Flexible sigmoidoscopy enables the physician to look at the inside of the large intestine from the rectum to the left side of the colon, called the sigmoid.

[1] With flexible sigmoidoscopy, the physician can see intestinal bleeding, inflammation, abnormal growths, and ulcers in the descending colon and rectum.

However, although in absolute terms only a relatively small section of the large intestine can be examined using sigmoidoscopy, the sites which can be observed represent areas which are most frequently affected by diseases such as colorectal cancer, for example the rectum.

The scope transmits an image of the inside of the rectum and colon, so the physician can carefully examine the lining of these organs.

If anything unusual is in the rectum or colon, like a polyp or inflamed tissue, the physician can remove a piece of it using instruments inserted into the scope.

The colon and rectum must be completely empty for flexible sigmoidoscopy to be thorough and safe, thus the patient must drink only clear liquids for 12 to 24 hours beforehand.

A 2010 British study[3] showed sigmoidoscopy reduced "overall colorectal cancer incidence and mortality by 31 percent", and "incidence of cancer in just the lower part of the colon (or distal colon) was reduced by approximately 50 percent for those who underwent screening compared with those in the control group.

Although generally considered quite safe, sigmoidoscopy does carry the very rare possibility of tearing of the intestinal wall by the instrument, which could require immediate surgery to repair the tear; in addition, removal of a polyp may sometimes lead to localized bleeding which is resistant to cauterization by the instrument and must be stopped by surgical intervention.