Colorectal Cancer Prevention (PDQ®): Prevention - Health Professional Information [NCI] - Description of the Evidence
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A systematic review of 46 observational studies of ASA and CRC in 2007 found a reduction in CRC (odds ratio [OR] for any use 0.80 [0.73–0.87]). A large cohort study (301,240 people with 3,894 colorectal cancer cases) published after this systematic review found an association between weekly or daily ASA use and reduced 10-year incidence of distal and rectal (but not proximal) colorectal cancer, with an HR of 0.76 (95% CI, 0.64–0.90) for rectal cancer for daily use. However, use was assessed at only one time, and there is no information about dose or duration of use.
In the Physicians' Health Study, 22,000 men aged 40 to 84 years were randomly assigned to receive placebo or ASA (325 mg every other day) for 5 years. There was no reduction in invasive cancers or adenomas at a median follow-up of 4.5 years. In a subsequent analysis of more than 12 years, both randomized and observational analyses indicated that there was no association between the use of ASA and the incidence of CRC. The low dose of ASA and the short treatment period may account for the null findings.
In a randomized study of 635 patients with prior CRC (T1–T2 N0 M0) who had undergone curative resection, ASA intake at 325 mg/day was associated with a decrease in the adjusted RR of any recurrent adenoma as compared with the placebo group (0.65; 95% CI, 0.46–0.91) after a median duration of treatment of 31 months. The time to detection of a first adenoma was longer in the ASA group than in the placebo group (HR for the detection of a new polyp, 0.54; 95% CI, 0.43–0.94, P = .022). Harms of treatment included upper gastrointestinal hemorrhage and hemorrhagic stroke. In a study of 1,121 patients with a recent history of colorectal adenomas, after a mean duration of treatment of 33 months, the unadjusted RRs of any adenoma (as compared with the placebo group) were 0.81 in the 81-mg ASA group (95% CI, 0.69–0.96) and 0.96 in the 325-mg ASA group (95% CI, 0.81–1.13). For advanced neoplasms (adenomas measuring at least 10.0 mm in diameter or with tubulovillous or villous features, severe dysplasia, or invasive cancer), the RRs were 0.59 (95% CI, 0.38–0.92) in the 81-mg ASA group, and 0.83 (95% CI, 0.55–1.23) in the 325-mg ASA group. Harms of treatment were similar in the two groups and included upper gastrointestinal bleeding and hemorrhagic stroke.