Doctors: Who Should Take Aspirin, and When Doctors put out new guidelines on when to start daily baby aspirin.

March 16, 2009 -- An aspirin a day keeps the doctor away...

That's not the saying, but doctors have agreed, for about a generation, that an aspirin a day is good for you. It may reduce the risk of heart attacks or strokes by 20 percent or more.

But at what age should you start? How much should you take? And is it different for men than women? The answers -- coming from different studies, interest groups, and insurance companies, were all over the map.

Now, the the U.S. Preventive Services Task Force, an independent panel convened by the Department of Health and Human Services, has published guidelines it says should end the confusion.

The key points:

Men should start a daily aspirin at age 45, mainly to protect against heart attacks.

Women should start at 55, mainly to protect against stroke.

For both sexes, a baby aspirin -- typically 81 milligrams a day -- will do the job. There is no evidence that a larger dose makes a difference.

And both sexes should stop by age 80, unless their doctors say otherwise. As you get older, there's a greater risk of bleeding in the brain or the digestive system -- a risk that is small but can be fatal in some cases.

General guidelines do not always generate much response, but these are on a hot topic.

Doctors said they come from an influential group, and they're being published in a widely-read medical journal, the Annals of Internal Medicine.

Numbers are hard to come by, but few people who would benefit from a daily baby aspirin appear to be taking one. In one recent study, only 16.6 percent of those eligible were taking aspirin.

If they start, doctors say their risk of heart attack or stroke will drop by about 20 percent.

"People may ask themselves, 'Am I at risk for a heart attack or a stroke,'" said Dr. Randal Thomas, director of cardiovascular health at the Mayo Clinic in Minnesota. "If you're above age 45 and male, if you're above age 55 and female, the answer is most likely yes, and you will most likely benefit from taking a small dose of aspirin a day."

He pointed out that more people take statin drugs -- cholesterol-blocking drugs, such as Lipitor, Crestor or Zocor -- than take aspirin. Aspirin costs far less, and may do as much or more to protect one's health.

"I put people on aspirin at the drop of a hat," said Dr. Linda Prine, a family physician in New York City.

There are other, more natural ways to protect oneself, said Dr. Dean Ornish, the bestselling author who now heads the Preventive Medicine Research Institute at the University of California, San Francisco.

"Making comprehensive lifestyle changes can reduce the likelihood of blood clots forming where you don't want them to form without reducing the capacity of blood to clot where you do want it to occur," he said in an e-mail to ABC News. "This seems more productive than trying to parse out in which patients the benefits of aspirin may outweigh the risk, because there is still significant risk even in these patients."

Actually, some doctors said, starting a daily aspirin is not a decision to be taken lightly, because the risk of bleeding is not to be ignored. Some doctors recommend aspirin to patients at age 40, or even in their 30s, when the benefit is small.

"If the decision is difficult for the doctor, imagine how difficult it is for the patient to decide," said Dr. Albert Levy of New York's Mt. Sinai School of Medicine. "The patient will, at the end of the day, decide to take or not to take the doctor's advice. The doctor should explain the pros and cons and document the discussion as it is a very tricky issue. Aspirin is great but also dangerous.

"I agree with the new recommendations," he concluded.