Women who take a low-dose aspirin every other day have a reduced risk of colon cancer, according to the results of a study published in the Annals of Internal Medicine—however, the benefit carries with it a risk of gastrointestinal bleeding.
Colorectal cancer remains the second leading cause of cancer-related death in the United States. Some studies have found a link between daily aspirin and reduced cancer incidence and mortality and some suggest that aspirin appears to reduce the risk of hereditary colon cancer. The U.S. Preventive Services Task Force (USPSTF) recommends that women aged 55 through 79 take daily aspirin only if the potential benefits outweigh the harms.
This most recent evidence is the result of 18 years of follow-up in the Women’s Health Study, which was a 10-year randomized trial that evaluated the effects of aspirin and vitamin E on cardiovascular disease and cancer risk. The study included 35,876 women aged 45 or older with no history of cardiovascular disease or cancer. The women were randomly assigned to take 100 mg of aspirin or placebo every other day for 10 years. The extended follow-up includes data from 33,682 women.
Overall, there were 5,071 confirmed cancer cases (including 2,070 breast, 451 colorectal, and 431 lung). There were 1,391 cancer deaths. Women in the aspirin group had a 20 percent reduced risk of colon cancer, but the benefit didn’t appear until after a decade. What’s more, the aspirin group experienced a higher rate of gastrointestinal bleeding and peptic ulcers. There was no reduction in the overall risk of cancer or the risk of lung and breast cancer among women who took aspirin.
The researchers concluded that long-term use of alternate-day, low-dose aspirin may reduce the risk for colorectal cancer in healthy women; however, an accompanying editorial cautions against recommending widespread use of aspirin in healthy, middle-aged women because of the risk of bleeding and because aspirin hasn’t been shown to reduce the overall risk of cancer.
 Cook NR, Lee IM, Zhang SM, et al. Alternate-day, low-dose aspirin and cancer risk: Long-term observational follow-up of a randomized trial. Annals of Internal Medicine. 2013; 159(2): 77-85.
 Thun MJ, Jacobs EJ, Patrono C. The role of aspirin in cancer prevention. Nature Reviews Clinical Oncology. Published early online April 3, 2012. doi:10.1038/nrclinonc.2011.199
 Rothwell PM, Price JF, Fowkes FGR et al. Short-term effects of daily aspirin on cancer incidence, mortality, and non-vascular death: analysis of the time course of risks and benefits n 51 randomised controlled trials. Lancet. Early online publication March 21, 2012.
 Burn J, Gerdes A-M, Macrae F et al. Long-term effect of aspirin on cancer risk in carriers of hereditary colorectal cancer: an analysis from the CAPP2 randomised controlled trial. Lancet. Early online publication October 28, 2011.
 Rothwell PM. Alternate-day, low-dose aspirin and cancer risk. Annals of Internal Medicine. 2013; 159(2): 148-150.