Aspirin has a proven ability to prevent cancer which should be factored into new patient guidelines, according to the British lead author of a major new study.
Professor Peter Rothwell believes the evidence shows aspirin's anti-cancer benefits may be bigger 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 not only prevent cancer occurring, but also slow its progress.
One study showed that taking aspirin reduced the risk of developing cancer by about a quarter after just three years. From five years onwards, it cut the risk of dying from cancer by 37%.
Another in a series of three papers showed that aspirin reduced the chances of cancer spreading instead of staying in one place by almost 50%.
The deadly spread, or metastasis, of tumours to organs such as the liver and brain is usually what kills cancer patients.
Many people take a low 75mg dose of aspirin each day to guard against heart attacks and strokes.
Experts advise against this for "healthy" individuals at no special risk of heart and artery disease because of the possible long-term side effects of aspirin. The drug, which prevents blood clotting, can increase the likelihood of internal bleeding in the stomach, intestines and brain.
In some cases, such as pregnant women at risk of high blood pressure, the benefits of taking aspirin are said to outweigh the risks. However, to date cancer has not been part of this calculation.
Prof 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, all the guidelines have just been based on the prevention of strokes and heart attacks.
"This research really 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."
Two studies are published in The Lancet and one in The Lancet Oncology.
For one study, Prof Rothwell's team analysed data from more than 77,000 patients taking part in 51 randomised trials looking at aspirin's effect on rates of heart attacks and strokes.
Taking daily low-dose aspirin reduced the risk of cancer death by 15% overall compared with not taking the drug. For patients taking aspirin for five years or more, this figure rose to 37%. After just three years, aspirin reduced cancer incidence by 23% in men and 25% in women.
In absolute terms, taking aspirin lowered the proportion of patients developing cancer from 12 per 1,000 to nine per 1,000 after three years.
Both the heart and artery benefits and the risk of bleeding from taking aspirin diminished over time, so that after three years the reduced risk of cancer was the only significant effect seen.
"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 preventing heart disease and stroke - particularly with prolonged use," said Prof Rothwell.
The second study focused on aspirin's impact on cancer spread. Metastasis data were collected from patients diagnosed with cancer while taking part in five large randomised heart attack and stroke prevention trials.
Over an average period of 6.5 years, taking low-dose aspirin every day was found to reduce the overall risk of distant-spreading cancer by 36%. For common solid cancers, including bowel, lung and prostate, the risk was reduced by 46% and for bladder, kidney and other solid cancers by 18%.
Aspirin reduced the proportion of cancers that spread instead of staying localised by 48%.
The drug also reduced the risk of being diagnosed with a solid cancer that had already spread by 31%. For patients initially diagnosed with a local cancer, the risk of later metastasis was reduced by 55%.
Aspirin's ability to slow metastasis may be linked to its effect on platelets, the clotting bodies in the blood, said the scientists. The role of platelets in promoting cancer spread in mice was reported more than 40 years ago.
"Previously no drug has ever been shown to reduce metastasis as a specific effect," said Prof Rothwell. "It opens up a completely new therapeutic area."
The third study looked at observational data from 17 case-control studies which did not compare different groups of treated and non-treated patients. The findings appeared to support those from randomised trials.
Prof Rothwell suggested that the National Institute for Health and Clinical Excellence (Nice), which issues treatment guidelines for the NHS, should issue advice on the use of aspirin to combat cancer.
"I think it would be helpful in the fullness of time if a body like Nice, for example, would come up with an independent recommendation on the basis of all the evidence that has been gathered," he said. "That would give people confidence."
Nice has never been asked by the Department of Health to develop guidance on aspirin and cancer prevention. But aspirin is mentioned in the latest Nice guidance on colorectal (bowel) cancers. It says that "regular use of non-steroidal anti-inflammatory drugs (NSAIDs), notably aspirin, seems to reduce the risk of colorectal cancer".
Two American experts commenting in The Lancet pointed out some limitations to the research. Dr Andrew Chan and Dr Nancy Cook, from Harvard Medical School in Boston, said the analysis did not include findings from two large US trials that failed to show an anti-cancer benefit from taking aspirin every other day.
The researchers may also have missed the effect of pre-cancerous cells being removed from patients either through screening or the effects of aspirin-associated bleeding, they added. Some analyses were also limited by the quality of available data.
However the US experts concluded: "These caveats notwithstanding, Rothwell and colleagues show quite convincingly that aspirin seems to reduce cancer incidence and death..."
