A bowel cleansing is needed before a colonoscopy, and laxatives for bowel cleansing usually include cotrimoxazole, mannitol, and magnesium sulfate. 1. Copolyethylene glycol electrolytes: Copolyethylene glycol electrolytes are used as a bowel cleansing agent before colonoscopy. Compound Polyethylene Glycol Electrolyte Dispersant is mixed with a large amount of water and allowed to be drunk by the patient, as this medicine will not be absorbed by the intestines and will not enter the circulatory system, which will not have a great effect on the human body. 2. Mannitol: enteroscopy before, will often use mannitol, through its high osmotic effect, can make the water to the intestinal lumen to move, so that there is a large amount of water in the intestinal tract, can stimulate defecation to increase intestinal peristalsis, to play its role as a diarrhea. 3. magnesium sulfate: magnesium sulfate is not easy to be absorbed by the intestinal tract after oral intake, magnesium ions stay in the intestinal tract, through elevating the osmotic pressure in the intestines, so that the intestinal tract to absorb a large amount of water, increase the intestinal luminal volume expansion of the intestinal wall, increase intestinal peristalsis to stimulate defecation and conduct diarrhea. Patients can choose the appropriate laxative for bowel cleansing according to their own situation before doing colonoscopy.