Taking Aspirin can prevent colorectal cancer in people with a family history of the disease, a new study suggests.

Previous studies suggested Aspirin might protect against colorectal and other cancers. Friday's study in the journal Lancet is the best designed so far to test the hypothesis. 

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Cancer prevention experts aren't ready to say Aspirin is useful for the general public. (M. Spencer Green/Associated Press)

The international researchers followed 861 people with a known genetic predisposition to colorectal cancer for up to 10 years.

The participants had Lynch syndrome, a condition that accounts for about three per cent to five per cent of colon cancer cases. About one in 1,000 people have the syndrome.

In the randomized control trial, people were assigned to take 600 milligrams of Aspirin daily — about two regular strength Aspirin — or dummy pills of starch.

The protective effect began to be seen five years after patients started taking the Aspirin.

"If you gave two Aspirins a day for two years to people with hereditary bowel cancer then after five years their cancer risk would be reduced by more than half," said John Burn, a professor of clinical genetics at Newcastle University in England.

That important finding will change how doctors treat those with a genetic predisposition, said Dr. Des Leddin ,a gastroenterologist with Capital Health in Halifax.

"I think this is pretty convincing evidence that we'll be treating them with Aspirin," Leddin said. "But on an individual basis you always have to weigh up the risks and benefits."

Risks of daily Aspirin use

The risks of taking Aspirin daily include stomach bleeding or even stroke, which is why Leddin advised that people talk to their doctors.

Leddin and other experts cautioned the evidence doesn't show everyone should take Aspirin to prevent colorectal cancer. Research to test that idea that is continuing.

"We're not ready to say Aspirin is useful for the general public," said Asad Umar, a cancer prevention expert at the U.S. National Cancer Institute who was not linked to the study. "There are still a lot of toxicity concerns."

The study was paid for by groups including the European Union, Cancer Research U.K., Bayer Corporation, the original maker of Aspirin, and others.

Burn, the study's chief investigator, reported receiving a speaker's fee from Bayer last year. 

With files from CBC's Pauline Dakin and The Associated Press