Many studies have shown that the regular use of aspirin reduces the risk of colorectal neoplasms. Quite how this works is not clear but it is thought to be related to prostaglandin metabolism or, more specifically, to aspirin's ability to inhibit the enzyme cyclo-oxygenase-2 (COX-2).
If this is the mechanism by which aspirin reduces colorectal cancers then it would be cancers that over-express COX-2 that would occur less frequently in aspirin users. Chan et al (NEJM 2007;356:2131-42) looked at this theory by histochemical assays of cancers removed from men and women in two large surveys and matched these against aspirin intake. They found that cancers that over-express COX-2 were reduced by aspirin but not cancers that had weak COX-2 expression.
The effect was found relative to increasing aspirin dose and duration of use. Flossmann et al (Lancet 2007;369:1603-13) showed that 300mg per day for 5 years is effective in primary prevention but the latency time is 10 years so patients have to be dedicated to their health.
