That's the lesson of a new study suggesting that colonoscopies reduce the risk of dying from colorectal cancer not by up to 90%, as believed, but by perhaps 60% to 70%. Further, they're much better at finding problems in one side of the colon than the other. "Colonoscopy seems to prevent two-thirds of deaths on the left side," says Dr. Nancy Baxter, a colorectal surgeon and researcher at St. Michael's Hospital in Ontario, Canada, and lead author of the study. "But there's not much effect at all on the right."
The process is a bit like maneuvering a very narrow, floppy garden hose through a long tunnel, says Dr. Jacques Van Dam, professor of medicine and director of endoscopy at the Stanford University Medical Center.
Physiology also complicates the right-side picture. Experts suggest several explanations for the left- versus right-side difference. For one thing, it's harder to examine the right side of the colon. "The stool is more liquid on the right side," Baxter says. "So it coats the lining and makes polyps harder to see."