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updated 10/21/2010 7:20:08 PM ET 2010-10-21T23:20:08

A low dose of aspirin may reduce colon cancer cases by a quarter and deaths by a third, a new study found.

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But experts say aspirin's side effects of bleeding and stomach problems are too worrying for people who aren't at high risk of the disease to start taking the drug for that reason alone.

Previous studies have found a daily dose of at least 500 milligrams of aspirin could prevent colon cancer, but the adverse effects of such a high dose outweighed the benefits. Now, researchers say a low dose, equivalent to a baby or regular aspirin, also appears to work.

European researchers looked at the 20-year results of four trials with more than 14,000 people that were originally done to study aspirin's use in preventing strokes. They found people taking baby or regular aspirin pills daily for about six years reduced their colon cancer risk by 24 percent and that deaths from the disease dropped by 35 percent. That was compared to those who took a dummy pill or nothing. There seemed to be no advantage to taking more aspirin than a baby-sized dose.

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The study's conclusion that even low doses of aspirin can reduce colon cancer suggests the drug is inching its way toward being used for cancer prevention, though people should not start taking aspirin daily without consulting their doctor.

The studies used European baby aspirin of 75 milligrams and regular aspirin, 300 milligrams. US. baby aspirin is 81 milligrams and regular aspirin, 325 milligrams.

If taken in high doses over a long period, aspirin can irritate the stomach, intestines and bowel, causing lesions and major bleeding.

Some researchers said the drug would benefit certain people.

"Anyone with any risk factors such as a family history (of colon cancer) or a previous polyp should definitely take aspirin," said Peter Rothwell, a professor at the University of Oxford and one of the paper's authors. The finding also "tips the balance" for anyone considering aspirin to prevent heart attacks and strokes, he said.

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No funding was provided for the study and it was published online Friday in the journal Lancet. Rothwell and some of his co-authors have been paid for work by several drugmakers who make anti-clotting drugs like aspirin.

The trials analyzed in the Lancet paper were done before the widespread introduction of screening tests like sigmoidoscopies and colonoscopies, which cut a person's chances of dying from colon cancer from about 40 to 70 percent. Rothwell said taking aspirin would still help, because the drug seems to stop cancers in the upper bowel, not usually caught by screening tests.

The studies compared people who took a low dose of aspirin to those who took a placebo or nothing. Researchers followed the patients for almost 20 years and observed who got cancer by checking cancer registries and death certificates in Britain and Sweden, where the studies were done. Of 8,282 people taking a low dose of aspirin, 119 died of colorectal cancer. Among the 5,751 people who took a placebo or nothing, 121 died of the disease.

Scientists think aspirin works by stopping production of a certain enzyme linked to cancers including those of the breast, stomach, esophagus and colon.

Other experts warned against aspirin for the general population. "It's not for everybody," said Robert Benamouzig, of Avicenne Hospital in Bobigny, France, who co-authored a commentary in the Lancet. He said he would advise some of his high-risk patients to take aspirin, but only after explaining its side effects.

Colorectal cancer is the second most common cancer in developed countries, and there are about 1 million new cases and 600,000 deaths worldwide every year. The average person has about a 5 percent chance of developing the disease in their lifetime.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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