Clinically there is no such thing as the best time to drink mannitol before a colonoscopy, patients need to take mannitol 4-8 hours before the examination, and also need to pay attention to other related matters. Colonoscopy is mainly used to observe the internal structure of the colon and rectum. Usually, patients need to take a light laxative 1 night before the examination, and on the morning of the examination, take mannitol by mouth and drink plenty of water. The time to drink mannitol is usually 4-8 hours before the operation and should be taken orally within 30 minutes. At the same time, patients also need to start regulating the diet 3 days before the examination, try to consume semi-fluid food without slag or less slag, if the patient is suspected of having intestinal polyps and prepared for electrocautery, can not be used mannitol for intestinal preparation, and need to be changed to laxatives such as compound polyethylene glycol electrolyte dispersions. It is recommended that patients choose the time of taking mannitol reasonably under the guidance of the doctor, and actively cooperate with the doctor to carry out the preparation before colonoscopy and colonoscopy examination.