We identified four clinical trials with 2967 randomly assigned participants. Each trial evaluated aspirin for the
secondary prevention of colorectal adenomas. Doses of aspirin tested ranged from 81 to 325 mg/d. The average
age of participants at baseline was 58 years, and 60% were male. Median follow-up was 33 months.
A total of 2698 participants underwent colonoscopic follow-up and were included in the analysis of adenoma
occurrence and advanced-lesion occurrence after randomization. Among these participants, adenomas were
found in 424 (37%) of the 1156 participants allocated to placebo and in 507 (33%) of the 1542 participants
allocated to any dose of aspirin. Advanced lesions were found in 12% of participants in the placebo group
and in 9% of participants allocated to any dose of aspirin. The pooled risk ratio of any adenoma for any dose
of aspirin vs placebo was 0.83 (95% CI = 0.72 to 0.96). This corresponded to an absolute risk reduction of
6.7% (95% CI = 3.2% to 10.2%). For any advanced lesion, the pooled risk ratio was 0.72 (95% CI = 0.57 to
0.90). We found no statistically significant effect modification for any of the baseline factors studied.