The study published in The Lancet was conducted by a team led by Professor Peter Rothwell, from the Department of Clinical Neurology, in collaboration with the original investigators including Sir Richard Doll, Sir Richard Peto and Charles Warlow.

Professor Rothwell’s team followed up two large randomised trials of aspirin carried out at Oxford University in the late 1970s and early ‘80s where patients were given varying doses of aspirin or a dummy pill for five to seven years. Researchers have logged the results of the delayed effect of aspirin on colon cancer of those patients who took part in the British Doctors’ Aspirin Trial and the UK-TIA Aspirin Trial.

Adenomas – the pre-cancerous growths that aspirin is thought to reduce – take at least 10 years to develop into cancers – which is why the effect of aspirin (which prevents development of adenomas) takes about 10 years to appear as a reduction in incidence of cancers.

In an accompanying review of observational studies, the researchers also showed that taking medium to high doses of aspirin for 10 years or more reduced the risk of colon cancer by between 50 and 70 per cent. This effect was seen regardless of age, sex, race or country of origin. The benefit was also seen in individuals with a high risk of cancer. A family history of colon cancer normally increases that individual’s chance of acquiring the disease by two to four times.

The authors concluded: ‘Use of 300 mg 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.’

Professor Rothwell stressed: ‘These results do not mean that we should all start taking aspirin – the risk of bleeding with long-term treatment will offset benefit in people at low risk of colon cancer. However, the benefit will probably outweigh the risks in individuals known to be at high risk of bowel cancer, such as those with a strong family history or those who have had an adenoma in the past. Anybody who thinks that they might benefit should discuss the pros and cons with their family doctor.’

Source: University of Oxford