Depending on the mode of colon visualization, whether colonoscopy, virtual colonoscopy, barium enema, or flexible sigmoidoscopy, patients being screened for colorectal cancer are exposed to some risks from the oral cathartic preparations, including: nausea, vomiting, weakness, dizziness, severe diarrhea, acute renal failure (secondary to the dehydration and/or acute phosphate nephropathy). For colonoscopy, there are additional risks, including adverse reactions to the sedative and/or analgesia , perforation of the bowel (approximately 1 patient in 1000 procedures), and excessive bleeding from any polypectomy site (1–2 patients per 100 procedures). Risk factors for immediate post-polypectomy bleeding include age over 65 years, anticoagulant use, co-morbid cardiovascular and chronic renal disease, polyp size larger than 1 centimeter, and poor bowel preparation.