You’ve probably seen low-dose aspirin in the drugstore, in packages with a red heart on them, as well as ads promoting aspirin for its heart benefits. You may even be taking “baby” aspirin, following your doctor’s advice—or on your own, “just to be safe.” So you may be surprised to learn that there’s still controversy about low-dose aspirin as a preventive for heart disease.

The evidence is indeed solid concerning aspirin therapy for secondary prevention—that is, for preventing recurrences in people who’ve already had a heart attack, angina, or ischemic stroke (the most common type of stroke, caused by a blood clot). That’s the only explicit heart-health claim the Food and Drug Admininstration (FDA) allows aspirin companies to make. However, for healthy people without symptoms or a history of cardiovascular disease (CVD)—that is, for primary prevention—the benefit is far from clear, and recent research has made it more uncertain.

Aspirin helps prevent heart attacks and ischemic strokes by decreasing the tendency of blood to clot. On average, all it takes is about 81 milligrams (one-quarter of a standard 325-milligram tablet) a day to accomplish this, though some experts advise 162 milligrams a day or 162 to 325 milligrams every other day.

Unfortunately, even at low doses, aspirin can cause stomach or intestinal bleeding and ulcers. It also increases the risk of hemorrhagic (“bleeding”) stroke slightly, at least in men. If aspirin were developed today, there’s little chance it would be approved for over-the-counter sale because of these risks. (In fact, all pain relievers have serious risks.) Stomach bleeding may not sound that scary, at least compared to a heart attack, but it can lead to potentially deadly complications, especially among older people and heavy drinkers.

So the trick is to figure out for whom the potential CVD benefits of aspirin therapy outweigh the risks. Experts disagree about which high-risk people are good candidates. And some believe that no one should be taking aspirin for primary prevention.

The Task Force weighs in

In 2009 the U.S. Preventive Services Task Force, a government-appointed panel of experts that evaluates medical research, expanded its recommendations about aspirin for primary prevention. It gave different advice for men and women, since CVD affects them differently, and thus aspirin has different potential benefits in them. Notably, men have a higher risk of heart attack at younger ages; women have a higher lifetime risk of stroke.

The Task Force concluded that doctors should encourage low-dose aspirin in men age 45 to 79 to help prevent heart attacks, and in women age 55 to 79 to help prevent ischemic strokes—but only when the potential benefit outweighs the risk of gastrointestinal bleeding. To weigh your risks and benefits, it advised consulting your doctor, as well as using an online CVD risk assessment tool. Because of insufficient evidence, the Task Force gave no advice for people 80 or older, for whom the potential benefits and risks are greater.

The American Heart Association gives no specifics—it merely recommends aspirin therapy for primary prevention in people at high CVD risk. That means, again, talking to your doctor.

The Task Force based its guidelines for primary prevention on nine studies, including the well-known Women’s Health Study, which helped identify aspirin’s anti-stroke effect in women. Other researchers, however, have looked at some or all of these studies and concluded that the data do not support the use of aspirin in healthy people, even those at higher risk—either because the overall results were not significant, or because the very modest benefit was offset by adverse effects. The doubters include experts at the FDA, who a few years ago decided not to allow the labeling of aspirin for primary prevention of CVD. Both sides make good arguments, which hinge largely on methodological issues of the studies.

Adding to the doubts about aspirin therapy for primary prevention are several recent studies that focused on people with diabetes, published too late to be included in the Task Force’s review. People with diabetes are at least twice as likely to have a heart attack or stroke, so they are obvious high-risk candidates for aspirin therapy. That’s why the American Diabetes Association and American Heart Association have for years advised aspirin for primary prevention in most people with diabetes. But the new studies raised questions about this advice. This June, in a revised position statement, the two associations concluded that results have been inconsistent and overall suggest only a modest benefit for people with diabetes, which may be outweighed by the risk of bleeding. Thus, they now recommend aspirin primarily for men over 50 and women over 60 with diabetes, if they have at least one other CVD risk factor and are not at high risk for bleeding.

If you do not have diabetes, you may wonder what this has to do with you. But if there are uncertainties about aspirin’s benefit for these people at high risk, that raises questions about primary prevention for everyone.

Risky confusion

If you already take low-dose aspirin, make sure your doctor knows, and review this decision periodically. Do not start taking aspirin on your own. A few years ago a survey in the American Journal of Preventive Medicine found that 30 percent of people over 40 who are at low CVD risk were taking low-dose aspirin, which makes no sense. And 31 percent of people with a history of CVD, most of whom should be taking aspirin, were not taking it.

It’s a familiar refrain: talk to your doctor about your risk factors for heart disease and stroke. That survey also found that 28 percent of people who are at high risk for CVD thought they were at low risk, while 44 percent at low risk thought they were at high risk. It may help to fill out one of the risk assessments with your doctor. If that suggests you’re a good candidate, and you’re not at elevated risk for bleeding, you might consider aspirin therapy.

Bottom line: It’s far more important to take a statin if you have undesirable cholesterol levels, and an antihypertensive drug if you have high blood pressure, than to take aspirin. Even better, try to correct these problems by losing weight, exercising more, improving your diet and quitting smoking.