ASPIRIN has a proven ability to prevent cancer that should be factored into new patient guidelines, according to the British lead author of a new study.

Professor Peter Rothwell says evidence shows aspirin's anti-cancer benefits may be more substantial than its protective effect on the heart and arteries.

His research, published in two Lancet journals, suggests that low daily doses of the painkiller may prevent cancer occurring as well as slow its progress.

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One study showed that taking aspirin reduced the risk of developing cancer by about one-quarter after three years. From five years onwards, it cut the risk of dying from cancer by 37%.

Another showed that aspirin reduced the chances of cancer spreading by almost 50%. The spread, or metastasis, of cancerous tumours to organs such as the liver and brain is usually what kills cancer patients.

Many people take a low 75 milligram dose of aspirin each day to guard against heart attacks and strokes, although experts advise against this for "healthy" individuals at no risk of heart and artery disease because of the possible long-term side effects. The drug, which prevents blood clotting, can increase the likelihood of internal bleeding.

However, to date cancer has not been part of aspirin's benefits-versus-risk calculation.

Mr Rothwell, from Oxford University, said: "It's certainly time to add prevention of cancer into the analysis of the balance of risk and benefits of aspirin. So far, the guidelines have just been based on the prevention of strokes and heart attacks.

"This research shows that the cancer benefit is as large, if not larger, than the benefit in terms of preventing heart attacks and strokes. It does change the equation quite drastically."

For one study, Mr Rothwell's team analysed data from more than 77,000 patients in 51 randomised trials looking at aspirin's effect on rates of heart attacks and strokes. It showed that taking daily low-dose aspirin reduced the risk of cancer death by 15% overall compared with not taking the drug.

"The benefits of daily aspirin in reducing the long-term risk of developing new cancers and in reducing the spread of existing cancers that have not yet presented are larger in absolute terms than the benefits in pre-venting heart disease and stroke, particularly with prolonged use," said Mr Rothwell.

The second study focused on aspirin's impact on cancer spread. Aspirin was shown to reduce the proportion of cancers that spread instead of staying localised by 48%. "Previously no drug has ever been shown to reduce metastasis as a specific effect," added Mr Rothwell.

Aspirin's ability to slow metastasis may be linked to its effect on platelets, the clotting bodies in the blood, said the scientists.