Review: evidence from observational studies, but not randomized trials, suggests that long-term aspirin prevents colorectal cancer.

M e t h o d s Data sources: MEDLINE (to December 2006), preMEDLINE (to April 2005), EMBASE/Excerpta Medica (to April 2005), the Cochrane Library (Issue 4, 2004), and the PubMed Cancer subset. Study selection and assessment: Englishlanguage studies that evaluated the efficacy of aspirin for primary prevention of colorectal cancer in persons at “average” risk and systematic reviews that assessed the harms of long-term aspirin use. Studies of familial colon cancer were excluded. 4 randomized controlled trials (RCTs) (n = 63 340), 9 cohort studies (n = 1 303 573), 13 case–control studies (n = 1 148 086), and 12 reviews met the selection criteria. Outcomes: Colorectal cancer mortality, colorectal cancer, colorectal adenoma, hemorrhagic stroke, and gastrointestinal bleeding.