Aspirin cuts the chances of developing bowel cancer by more than half in people with a family history of the disease, British scientists said Friday.

"We've now got the cherry on the cake -- the randomized controlled trial that sets out to try to prove that [aspirin prevents cancer] and did so," said Sir John Burn, of Newcastle University, northeastern England, who led the study.

His team looked at 861 people with Lynch syndrome, an inherited predisposition to cancer. Even though aspirin can cause internal bleeding, Burn said that the case for such people to start taking it was "overwhelmingly strong."

"If you give them all aspirin, you prevent 10,000 cancers but cause 1,000 ulcers," he said. "That's a good deal."

A large study last year concluded that a daily dose of just 75 milligrams of aspirin could cut death rates for all cancers by a third. Burn's team gave patients 600 milligrams of aspirin a day, which they believed was likely to show a bigger effect in preventing cancer. He is now starting a trial to determine the ideal daily dose.

Burns said that he was already taking aspirin, which was likely to be most effective if taken from a patient's late 40s or 50s.

"Before anyone begins to take aspirin on a regular basis they should consult their doctor as aspirin is known to bring with it a risk of stomach complaints, including ulcers," he said. "However, if there is a strong family history of cancer then people may want to weigh up the cost benefits."

Half of the 861 people in the study took two aspirins (600mg) a day, for varying lengths of time. Ten years after they began taking the pills, there had been 19 cancers among people who had taken aspirin, and 34 among those taking a placebo. Among those who took the drug for at least two years, there were 10 cancers in the aspirin group and 23 in the placebo group.

The effect began to be seen five years after they started taking aspirin and persisted well after they stopped, researchers reported in The Lancet medical journal.

How exactly aspirin prevents cancer is unclear, but Burns believes that compounds found in the drug trigger genetically damaged cells to destroy themselves at a very early stage.