The new study compares 10,000 patients who died of colon cancer with more than 50,000 healthy people. What they found is that colonoscopy dramatically cut the death risk for cancers on the left side of the colon. But it had no effect on cancers on the right side of the colon.
As a result, researchers estimate colonoscopy may be only 60 to 70 percent effective for reducing cancer death. That's compared with other studies putting the screen at 90 percent effective.
The report is being released online in the Annals of Internal Medicine.