CHICAGO -- A large study of women appears to have weakened the view that low doses of aspirin may be an easy way to prevent colorectal cancer.
Aspirin helps, but its effect is significant only after a decade of use, according to a 20-year study of almost 83,000 nurses that was published in today's Journal of the American Medical Association.
Low doses of aspirin did not lower the cancer risk significantly, the study found. High doses of aspirin, such as two or more per day, reduced the risk of colon cancer by one-third, but such dosages also were associated with possible dangerous bleeding.
A high-dose aspirin regimen that prevented one or two cases of colorectal cancer also could cause eight cases of serious bleeding from irritation of the stomach or the intestines, the researchers said.
''We can't make a recommendation that you could take an aspirin a day to prevent both heart disease and colorectal cancer," said a coauthor, Dr. Andrew Chan of Harvard Medical School.
The best prevention is screening, exercise, and a high-fiber diet, said Chan and researchers not in the study.
For average-risk people who get colonoscopies, the benefit of ''aspirin is very small," said Dr. Robert Sandler at the University of North Carolina at Chapel Hill. He was not involved in the study.
Previous studies suggested that taking one baby aspirin a day might prevent precancerous polyps that sometimes become colorectal cancer, the second-deadliest cancer for Americans after lung cancer.
Those studies raised hopes that the same small dose of aspirin shown to fight heart disease could also be a guard against cancer. According to those theories, aspirin might inhibit an enzyme linked to tumor formation.
Long-term use of high doses of aspirin are not advisable for most people, said Dr. Julie Buring of Brigham and Women's Hospital, which is affiliated with Harvard. Buring has conducted similar research, but who was not involved in the new study.
Buring, in a study published last month, found that women who took low doses of aspirin over 10 years had no lower risk of colorectal cancer and several other cancers than women who did not.
''If we're talking about high doses over long periods, you have to talk to your doctor for this," she said.