Women who take aspirin regularly might reduce their risk of colon cancer but at the risk of serious side effects, according to a study published yesterday in the Journal of the American Medical Association.

Researchers looked at 20 years of data from nearly 83,000 women enrolled in the Nurse's Health Study and found that women who took at least 10 aspirin a week for more than a decade were 23 percent less likely to develop colon cancer than women who took less aspirin or none at all. Rectal cancer did not seem to be affected by aspirin, though the researchers said the null result might have been because of the low number of rectal cancer cases.

But aspirin may not be a feasible colon cancer prevention strategy, because the doses needed for protection have significant side effects, principally gastrointestinal bleeding. The incidence of gastrointestinal bleeding requiring either hospitalization or a blood transfusion was double for women who reported taking 14 or more aspirin per week, compared with women who did not take aspirin. Low doses of aspirin did not substantially increase bleeding risk, nor did they reduce the risk of colon cancer.

This study included only women, though previous studies have shown similar effects on men.

For consumers, the "use of aspirin at any dose to prevent cancer is not recommended by the American Cancer Society, or by any other group, because of the potential for serious side-effects, particularly gastrointestinal bleeding" said Eric Jacobs, a senior epidemiologist with the American Cancer Society who was not involved in the study. He said in a statement from the ACS that for researchers, the study may help them figure out why aspirin and other anti-inflammatory medications have a preventative effect against cancer.

Other anti-inflammatory medications, such as naproxen or ibuprofen, which are known as nonsteroidal anti-inflammatories, had an effect similar to that seen in aspirin. Acetaminophen (Tylenol) did not have any effect, though, because it works differently in the body than other medications, the researchers did not expect it to.

Find out more: Go to the U.S. News guide on colorectal cancer to get in-depth information about symptoms, tests, treatment, and prevention.