By Ben Hirschler

LONDON (Reuters) - Taking two aspirin a day for two years reduces the long-term risk of bowel cancer in people with a family history of the disease by around 60 percent, according to a British study published Friday.

The finding could also have implications for the wider population, though more research is needed to determine an ideal dose for different groups of people.

In recent years there has been an intense debate about the merits of routinely taking aspirin, which increases the risk of stomach ulcers and internal bleeding, but also protects against heart problems.

John Burn of Newcastle University believes his study -- the first randomized controlled trial into the effect of aspirin on cancer outcomes -- is a key piece of evidence validating the case for aspirin.

Previous research into cancer and aspirin, a cheap drug originally developed by Bayer more than a century ago, has been based on less robust observational studies.

Burn's study looked at people with Lynch syndrome, a genetic condition predisposing them to a range of cancers. It affects at least one in 1,000 people and around half of them develop cancer, mainly in the bowel and womb.

His data suggests that for every 10,000 cancers prevented, there could be 1,000 extra ulcers due to aspirin.

"There's a trade-off. If we could get by on a smaller dose, then we could potentially avoid a lot of those ulcers," Burn told a news conference.

For the general population, taking low-dose aspirin might be a more sensible option, and a major new multi-dose trial should shed more light on this in the next few years, he added.

DELAYED EFFECT

Burn and colleagues studied 861 people with Lynch syndrome, who began a two-year course of either 600 milligrams a day of aspirin or a placebo between 1999 and 2005.

An initial analysis in 2007 found no difference in cancer rates between the groups. But it turned out the effect was delayed and in 2010 there was a clear divergence, with 19 new bowel cancers among those on aspirin and 34 in the placebo arm.

What is more, among those patients who stuck to the full two-year regimen of pill-taking -- some 60 percent of the total -- the effects were more pronounced, and strongly statistically significant, with a 63 percent reduction in bowel cancer cases from 23 in the placebo group versus 10 in the aspirin group.

"What we have finally shown is that aspirin has a major preventative effect on cancer but this doesn't become apparent until years later," Burn said.

How exactly aspirin provides protection is unclear, but Burn said the delayed effect suggested aspirin may hit faulty stem cells before they mutate into pre-cancerous cells.

The study, which was published online by the Lancet medical journal and part-funded by Bayer, also found some evidence of a reduction in other solid cancers linked to Lynch syndrome.

"This is a really important study showing that aspirin can significantly reduce the risk of bowel and other cancers in patients genetically at high risk of developing cancer," said Chris Paraskeva of the University of Bristol, who was not involved in the research.

In a commentary in the Lancet, Andrew Chan of Harvard Medical and Scott Lippman of the MD Anderson Cancer Center in Houston said the findings were "compelling" and arguably supported more general recommendations to consider aspirin for preventing bowel cancer, based on individual patients' risks.

(Reporting by Ben Hirschler; Editing by Will Waterman)