In the four trials of aspirin versus control (mean duration of treatment 6 years), 2·8% of 14,000 patients had colorectal cancer during a follow-up of 18 years.
Aspirin reduced the 20-year risk of colon cancer (incidence hazard ratio [HR] 0·76, but not rectal cancer (0·90).
Where subsite data were available, aspirin reduced risk of cancer of the proximal colon (0·45), but not the distal colon (1·10). Benefit increased with duration of treatment - aspirin taken for 5 years or longer reduced risk of proximal colon cancer by 70% and also reduced risk of rectal cancer (0·58).
There was no increase in benefit at doses of aspirin greater than 75 mg daily. However, risk of fatal colorectal cancer was higher on 30 mg versus 283 mg daily.
Aspirin taken for several years at doses of at least 75 mg daily reduced long-term incidence and mortality due to colorectal cancer. Benefit was greatest for cancers of the proximal colon, which are not otherwise prevented effectively by screening with sigmoidoscopy or colonoscopy.
Long-term effect of aspirin on colorectal cancer incidence and mortality: 20-year follow-up of five randomised trials. The Lancet, Volume 376, Issue 9754, Pages 1741 - 1750, 20 November 2010.
Image source: Colon (anatomy), Wikipedia, public domain.