If you're taking a daily aspirin for your heart, you may want to reconsider.

For years, many middle-aged people have taken the drug in hopes of reducing the chance of a heart attack or stroke. Americans bought more than 44 million packages of low-dose aspirin marketed for heart protection in the year ended September, up about 12% from 2005, according to research firm IMS Health.

Now, medical experts say some people who are taking aspirin on a regular basis should think about stopping. Public-health officials are scaling back official recommendations for the painkiller to target a narrower group of patients who are at risk of a heart attack or stroke. The concern is that aspirin's side effects, which can include bleeding ulcers, might outweigh the potential benefits when taken by many healthy or older people.

"Not everybody needs to take aspirin," says Sidney Smith, a professor at the University of North Carolina who is chairing a new National Institutes of Health effort to compile treatment recommendations on cardiovascular-disease prevention. Physicians are beginning to tailor aspirin recommendations to "groups where the benefits are especially well established," he says.

Doctors generally agree that most patients who have already suffered a heart attack or ischemic stroke, the type caused by a clot or other obstruction blocking an artery to the brain, should take regular low-dose aspirin. But for people without heart disease, the newest guidelines from the U.S. Preventive Services Task Force spell out much more clearly than before when aspirin should be administered.