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A large study of women reduces hopes that low doses of aspirin could 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 published in last month's Journal of the American Medical Association.
Low doses of aspirin did not lower cancer risk significantly. High doses of aspirin - such as two or more aspirin a day - reduced colon cancer risk by one-third but also were linked to dangerous bleeding.
A high-dose aspirin regimen that prevented one or two cases of colorectal cancer also would cause eight additional cases of serious bleeding from irritation of the stomach or intestines, the researchers estimated.
"We can't make a recommendation that you could take an aspirin a day to prevent both heart disease and colorectal cancer," said study co-author Dr. Andrew Chan, a researcher at Harvard Medical School.
The best prevention is regular screening, exercising and eating a high-fiber, low-fat diet, Chan and other researchers not involved in the new study said.
"For average-risk people who are getting screening colonoscopies, the added benefit of taking aspirin is very small," said Dr. Robert Sandler at the University of North Carolina-Chapel Hill.
Previous studies suggested that a once-a-day baby aspirin could prevent precancerous polyps that sometimes become colorectal cancer, the second deadliest cancer for Americans after lung cancer.
Those prior studies raised hopes that the same small dose of aspirin shown to fight heart disease could do double duty against cancer. Aspirin might inhibit an enzyme linked to tumor formation.
Long-term use of high doses of aspirin is not right for most people, cautioned Dr. Julie Buring of Harvard's Brigham and Women's Hospital, who has conducted similar research but was not involved in the new study.
Buring published a study last month showing that women who took low doses of aspirin over 10 years had no lower risk of colorectal cancer and several other cancers.
"This would not be something you would choose to do on your own," she said. "If we're talking about high doses over long periods, you have to talk to your doctor for this."
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