For years, the advice for preventing heart disease has been simple: take an aspirin every day; it can't hurt. But new research suggests that patients taking the pills and doctors prescribing them may need to think twice about that advice.
The study, published today in the Journal of the American Medical Association, found that taking 300 milligrams or less of aspirin increases the risk of major stomach or brain bleeding by 55 percent, greater than previous research has suggested.
Italian researchers studied more than 370,000 hospitalized patients, looking for differences in how aspirin affects patients with diabetes and those without. But their findings apply to millions of people hoping to prevent heart problems with aspirin.
Of the more than 186,000 participants taking a daily dose of aspirin, researchers counted nearly 2,300 cases of gastrointestinal bleeding and nearly 1,300 cases of brain bleeding.
Dr. Antonio Nicolucci, one of the study's authors, said the results show that the risks of bleeding are much higher than what doctors had previously suspected after several clinical trials and should prompt doctors to carefully consider a patient's individual health before prescribing aspirin.
"If the risk of having a cardiovascular event is low, then the risk of bleeding will likely offset any beneficial effect of aspirin," he said.
Doctors have known for years that aspirin increases the risk of bleeding. But many say the study highlights the issue of calculating risks and benefits of any medical treatment, even one as seemingly harmless as taking an aspirin a day.
"No preventive approach is without risk," said Dr. Thomas Schwenk, Dean of the University of Nevada School of Medicine. "If the benefits are barely measurable but the risks are real and possibly greater, then the decision making may shift against the use of aspirin."
For people with a history of heart attacks or stroke, doctors agree that taking aspirin is a key part of preventing a second problem. The American Heart Association, the American Diabetes Association and the U.S. Preventive Services Task Force all recommend a daily, low dose of aspirin for people who are at a 20 percent increased risk of heart problems. The benefits of preventing a second heart attack outweigh the risks of bleeding for most people.
In an editorial about the study, Dr. Jolanta Siller-Matula, of the Medical University of Vienna, put the numbers this way: for a group of 10,000 patients taking aspirin to prevent a second heart attack or stroke, data suggest the pill would prevent about 250 heart problems but would cause about 40 cases of brain bleeding.
"Thus the net benefit of aspirin for secondary prevention would substantially exceed the bleeding hazard," she wrote.
But the issue gets murkier for those at a lower risk of heart disease, such as people who have some risk factors but have never had a heart attack. Siller-Matula wrote that for 10,000 of those patients, aspirin would be expected to prevent seven heart attacks or strokes, but cause problems in another four patients.
"When the cardiovascular risk is low, the adverse effects of aspirin overwhelm any benefit," said Dr. Steve Nissen, chair of cardiovascular medicine at the Cleveland Clinic. "Unfortunately, many patients taking aspirin represent the 'worried well' rather than individuals with a high risk of coronary artery disease."
Doctors say patients should talk with their physicians about their risks and how taking an aspirin may benefit or harm them. For many, dealing with stomach bleeding may be better than having a heart attack. Dr. Gerard Mullin, an associate professor of medicine at Johns Hopkins University School of Medicine, noted that for patients who would benefit from daily aspirin, stomach bleeding can be fairly easily controlled with drugs called proton pump inhibitors, which protect the lining of the stomach. But of course, prescribing the PPIs for too many people would create a separate set of problems.
"We need to identify those patients who would benefit most by the use of drugs rather than generalize," Mullin said.
Dr. Scott Nelson, a family physician in Mississippi, said he has always been a "strong believer" of the value of using aspirin for prevention in his patients.
"I may now re-think my position on 'blanket' prescribing of aspirin to all of my at-risk patients and pay more attention to the overall risk-benefit picture," Nelson said.
ABC News' Dr. Sheila Reddy contributed to this report.