What are the risks and potential downsides of having a colonoscopy?

Bleeding,perforation: Main risks are bleeding and possible perforation. Surgery is required to fix the latter. While there are less invasive tests e.G ct colonography and barium enema, only colonoscopy gives you the opportunity to simultaneously biopsy a lesion and also potentially cure low grade premalignant or even limited malignant lesions.

Low risk: Bleeding and perforation are indeed risks however combine risk in the hands of an experienced endoscopist is probably less than 1%, so it is safe and by far the most efficient test for polyps/cancer of the colon as it is both diagnostic and therapeutic, which none of the alternative exams are.