at age 75. Corresponding numbers for women were 2 days, 12 days, and 38 days.

The researchers conclude that for most women aspirin treatment results in increased health care costs and worse health outcomes. However, for women 65 years or older with 5-times-increased cardiovascular risk, aspirin may have a favourable benefit/risk ratio. The benefits for healthy men are not impressive until age 75, but daily aspirin would generally seem to be beneficial for men with moderate or high risk for cardiovascular disease.

Greving, JP, et al. Cost-effectiveness of aspirin treatment in the primary prevention of cardiovascular disease events in subgroups based on age, gender, and varying cardiovascular risk. Circulation, Vol. 117, June 3, 2008, pp. 2875-83
Mosca, L. Aspirin chemoprevention – one size does not fit all. Circulation, Vol. 117, June 3, 2008, pp. 2844-46

Editor's comment: The major "take-home" message from this study is that one size does definitely not fit all when it comes to using aspirin for prevention of a first cardiovascular event. In general, aspirin may benefit men with a 10-year cardiovascular disease risk greater than 10% and women with a 10-year cardiovascular disease risk greater then 15%.