After a colonoscopy two years ago, Patti Damare felt so delirious and weak that she couldn’t stand on her own. That was on a Friday, and she chalked up her symptoms to lingering effects of anesthesia.

If you’re between the ages of 45 and 85, you should have a colorectal cancer screening routine in place, per the American Cancer Society (ACS). But a colonoscopy—in which your doctor uses a special ...