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 Wednesday'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 per day — reduced colon cancer risk by a third, but also were linked to dangerous bleeding.
"The hope was we might be able to find something, like aspirin, that would protect a large segment of the population," study co-author Dr. Andrew Chan, a researcher at Harvard Medical School,
"We found regular aspirin use in general does seem to protect against colon cancer, but it looks like (it was women on) the highest doses, women taking more than 14 standard aspirin tablets a week, who enjoyed the greatest benefit."
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 Chan.
"Research suggests aspirin works," he told Smith. "Again, there are trade-offs from taking aspirin at these higher doses. So it may not be the answer, even for people at greater risk."
That said, "There still may be some people in the population who have particular risks who may decide that weighing the risks and benefits with the physician, it might make sense, but that has to be looked at."
He tells CBS News, "One of the exciting things about the study is to show that aspirin or aspirin-like drugs can prevent cancer. It does suggest that there is an opportunity for the development of future treatments that hopefully don't have the same side effects as aspirin.