When would you use a sigmoidoscopy over a colonoscopy?

The advantage a sigmoidoscopy has over a colonoscopy is that the bowel does not need to be prepared. In other words, whereas a colonoscopy requires the patient to clean their colon with bowel prep, a sigmoidoscopy can be performed on an unprepped patient. Therefore, if an emergency or unstable patient requires direct visualization of the distal bowel, a sigmoidoscopy would be ideal (suspected c. difficil colitis, massive bleed from the rectum, etc).