Previously, research has pointed to Aspirin's potential benefits for preventing colorectal cancer in people who are genetically predisposed to the malignancy.
Since less was known about taking low doses on alternate days, researchers in the U.S. randomly assigned 38,876 women aged 45 years or older to take either 100 milligrams of Aspirin or a placebo every other day.
"After a median follow-up of 18 years, a difference in colorectal cancer by Aspirin group emerged," Julie Buring of Brigham and Women's Hospital in Boston and her co-authors concluded in Monday's issue of the Annals of Internal Medicine.
The incidence of colorectal cancer was 20 per cent lower in the Aspirin group than in the placebo group.
The long-term difference in cancer rates could be because of the long period between early stage disease and clinical presentation of colorectal cancer, the researchers said.
Even with low dose Aspirin, there was a net increase of 193 self-reported cases of gastrointestinal bleeding and 214 reported peptic ulcers, although some were minor, the researchers cautioned.
The study provides the first reliable data in women, Dr. Peter Rothwell of the University of Oxford said in a journal editorial accompanying the study.
But the lack of a reduction in death from any cause or risk for all cancer "should temper any recommendations for widespread use of Aspirin in healthy middle-aged women," Rothwell cautioned.
Cancers of the colon, esophagus and stomach together accounted for about eight per cent of cancer cases in the women's study compared with 23 per cent of all cancer deaths in a similar study in men, which Rothwell said reinforces the need to consider the risks of benefits of Aspirin separately for men and women.
The study was funded by the U.S. National Heart, Lung and Blood Institute and the National Cancer Institute.
The Women's Health Study started in 1993 and ended in 2004 to look at the benefits and risks of low-dose Aspirin and vitamin E for cardiovascular disease and cancer.