It depends on whether you have been diagnosed with coronary artery disease or cerebrovascular disease. For people who have angina or a history of a heart attack, stroke, or transient ischemic attack, the effort to prevent a second one is referred to as secondary prevention. (An attempt to prevent a first heart attack or stroke is known as primary prevention.)

For secondary prevention, substantial evidence indicates that taking a baby aspirin daily, or an adult aspirin every other day, significantly lowers the likelihood of another heart attack or stroke. As a result, the U.S. Food and Drug Administration (FDA) allowed aspirin to be labeled as effective for secondary prevention.

In contrast, the results of a number of trials have shown no benefit of aspirin for primary prevention. In 2003, the FDA reviewed a proposal to label aspirin as effective for primary prevention. Based on the negative findings in 5 primary-prevention trials, the FDA did not allow aspirin labels to say aspirin was beneficial in primary prevention.

Since that time, a trial of nearly 40,000 healthy participants in a women's health study showed no benefit of regular aspirin for the primary prevention of non-fatal heart attacks or strokes, or of death from cardiovascular causes. This study also failed to find benefits of aspirin for individuals with diabetes who had no symptoms of vascular disease.

A relatively small study performed by Scottish researchers and reported in a recent issue of the British Medical Journal showed that taking 100 mg daily aspirin over a 7½-year period in about 1,300 adults conferred no primary-prevention benefits. These participants were all age 40 or older and had no symptoms. Interestingly, however, all these individuals had either type 1 or type 2 diabetes, as well as poor blood supply to their legs, and so were at high risk for vascular disease.

Despite these negative findings, the American Heart Association (AHA) has recommended that people with diabetes take regular aspirin as primary prevention, since this group is known to have an exceptionally high rate of death from heart attacks and strokes.

I tend to agree with these AHA guidelines for people with diabetes, at least to the extent that physicians should take all risk factors into consideration when deciding whether to prescribe aspirin for primary prevention in their patients with diabetes.

But even though the overall findings do not favor aspirin for primary prevention, an examination of the results from individual trials does show some benefits for specific groups of subjects.

• One "negative" overall study did find that aspirin lowered the risk of non-fatal heart attacks in healthy men.
• Another found that aspirin reduced strokes and heart attacks in women over the age of 65.

The inconsistencies of the results have led me to take a daily baby aspirin for primary prevention for many years, and I suspect that millions of healthy Americans are also taking aspirin regularly for primary prevention, with or without the blessings their doctors.

We tend to think that aspirin is quite safe, but it is associated with a significant risk of gastrointestinal bleeding, especially in those older than 65 and in individuals with a prior history of peptic ulcer or gastritis. And so I don't believe it's wise for healthy Americans to take aspirin regularly unless their doctors have made a thoughtful recommendation in favor of its use.

In fact, I frequently wonder whether I should stop my own habit of a daily aspirin, but I generally decide to continue, reasoning that it may help and, besides, I've had no evidence yet of intestinal bleeding during all these years. To be truthful, though, I have not had a proper discussion of the matter with my own doctor.