But a new Canadian study suggests the procedure may detect polyps and early cancers only 60 to 70 percent of the time , reports CBS News medical correspondent Dr. Jon LaPook.
According to the study, whether a colonoscopy saves a life may depend on exactly where in the colon the cancer starts
"It seemed to prevent two-thirds of the deaths from left sided colon cancer," said Dr. Nancy Baxter of the University of Toronto. "But it didn't seem to prevent any of the deaths from right-sided colon cancer."
One possible explanation is that polyps on the left side of the colon are more likely to be raised and easy to detect, while those on the right side tend to be flatter and easy to miss.
The results are being debated by doctors. Only about 30 percent of the colonoscopies in this Canadian study were performed by gastroenterologists.
"We don't know the quality of the colonoscopies, the skill of the examiners, the preparation of the patients, the completeness of the colonoscopies," warned Dr. Sidney Winawer of the Memorial Sloan-Kettering Cancer Center.
Other studies have found doctors who rush through the procedure can miss polyps. It's also important for patients to follow the instructions for colon cleansing to the letter.
And even if colonoscopies are less accurate than previously thought, they're still the best way to prevent colon cancer.
"Colonoscopy can reduce mortality from colon cancer," Winawer said.
Colon cancer screenings are recommended for most people starting at age 50. Patients should choose experienced doctors and make sure that they follow their doctors' directions to the letter when preparing for the procedure.