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A systematic review was conducted to determine the effect of nonsteroidal anti-inflammatory drugs for the prevention or regression of colorectal adenomas and cancer.Randomized, controlled trials through September 2003 were identified. Nonsteroidal anti-inflammatory drugs were the interventions. The primary outcomes were the number of patients with at least one colorectal adenoma, a change in polyp burden, or colorectal cancer. The secondary outcome was adverse events. Two reviewers independently extracted data and assessed trial quality. Dichotomous outcomes were reported as relative risks with 95 percent confidence intervals. The data were combined if clinically and statistically reasonable.Nine trials with 150 familial adenomatous polyposis and 24,143 population patients met the inclusion criteria. The interventions included sulindac, celecoxib, or aspirin. From the combined results of three trials, significantly fewer patients in the aspirin group developed recurrent sporadic colorectal adenomas (relative risk, 0.77 (95 percent confidence interval, 0.61, 0.96), number needed to treat 12.5 (95 percent confidence interval, 7.7, 25)) after one to three years. In another three trials, patients with familial adenomatous polyposis who received nonsteroidal anti-inflammatory drugs had a greater proportional reduction (range, 11.9-44 percent) in the number of colorectal adenomas compared with those in the control group (range, 4.5-10 percent). There was no significant difference for the outcomes of colorectal cancer or adverse events in any of the trials.There is combined evidence from three randomized trials that aspirin significantly reduced the recurrence of sporadic adenomatous polyps. There was evidence from short-term trials to support regression, but not elimination or prevention, of colorectal polyps in familial adenomatous polyposis.