Long term use of 300mg or more of aspirin a day for five years can prevent colorectal cancer, conclude authors of a study published in this week's special gastroenterology edition of The Lancet.
But the authors of the study and an accompanying comment stress that the potential risks of long term aspirin use at this dose and the availability of alternative prevention strategies mean that widespread use of aspirin for cancer prevention cannot be recommended in the general population.
However, the benefits are likely to outweigh the risks in individuals at increased risk of colon cancer. The findings are also likely to influence the choice of antiplatelet drug in patients who require long-term treatment because of vascular disease.
The study was conducted by Professor Peter Rothwell, University Department of Clinical Neurology, Radcliffe Infirmary, Oxford, UK and colleagues. In collaboration with the original investigators (Sir Richard Doll, Sir Richard Peto and Charles Warlow), they determined the delayed effect of aspirin by following-up patients from two large randomised trials of aspirin performed in the late 1970s and early 80s - the British Doctors' Aspirin Trial and the UK-TIA Aspirin Trial.
The researchers were particularly interested in long term follow-up due to the likely time delay in any effect of aspirin on colorectal cancer. Adenomas (the pre-cancerous growths that aspirin is thought to reduce) take at least 10 years to develop into cancers. The study showed that use of Aspirin for five years reduced the subsequent incidence of colorectal cancer by 37% overall, and by 74% during the period 10-15 years after treatment was started.
In an accompanying analysis of observational studies, the risk of colorectal cancer also appeared to be reduced by between 50-70% in patients taking medium-high doses of aspirin for 10 years or more. This analysis also showed that the effects of aspirin were consistent regardless of age, sex, race or country of origin of patients studied (all of which affect the general rate of colorectal cancer) and that the effect was also seen in individuals with a family history of colorectal cancer in a first degree family relative (which increases lifetime risk of an individual acquiring the disease by two to four times).
The authors conclude: "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 10 years, which is consistent with findings from observational studies.
"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, Dr Andrew Chan, Gastrointestinal Unit, Massachusetts General Hospital, Boston, USA, says: "Rothwell and colleagues' results, when viewed in the context of the preponderance of laboratory studies, epidemiological data, and adenoma recurrence trials, 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."