Nearly a third of middle-aged Americans regularly take a baby aspirin in the hope of preventing a heart attack or a stroke or lowering their cancer risk. But new research shows that aspirin is not for everyone, and that, in some patients, this wonder drug is doing more harm than good.

"I stop a lot more aspirin than I start," said Alison Bailey, director of the cardiac rehabilitation program at the University of Kentucky's Gill Heart Institute. "People don't even consider aspirin a medicine, or consider that you can have side effects from it. That's the most challenging part of aspirin therapy."

Last week, researchers in London reported that they had analyzed nine randomized studies of aspirin use in the United States, Europe and Japan that included more than 100,000 participants. The study subjects had never had a heart attack or stroke; all regularly took aspirin or a placebo to determine whether aspirin benefits people who have no established heart disease.

In the combined analysis, the researchers found that regular aspirin users were 10 percent less likely than the others to have any type of heart event, and 20 percent less likely to have a nonfatal heart attack. While that sounds like good news, the study showed that the risks of regular aspirin outweighed the benefits.

Aspirin users were about 30 percent more likely to have a serious gastrointestinal bleeding event, a side effect of frequent aspirin use. The overall risk of dying during the study was the same among the aspirin users and the others. And though some previous studies suggested that regular aspirin use could prevent cancer, the new analysis showed no such benefit.

Overall, for every 162 people who took aspirin, the drug prevented one nonfatal heart attack but caused about two serious bleeding episodes.

"We have been able to show quite convincingly that, in people without a previous heart attack or stroke, regular use of aspirin may be more harmful than it is beneficial," said Sreenivasa Seshasai of the Cardiovascular Sciences Research Center at St. George's, University of London.

But Dr. Seshasai said the new findings didn't necessarily mean that healthy men and women should immediately stop taking aspirin. People with a strong family history of heart attack or stroke may benefit by continuing the regimen, and they should raise the question with their doctors.

"The decision to treat such individuals with aspirin should be made on a case-by-case basis, taking into account the likely risk of heart attack or stroke in the future," he said.