Background: Aspirin for prevention of colorectal cancer is controversial.
Purpose: To examine the benefits and harms of aspirin chemoprevention.
Data Sources: MEDLINE, 1966 to December 2006; EMBASE, 1980 to April 2005; CENTRAL, Cochrane Collaboration's registry of clinical trials; Cochrane Database of Systematic Reviews.
Study Selection: Two independent reviewers conducted multilevel screening to identify randomized, controlled trials (RCTs), caseâ€“control studies, and cohort studies of aspirin chemoprophylaxis. For harms, systematic reviews were sought.
Data Extraction: In duplicate, data were abstracted and checked and quality was assessed.
Data Synthesis: Regular use of aspirin reduced the incidence of colonic adenomas in RCTs (relative risk [RR], 0.82 [95% CI, 0.7 to 0.95]), caseâ€“control studies (RR, 0.87 [CI, 0.77 to 0.98]), and cohort studies (RR, 0.72 [CI, 0.61 to 0.85]). In cohort studies, regular use of aspirin was associated with RR reductions of 22% for incidence of colorectal cancer. Two RCTs of low-dose aspirin failed to show a protective effect. Data for colorectal cancer mortality were limited. Benefits from chemoprevention were more evident when aspirin was used at a high dose and for periods longer than 10 years. Aspirin use was associated with a dose-related increase in incidence of gastrointestinal complications.
Limitations: Important clinical and methodological heterogeneity in the definitions of regular use, dose, and duration of use of aspirin necessitated careful grouping for analysis.
Conclusions: Aspirin appears to be effective at reducing the incidence of colonic adenoma and colorectal cancer, especially if used in high doses for more than 10 years. However, the possible harms of such a practice require careful consideration. Further evaluation of the cost-effectiveness of chemoprevention compared with, and in combination with, a screening strategy is required.