The conventional wisdom that taking a daily aspirin will help prevent heart attacks and strokes in otherwise healthy people is under renewed challenge.

A major international study led in part by a Minnesota physician has found that healthy people over age 70 got no preventive benefit from aspirin and were at greater risk for harm such as stomach bleeding. It's the third major study this year to reach a similar conclusion.

"We hope this shifts the paradigm for people 70 and older who are considering taking aspirin," said Dr. Anne Murray, a geriatrician at Hennepin Healthcare in Minneapolis who led the research along with Dr. John McNeil in Australia.

"We found no measurable benefit to taking aspirin to prolong healthy life, and actually it may induce harm," Murray said.

The study did not include people who have heart disease, and Murray cautioned that anyone currently taking aspirin, especially if they are at high risk, should consult their doctor before making any changes. The study, released Sunday in the New England Journal of Medicine, involved 19,000 older healthy people enrolled for nearly five years.

In August, two other large studies produced similar results. One tracked 12,000 middle-aged people with cardiovascular risk factors, the other looked at 15,000 patients with diabetes.

To varying degrees, the research found that those taking low-dose aspirin had a higher risk for developing bleeding in the stomach or the brain, both of which could require hospitalization.

There was little benefit to their heart health for people at low or moderate cardiovascular risk.

Aspirin's reputation as an inexpensive and effective medicine to prevent heart attacks and strokes came from studies conducted decades ago.

Since then, the profession has placed a greater emphasis on prevention, including new medications to control risk factors and an emphasis on a better diet and less use of tobacco, which addressed the risks for many of those patients.

"The rate of heart attack and stroke has decreased dramatically as we got better at controlling high blood pressure and high cholesterol," said Dr. Michael Miedema, a cardiologist at Allina Health in Minneapolis.

"At some point, we were considering recommending aspirin for everyone," said Miedema. "The benefits that we had initially seen, we just don't see any more."

Aspirin helps dissolve clots that form in arteries of those with heart disease.

But those same anti-clotting properties can aggravate internal bleeding. Aspirin also irritates the lining of the stomach, even the low-dose aspirins that come with a protective coating.

"We have a lot of patients that say 'I take an aspirin to be safe,' " said Miedema. "Based on recent data, that may not be the case. Just because it is over-the-counter doesn't mean it is safe."

He said, however, that patients with known heart disease should not stop taking aspirin.

Murray said many patients she sees take low-dose aspirin even though a doctor had not recommended it.

In her study, which looked at both Americans and Australians, about 37 percent of the Americans had already been taking aspirin, compared to just 8 percent of Australians.

Murray did not routinely prescribe aspirin for elderly patients because there had been no published evidence that it was effective, which is one reason she and the Berman Center for Outcomes and Clinical Research, which is affiliated with Hennepin Healthcare, pursued the study.

"Keep in mind that geriatricians are trained to be very aware of potential adverse events or side effects of any medication," she said. "One of the first things we do when we see a new patient is wean them down from their medication lists."

The study released Sunday also tested whether aspirin helped prevent dementia and disabilities, two major conditions that affect quality of life in the elderly. It found no preventive effect.

The findings are likely to touch off reviews by medical associations and government agencies about revising existing guidelines.

The U.S. Preventive Services Task Force, a group convened by the federal government to make recommendations on preventive care, currently says that people between the ages of 50 and 59 should take low doses of aspirin to prevent cardiovascular problems, as well as colorectal cancer, if they have some risk factors.

The recommendation for those aged 60 to 69 is that it should be an individual choice, while there was no recommendation for those 70 and above.

Miedema, who sits on an advisory committee for the American Heart Association, said discussions about altering its recommendations have already begun, although he said it is too early to comment.