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A colonoscopy is a visual examination of the colon or sections thereof. This is achieved by carefully inserting and slowly advancing a very flexible, tube-like instrument (endoscope) into the intestine.  At the same time, air is introduced to inflate the intestine. The endoscope is fitted with a cold light source and an optic lens. Thus, the images produced can be shown on a monitor and studied in detail by the doctor. Any changes in the intestinal mucosa can be clearly identified and can be removed directly during the examination.    

Many people are afraid of a colonoscopy.  A colonoscopy must not necessarily be painful, however, the preparation for this procedure can be unpleasant. Two days prior to a colonoscopy, the food intake should be changed to a light diet and one day before, a laxative must be taken in order to empty and clean the intestinal tract. Only then does the doctor have a clear view of the intestinal mucosa which is under investigation. The colon has several  anatomic sites like bends and curves and pushing foreward the instrument therefore can be unpleasant without anaesthesia. A general anaesthesia is not necessary however, there are highly effective sedatives available which induce a sleep-like condition.     

Usually, a colonoscopy enables the doctor to find all polyps. Sometimes, very small polyps hide in places which are not clearly visible, however, this rarely occurs. 

The doctor must in any case examine the entire colon. In the past, clinicians thought most polyps would develop in the rectum, hence it would be sufficient to examine that part only but this has been shown to be false.  As additional reassurance, the patient can request a photo of the appendix  (located where the small intestine meets the colon) as proof that the doctor advanced the endoscope to the small intestine. The doctor, however, can also use a tissue sample as proof that the entire colon was examined.        
 
Polyps discovered through a colonoscopy are always removed. These are then examined by the  pathologist to determine whether they are harmless or already cancerous. 

If a colonoscopy is performed as part of an early detection program or suspicion of intestinal cancer, and a tumour is found, it is not, like polyps, removed immediately. Instead, tissue samples are taken to confirm the diagnosis. 

Because the intestine is inflated with air during a colonoscopy, it is not uncommon to experience slight flatulence  after the procedure. The best remedy for this side effect is to go to the toilet and pass wind.  Other side effects e.g. bleeding or penetration of the intestinal wall are very rare. 

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