It is recommended to take laxatives 4~6 hours before the examination to have a colonoscopy at 2pm. Do colonoscopy need to take laxatives 4~6 hours in advance to achieve the purpose of intestinal cleansing, clinically commonly used laxatives are generally compounded polyethylene glycol electrolyte dispersion, can be in the intestinal cleansing at the same time will not cause electrolyte disorders, after taking the drug to promote the discharge of intestinal contents until the pull out of the fecal debris-free, water until the bowel to achieve the standard of readiness. It is also important to note that before the bowel examination, it is necessary to fast for 6-8 hours, 4 hours of water, the day before the examination is recommended to eat a non-dregs, low-fiber food, which will help the doctor to be able to directly and clearly observe the digestive tract. Patients who need to have a colonoscopy should do so under the guidance of a doctor.