Aspirin is less helpful in preventing heart attacks for women

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Gender accounts for a substantial proportion of the variability in the efficacy of aspirin in reducing myocardial infarction (MI), and women may be less responsive to aspirin than men, according to study results published online on Oct. 18 in BMC Medicine.

The results of a meta-analysis showed that trials predominantly with men showed the largest risk reduction in nonfatal MI (38 percent), whereas those with mostly women showed no significant benefit.

For fatal MI, no significant effect in prevention was seen regardless of gender, reported Don Sin, MD, of the University of British Columbia and St. Paul's Hospital, and colleagues.

Aspirin reduces the risk of MI by about 25 percent on average, but trials have shown considerable variation in the effect sizes, from zero to 50 percent compared with placebo, the investigators said.

The meta-analysis of the researchers suggested more than a quarter of the variation may reflect lower aspirin response among women.

The researchers conducted a systematic review of randomized placebo-controlled clinical trials, published in the PubMed database from 1966 through October 2006, and examined the efficacy of aspirin therapy primarily for cardiovascular prevention, or when taken as a pain reliever. They found 23 trials with a total of 113,494 participants, 49.3 percent of whom were men.

All but two of the trials reported on non-fatal MI outcomes, 15 reported fatal MI and 17 reported on both. Daily aspirin dosage ranged from 75 mg to 1,500 mg, and the patient follow-up lasted from one to 10 years.

Overall, a total of 27 percent of the variation in the non-fatal MI results could be accounted for by considering the gender mix of the trials. Trials that recruited predominantly men demonstrated the largest risk reduction in non-fatal MI (70 to 89 percent), while trials that contained predominately women failed to demonstrate a significant risk reduction (66 percent) in non-fatal MI.

The researchers said that the findings were similar for the combined endpoint of non-fatal and fatal M. However, aspirin resistance is 2.3 to 2.5 times more common among women than men, according to previous studies.

"Moreover, women who develop atherosclerosis tend to be older, have more co-morbid conditions and more extensive disease at the time of diagnosis, which might also interfere with the actions of aspirin," Sin's group reported.