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Does an aspirin a day keep cancer at bay? After carving out a role in heart-attack prevention, mounting studies suggest aspirin may have a role in preventing colorectal cancer as well.

The latest evidence comes from the Nurses' Health Study of nearly 90,000 women by Harvard University researchers, who found rates of cancer were lower among women who took four to six aspirins a week for 10 to 20 years than among those who didn't.A year ago, researchers reported similar benefits in men. Colon cancer is expected to strike 138,200 Americans this year and kill 55,300.

So why not start the cure? It isn't that simple.

"There's not enough out there yet for a global recommendation," says Michael Thun of the American Cancer Society.

Aspirin is undeniably versatile. In its traditional role, it lowers fever, cools inflammation and relieves pain. Further, aspirin binds to sticky blood cells called platelets, preventing clumping and clotting that trigger a heart attack.

Now, it's theorized, aspirin's power to block prostaglandins, substances that regulate cell growth, may give it the power to thwart tumors. But aspirin can also inflict stomach irritation, gastrointestinal bleeding and cerebral hemorrhage.

The story of aspirin in heart disease is more conclusive. In people with heart disease, aspirin now is part of standard therapy. In healthy people, aspirin's ability to prevent heart attacks prompted the U.S. Preventive Services Task Force to urge men over 40 and at coronary risk to consider taking low-dose aspirin therapy - unless other risks rule them out.

For women, the picture isn't as clear. While aspirin prevents female cardiac disease, women are more at risk of cerebral hemorrhage than men.

Now we open aspirin's newest chapter, where regular aspirin users appear to have 30 percent to 44 percent less colon cancer. It wasn't a quick fix. It took a decade for protective benefits to appear at all, and the benefits became statistically significant only after two decades.

Doubts persist about dosage. Though a very low dose in the baby-aspirin range (80 milligrams a day) seems to suffice, the Harvard researchers admitted more information is needed before any recommendation is issued.

"It would be great if a baby aspirin took care of both cardiovascular and colon protection, but we don't know that yet," Thun says.

But some find the story convincing enough to justify action.

"We as physicians must advise our patients now," wrote Aaron Marcus, of the New York V.A. Medical Center, in an editorial in the New England Journal of Medicine. He recommended that people at risk for colorectal cancer take a single, regular-strength aspirin tablet (325 mg) every other day. "The ravages of colorectal cancer far exceed the possible complications of long-term therapy with aspirin," he wrote.

Carmen Allegra of the National Cancer Institute's NCI-Navy Oncology Branch adds, "There's a lot of good data to support the use of aspirin." Though the cancer institute avoids making recommendations, Allegra says he personally believes "it's reasonable to go ahead and take aspirin." He does so himself, he says.

Thun suggests people talk to their doctors before starting on aspirin. "Anyone who's got aspirin sensitivity, a history of ulcers or bleeding definitely should not take aspirin," he stresses. Anemia, hemophilia and use of other anti-clotting drugs also preclude aspirin use.

Don't despair if you're one of the people who can't tolerate aspirin. "It's not like this is the only tool against colorectal cancer," Thun points out. "A diet high in fruits, vegetables and fiber and low in fat, plus exercise and regular screening after age 50, all have proven efficacy."

Mass conversion to aspirin without figuring one's individual odds could be dangerous, argues Peter Gann, assistant professor of medicine at Northwestern University.

It may benefit those with family histories of the cancer, "but for millions of people to rush and take aspirin without adequate planning and supervision, there could be a price to pay," Gann says.

John Baron, professor of medicine at Dartmouth University, believes in aspirin's promise but retains his skepticism about its miracles. "Aspirin has probably been oversold as primary prevention," he says. While aspirin prevented heart attacks in healthy men, it didn't change their eventual overall death rate, suggesting death from other causes, including bleeding complications from aspirin, offset the ultimate goal of a longer life.

Baron is now investigating aspirin's power to prevent colon polyps. He calculates, however, that "to avoid one colon cancer, you have to give aspirin to 1,000 people for 10 years. That's a lot. You have to be careful. These are people who aren't sick, who are perfectly well." You don't want to make them sick by prompting bleeding, he says.

A pill for prevention is so seductive that many would like to jump on the aspirin bandwagon. People at risk for heart disease or colon cancer may be candidates, but everyone still needs to balance potential benefits against the risks of bleeding or cerebral hemorrhage.

Says Thun, "It's a weighing game."