Taking aspirin regularly can cut the long-term risk of cancer, according to the first major study of its kind.
British researchers found it can reduce the risk by 60 per cent in people with a family history of the disease.
The landmark research covering 16 countries is the first proof that the painkiller has a preventive action that is likely to benefit anyone using it every day.
Millions who take low-dose aspirin to prevent heart disease will gain from its anti-cancer properties, while healthy people may follow the example of increasing numbers of doctors who take it for insurance.
In the study of 861 patients with Lynch syndrome, a genetic fault leading to bowel and other cancers at an early age, half were given two aspirins a day, 600 mg in total, for two years.
|British researchers found taking aspirin regularly can reduce the risk by 60 per cent in people with a family history of the disease
The remainder were given placebo, or dummy, pills, says a report published online in The Lancet medical journal.
Initially, the researchers found no change in cancer rates between the groups. But when they followed up the study after five years, they detected a significant difference.
By 2010 a total of 19 new bowel cancers had been identified among those given aspirin and 34 among the placebo group - a cut of 44 per cent among those taking the drug.
When researchers focused on the 60 per cent of patients who they were certain had conscientiously taken aspirin for at least two years they found an even more striking result.
Just ten cancers were discovered in the aspirin group compared with 23 in the placebo group, a cut of 63 per cent.
Rates of other cancers linked to Lynch syndrome were almost halved by taking aspirin.
Professor Sir John Burn from Newcastle University, who led the research, said: 'What we have finally shown is that aspirin has a major preventive effect on cancer but it doesn't become apparent until years later.'
The study is being hailed as the last piece of the jigsaw after years spent trying to prove that aspirin has a direct effect in stopping tumours.
A big step forward came last year with a study which showed that low-dose aspirin cuts overall death rates by a third after five years' use.
However, it used records to look at the incidental benefits for patients taking it to stave off further heart attacks and strokes. The latest trial actually set out to prove that cancer could be prevented in people taking it for no other reason. Experts say healthy middle-aged people who start taking aspirin around the age of 45 or 50 for 20 to 30 years could expect to reap the most benefit because cancer rates rise with age.
There is widespread concern that side-effects such as stomach bleeding and haemorrhagic stroke outweigh any advantage among healthy people. Sir John, who takes aspirin every day, estimates there are 30,000 people with Lynch syndrome in the UK who might benefit from aspirin treatment.
He said: 'If we put them all on two aspirins a day now, in the next 30 years or so we would prevent 10,000 cancers. On the other hand, this would cause around 1,000 ulcers.
'If we can prevent 10,000 cancers in return for 1,000 ulcers and 100 strokes, in most people's minds that's a good deal, especially if you've grown up in a family with three, four, five, six people who have had cancer.
'On the other hand, if you're just in the general population and you don't have cancer in your family, then that's going to be a much finer balance.'
Further research will take place, he said, to discover the ideal dose of aspirin.
© Daily Mail, London