Taking a baby aspirin may prevent heart attacks in men, but it does little to ward off a first Heart Attack in women aged 45 to 64, researchers reported Monday.

However, the low-dose aspirin therapy widely recommended for both men and women may reduce the risk of Stroke caused by a blocked blood vessel in the brain, according to the findings.

The results do not apply to people who are taking aspirin because they have already survived a heart attack.

Until now, doctors have widely recommended low-dose aspirin therapy for both genders, even though that advice was based on studies that mostly included men.

But when researchers tested aspirin on nearly 40,000 women as part of the Women’s Health Study, they found the women who received a placebo were no more likely to have a first heart attack than those who regularly took aspirin for 10 years.

“Aspirin had no significant effect on the risk of fatal or nonfatal myocardial infarction (heart attack),” said the team, led by Paul Ridker of Brigham and Women’s Hospital in Boston.

But women who took 100 milligrams of aspirin every other day - the equivalent of taking one baby aspirin each day - were 24 percent less likely to have an ischemic stroke, the most common type of stroke caused when blood can’t get to the brain.

That benefit came with a price.

The women who were taking aspirin were 40 percent more likely to develop serious stomach or intestinal bleeding that required a transfusion.

Among the 4,097 women in the study over 64, regular aspirin use began to show a clear benefit, cutting the risk of ischemic stroke by 30 percent and the chance of heart attack by 34 percent.

“Age significantly modified the effect of aspirin,” the researchers said.

Previous research on men showed regular aspirin use reduced the risk of having a heart attack by 32 percent, yet did not seem to affect the likelihood of a stroke.

Ridker and his colleagues said the findings “clearly demonstrate the importance of studying women as well as men in major cardiovascular clinical trials.”

The study, which will be published later this month in The New England Journal of Medicine, was released early to coincide with a presentation at a meeting of the American College of Cardiology.