The results are opposite what is known about aspirin in men, where its benefit for stroke is limited and its ability to prevent heart problems is legendary. Since women proportionately suffer more strokes and men more heart attacks, this is generally good news, specialists said.
Researchers also found that taking vitamin E did no good for women of any age, confirming a study last fall that concluded supplements of this nutrient could even be harmful.
The new information comes from the Women's Health Study, the first rigorous, scientific test of whether long-term use of aspirin or vitamin E made a difference in cardiovascular risk in females. Previous research has been almost exclusively in males.
Findings were reported Monday at an American College of Cardiology meeting in Orlando. They also were being published online by the New England Journal of Medicine and will be in the March 31 print edition.
"In contrast to men, aspirin did not reduce the risk of nonfatal or fatal heart attacks in women of all ages but did so in women over age 65," she said.
The study was led by Julie Buring, Dr. Paul Ridker and others at Brigham and Women's Hospital and Harvard Medical School. Many have been consultants to aspirin makers, but the companies did not run the federally funded study. Bayer supplied aspirin for it and the Natural Source Vitamin E Association supplied that nutrient.
In the study, 40,000 female health professionals 45 and older were randomly assigned to take either fake pills or 100 milligrams of aspirin — slightly more than the 81-milligram "baby aspirin" pills commonly sold — every other day.
After 10 years, aspirin users had a 17 percent lower risk of stroke, and a 24 percent lower risk of strokes caused by blood clots, owing, researchers believe, to aspirin's well-known anti-clotting properties.
Women 65 and older got even more benefit: They were 30 percent less likely to have a stroke caused by a blood clot and 34 percent less likely to suffer a heart attack.
Aspirin's benefits were greatest for nonsmokers and former smokers, and didn't vary among women who did or did not use hormones after menopause.
But the benefits did come with a cost. Stomach or intestinal bleeding requiring a blood transfusion occurred in 127 women on aspirin and in 91 women taking dummy pills.
"Is it manageable? Yes. Is it worth it? That's an individual decision," Nabel said, adding that anyone considering taking aspirin should talk with his or her doctor about the relative risks.
In 2002, the federal government and the American Heart Association recommended aspirin for adults who have risk factors that raise their chances of developing a heart problem, but authors say their new study makes this somewhat questionable for women.
The study "demonstrates the importance of studying medical therapies among women as well as men," Buring said. "We finally have the evidence base needed for women to make rational decisions about the use of aspirin in preventing cardiovascular disease."
Reasons for the gender differences are unclear. Strokes usually occur from blood clots that form in neck arteries; heart attacks, from coronary artery clots. Aspirin may affect one or the other more in men and women.
The dose of aspirin in this study also was lower than what some previous studies in men used.
"It also may be due to a difference in biology between men and women that we simply don't understand. It could be that men and women respond differently to aspirin," Nabel said.
By Marilynn Marchione