For decades, gastroenterologists put colonoscopies on a pedestal. If everyone would get the screening just once a decade, clinicians believed it could practically make colorectal cancer “extinct,” said Michael Bretthauer, a gastroenterologist and researcher in Norway. But new results from a clinical trial that he led throw confidence in colonoscopy’s dominance into doubt.

The trial’s primary analysis found that colonoscopy only cut colon cancer risk by roughly a fifth, far below past estimates of the test’s efficacy, and didn’t provide any significant reduction in colon cancer mortality. Gastroenterologists, including Bretthauer, reacted to the trial’s results with a mixture of shock, disappointment, and even some mild disbelief.

“This is a landmark study. It’s the first randomized trial showing outcomes of exposing people to colonoscopy screening versus no colonoscopy. And I think we were all expecting colonoscopy to do better,” said Samir Gupta, a gastroenterologist at the University of California, San Diego and the VA who didn’t work on the trial. And, he said, it raises an uncomfortable question for doctors. “Maybe colonoscopy isn’t as good as we always thought it is.”

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