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Aspirin reduces the risk of colorectal cancer, but its effects seem confined to cancers that express the cyclo-oxygenase-2 (COX 2) enzyme, according to a large observational study. Regular use of aspirin was associated with a 36% decrease in the risk of colorectal cancers expressing COX 2 (multivariate relative risk 0.64, 95% CI 0.52 to 0.78) in more than 130000 health professionals in two long running cohorts. Protection increased with longer duration of use (becoming significant after five years) and with higher dose. Aspirin in any dose had no impact on cancers not expressing COX 2. The study looked at 636 incident cancers, nearly two thirds of which expressed the enzyme.
These findings suggest that the anticancer effects of aspirin are mediated through COX 2 inhibition, say the authors. It's also likely that the effect is greatest in the early stages of adenoma or cancer, as participants had to take aspirin for at least five years to benefit.
In the long run, the value of aspirin as a prophylactic against cancer will probably be limited by its well known and sometimes serious side effects. Hopefully, further investigation of the complex COX 2 pathways will reveal a more specific, more effective, and less troublesome agent, says an editorial (pp 2195-8).