A new study out of the University of Oslo is raising questions about just how effective colonoscopies are at preventing colorectal cancers.

"Beyond the U.S., it's hard to find anywhere else where the majority of screening is performed through colonoscopy," said Dr. Xavier Llor, professor at Yale School of Medicine.

Results from Norway, Sweden and Poland showed only an 18% reduction in colon cancer risk for those who were invited to be screened. Doctors at Yale Medicine say that data doesn't show the whole picture.

"The main analysis is giving us data of the entire cohort, and not considering the fact that the majority of patients who were invited did not actually have the colonoscopy," said Llor.

Fifty-eight percent of those who got the postcard never got screened. Llor says for those who got a colonoscopy, the data shows a different picture.

"It was a 50% reduction in colorectal cancer-related deaths, a 30% reduction in colorectal cancer," Llor said.

U.S. health authorities are still recommending colonoscopies every 10 years once patients pass a certain age.

"Right now, we're saying 45 for an average risk individual. It'll be sooner for someone with a family history," he said.

Most Western countries depend on less-invasive stool tests more than colonoscopies. Llor says the at-home tests might be a good option for those with no family history who find colonoscopy too invasive or expensive.

The Centers for Disease Control and Prevention says nearly 70% of Americans 50 to 75 are up to date with their colonoscopies.