Bowel preparation for colonoscopy refers to the laxatives taken before the procedure to clean the colon of fecal debris. A colonoscope is a long, flexible tube with a television camera on the tip. The camera can’t see through fecal debris. So any fecal debris left in the colon could obscure identification of a polyp or even a small cancer.
Several studies have shown that fewer small and large polyps are detected in patients with less-than-optimal bowel preparation. And poor preparation has several potential consequences during the procedure itself.
First, your colonoscopy may last longer because the doctor will need to take time to clear out debris.
Second, your doctor may lack confidence that the colon lining was seen adequately and may ask you to return for a subsequent screening earlier than would be otherwise recommended – say 1 year, rather than 5 or 10 years. This will subject you to increased costs and risk.
Finally, if the preparation is very poor, the doctor may have to stop the procedure entirely, and you will need to reschedule.

First, ask if the doctor recommends what is called split-dosing. Split-dosing refers to taking half the laxative prescription the night before colonoscopy, and the other half on the day of the procedure, usually about four to five hours before the procedure is scheduled. Several studies have shown that split-dosing significantly improves the quality of the preparation for colonoscopy.
In the past, it was common for doctors to have patients take all the prescription the night before colonoscopy. This effectively cleaned out the fecal debris, but left the potential for a different problem that can develop when the time between the end of the preparation process and the start of the colonoscopy is prolonged. Thick mucus and intestinal secretions empty out of the small intestine during that interval and stick to the first parts of the colon.