The study showed that taking 300mg of aspirin a day for five years could reduce the incidence of colorectal cancer by 37 per cent, and by 74 per cent during a period of 10-15 years after treatment was started.
But potential risks associated with the long-term use of high-dose aspirin suggest that screening remains the best option to prevent colorectal cancer, experts say.
For the study, researchers determined the delayed effect of aspirin by following up 7,582 patients from two large randomised trials of aspirin performed in the late 1970s and early 1980s.
The long-term study was necessary because the pre-cancerous growths that aspirin is thought to reduce take at least 10 years to develop into cancers.
The effects of aspirin were found to be consistent regardless of age, sex and race.
Lead researcher Peter Rothwell, professor of neurology at the University Department of Clinical Neurology, Radcliffe Infirmary, Oxford, said GPs should advise patients at high risk, such as those with a strong family or personal history of colorectal cancer, to take 300mg of aspirin daily.
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