Aspirin prevents colon cancer

Clinical Question

Does aspirin prevent colon cancer?

Bottom Line

In this study, taking at least 300 mg aspirin for approximately 5 years prevented colon cancer up to 20 years later. (LOE = 2b)


Flossmann E, Rothwell PM; British Doctors Aspirin Trial and the UK-TIA Aspirin Trial. Effect of aspirin on long-term risk of colorectal cancer: consistent evidence from randomised and observational studies. Lancet 2007;369:1603-1613.  [PMID:17499602]

Study Design

Systematic review




Outpatient (any)


The US Preventive Services Task Force recommends against the use of aspirin or nonsteroidal anti-inflammatory drugs to prevent colon cancer, in part because of safety concerns. These authors report 20 years of pooled follow-up data on more than 7000 patients from 2 randomized trials of aspirin versus no aspirin. Their rationale is that since colon cancer has a very long latency period (more than 10 years), most trials haven't followed up long enough to see real effects of preventive interventions. In the original trials, patients received daily doses of 300 mg, 500 mg, or 1200 mg of aspirin for 1 year to 7 years. It is unclear if the patients continued taking aspirin beyond the original study periods. Since the trials occurred in the United Kingdom, the researchers were able to use national registries to achieve nearly 100% follow-up after 20 years. They used intention to treat to analyze the rate of developing colon cancer. In addition to analyzing these data, they did a systematic review of case-control studies that assessed the association between aspirin use and development of colon cancer to determine if their data are consistent with observational studies. Patients taking aspirin had a lower rate of developing colon cancer (2.6%) than control patients (3.4%). One would need to treat 112 patients (95% CI, 57-1038) for 20 years to prevent 1 colon cancer. Among patients taking aspirin for at least 5 years, 2.5% developed colon cancer compared with 3.8% among control patients (number needed to treat = 75; 43-238) during the 20 years of follow-up. Finally, the authors don't report adverse effects related to aspirin use, so it is unclear who benefits.