Four RCTs (n=2,967 participants, range 272 to 1,121) were included. Median follow-up was around 33 months. Mean treatment compliance rates ranged between 69% and 92%.
There was a statistically significant reduction in risk of any adenoma for aspirin (any dose) compared with placebo (RR 0.83, 95% CI 0.72 to 0.96, I2=41.5%; four RCTs). The benefit was significantly better for lower doses than higher doses (RR 1.18, 95% CI 1.01 to 1.37; two RCTs). For advanced lesions, the pooled risk ratio was 0.72 (95% CI 0.57 to 0.90, I2=0%; four RCTs). The largest reduction in risk for all adenomas occurred during the first year of follow-up (RR 0.62, 95% CI 0.48 to 0.81; three RCTs). Aspirin had no effect on risk beyond 38 months (RR 0.99, 95% CI 0.78 to 1.26; four RCTs).
No statistically significant effect modification was found for any of the baseline variables studied, although the effect of aspirin (any dose) on advanced lesions was greater in those with a family history of colorectal cancer (RR 0.53, 95% CI 0.33 to 0.83) than in those without (RR 0.92, 95% CI 0.67 to 1.28). There were no differences between groups in adverse events (reported in three RCTs) for mortality, myocardial infarction, major bleeding and all-site invasive cancer, but there was a statistically significantly higher rate of strokes (most of which were thought to be ischaemic) among aspirin-treated participants than among those who received placebo. In the three studies that reported incidence of colorectal cancer (in patients with no history of colorectal cancer) there was no significant difference between treatment groups. Further results were reported.