Low-Dose Aspirin May Reduce Colon Cancer Risk

Women who regularly took low-dose aspirin had significantly reduced risk of colon cancer, but researchers are not yet ready to generally recommend the drug for primary prevention.

Aspirin has long been recommended to help prevent heart attacks and strokes, and new research suggests the drug may also reduce the risk for colorectal cancer in women.

The study, published in the July 16, 2013, issue of the Annals of Internal Medicine and funded by the National Institutes of Health, is a follow-up of the Women’s Health Study, which assessed the benefits and risks of aspirin and vitamin E in preventing cardiovascular disease and cancer in health professionals aged 45 and over.

The original study enrolled 39,876 women aged 45 or older to take either 100 mg of aspirin or placebo every other day, starting between 1993 and 1996 and continuing through 2004. Participants were mailed annual questionnaires on drug adherence, adverse effects, non-study aspirin use, clinical end points, and risk factors. The researchers observed no difference in colon cancer risk between the experimental and placebo groups during this original study period.

Participants in the original study did not receive aspirin or placebo after 2004, although some continued to take the drug on their own. The researchers continued to follow 33,682 of the original participants through March 2012 using the same annual survey used in the original study period.

After 18 years, 5071 cancer cases (including 451 colorectal cases) and 1391 deaths caused by cancer had been recorded among participants. Women who regularly took aspirin for at least 10 years had a 20% lower risk of developing colon cancer than those in the placebo group. This difference emerged during the period after the original study ended. Women from the aspirin group who continued to take aspirin after the end of the original study period had a post-study reduction in colon cancer risk of 43%, while those from the aspirin group who stopped taking aspirin after the end of the original study had a post-study reduction in risk of 33%. No difference between the groups was observed in the risk of developing breast or lung cancer.

The possible benefits of aspirin, however, came along with additional risks. Participants who received aspirin were 14% more likely to have stomach bleeding and 17% more likely to develop peptic ulcers than those in the placebo group.

In an accompanying editorial, Peter M. Rothwell, MD, PhD, of the University of Oxford in the United Kingdom, suggests that this increased risk may be related to the aspirin dosage used in the study. “In contrast to trials of daily aspirin, in which the risk for major bleeding decreased with increasing duration of aspirin use, the additional risk seemed to be maintained until the end of scheduled treatment,” Dr. Rothwell writes.

Given these risks, the study authors note that aspirin cannot yet be recommended for primary prevention of colon cancer in women and should be considered on an individual basis. As more research on the long-term effects of aspirin on cancer is completed, they write, making more general recommendations may become possible.

Dr. Rothwell makes similar recommendations in his editorial. “[These] findings therefore reinforce the need to consider the risks and benefits of aspirin separately in men and women, and treatment decisions in individuals should now reflect what is known about their risk for gastrointestinal cancer as well as their vascular risk.”