Scientists from the Radcliffe Infirmary, Oxford, analysed the long-term effects of aspirin by following up patients from two large trials performed in the 1970s and 1980s – the British Doctors' Aspirin Trial and the UK-TIA Aspirin Trial.
They found that the use of aspirin for five years reduced the subsequent incidence of colorectal cancer by 37 per cent overall and 74 per cent during the ten to 15-year period after treatment was started.
A further analysis of observational studies found that the risk of colorectal cancer appeared to be reduced by between 50 and 70 per cent in patients taking the medium-high doses of aspirin for ten years of more.
This effect was consistent regardless of sex, age, race or country of origin.
"Use of 300mg or more of aspirin a day for about five years is effective in primary prevention of colorectal cancer, with a latency of about ten years, which is consistent with findings from observational studies," the study's authors write in the Lancet medical journal.
"Long-term follow up is required from other randomised trials to establish the effects of lower or less frequent doses of aspirin."
In an accompanying comment in the Lancet, Dr Andrew Chan, of Massachusetts General Hospital, Boston, said: "[The results] do provide convincing evidence that aspirin, at biologically relevant doses, can reduce the incidence of colorectal cancer.
"However, with the concerns about the potential risks of long-term aspirin use and the availability of alternative prevention strategies (e.g. screening), these findings are not sufficient to warrant a recommendation for the general population to use aspirin for cancer prevention."
Colorectal cancer is diagnosed in about 35,000 people every year in the UK.
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