The study found that while aspirin use can reduce a woman's risk of ischemic stroke, taking more than 15 aspirin tablets per week doubles the risk of developing a hemorrhagic stroke, caused by bleeding in the brain. Ischemic stroke, the most common form of stroke, is caused by blood clots or other blockages in the arteries, thus restricting blood flow to the brain. Hemorrhagic strokes occur when a blood vessel or vessels inside the brain ruptures and bleeds and is more likely to be fatal than an ischemic stroke.

"This is the first large-scale detailed study of the relationship between aspirin use and the risk of principal types of stroke," said lead author JoAnn Manson, M.D., professor of medicine at Harvard Medical School and Brigham and Women's Hospital. "It suggests that low doses of aspirin are linked to a reduction in ischemic stroke risk and that heavy doses may increase the bleeding type of stroke, especially among older women or those with high blood pressure."

Researchers at Harvard Medical School and Brigham and Women's Hospital in Boston used data collected in the Nurses' Health Study to examine aspirin use and stroke risk in 79,319 healthy women ages 34 to 59. Participants were monitored over a 14-year period from 1980 to 1994 during which 295 ischemic strokes and 100 hemorrhagic strokes were recorded.

When taken regularly by heart attack and stroke survivors, aspirin has been shown to help prevent recurrences, says Manson, but medical debate continues about whether healthy people with no history of cardiovascular disease should routinely use aspirin as a method to prevent a first heart attack or stroke.

Women who took low doses of aspirin (one to six tablets per week) had a lower risk of ischemic stroke, whereas women who took higher doses of aspirin (more than 15 tablets per week) were approximately twice as likely to suffer hemorrhagic strokes. The risk of hemorrhagic stroke was tripled in older women with high blood pressure who took more than 15 aspirins per week compared to women who did not take aspirin or took lower doses. "This study indicates that it may be a good news-bad news situation in terms of the primary prevention of stroke," Manson said. "If low doses of aspirin reduce the risk of ischemic stroke in healthy individuals, this is important, since it is the most prevalent form of stroke. On the other hand, our findings suggest that taking too much aspirin could be dangerous."

Women who took one to six aspirins a week had a lower risk of stroke due to blockages in the large arteries of the brain than women who took no aspirin at all, Manson explained. Previous research has shown aspirin to be effective as a therapy to prevent a second ischemic stroke in both men and women.

However, Manson said the latest study does not answer the question of whether the high-dose aspirin risks apply to men as well. She added that even in the female population, the significance of this study must be confirmed in clinical trials that are currently underway.