Regular use of low-dose aspirin can cut the risk of colorectal cancer by a third, with benefits becoming apparent quickly, a large Scottish case-control study found.
After a just year, patients who took 75 mg/day of aspirin -- a lower dose than a standard baby aspirin -- reduced their risk of colon cancer by 13 percent compared with controls, according to Farhat V.N. Din of the University of Edinburgh and colleagues.
And after five years the relative risk reduction was 37 percent, the researchers reported online in Gut.
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Epidemiologic studies have shown that aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) lower the incidence of colorectal cancer by about 40 percent, a finding supported by some, though not all, randomized trials.
"The key finding of our case-control study is that the daily 75 mg dose of aspirin is associated with a lower incidence of [colorectal cancer] in the general population," Din and colleagues wrote.
Some previous studies have suggested that higher doses might be needed for prevention of colorectal cancer, but "our data provide reassurance that lower doses of aspirin do provide worthwhile risk reduction effects," they observed.
Aspirin has not as yet been recommended for primary chemoprevention of colorectal cancer, however, because of unanswered questions on dose, duration, and effects on survival.
To explore these questions, Din and colleagues prospectively recruited 2,279 cases and 2,907 matched controls ages 16 to 79 years who completed a questionnaire on lifestyle, medications used and cancer history.
Analysis of the data determined that overall, the risk of colorectal cancer was lower in users of any NSAID compared with nonusers after adjustment for factors such as age, sex, and physical activity.
The inverse association between the use of 75 mg aspirin and colorectal cancer was seen for both men and women, but was significant only in men.
There was no protective effect of aspirin or NSAIDs on all-cause mortality during nearly five years of follow-up. Nor was there an effect on colorectal-related mortality.
The lack of effect on survival may relate to sample sizes or limited duration of intake, according to the researchers.
Among cases taking aspirin or NSAIDs there were 224 deaths, six possibly related to the drugs (one gastrointestinal bleed and five cerebrovascular accidents).
Among the cases not using aspirin or NSAIDs there were 486 deaths, with three bleeds and three cerebrovascular accidents.
There was no difference between users and nonusers in deaths resulting from bleeds, although power to detect a difference may have been inadequate.
Strengths of the study include its prospective design, matched controls, and its being representative of the population at large, not only high-risk groups.
A shortcoming was that the investigators were not aware if cases continued taking aspirin or NSAIDs after their cancer diagnosis, which limits the conclusions that can be drawn.
In addition, plasma levels of the drugs were not measured.
The authors noted that their results are applicable to the general population, and called for well powered randomized trials to further clarify the effects on survival.