Mannitol bowel cleansing effect time varies from person to person, usually about one to two hours. Mannitol can be taken orally to play the effect of diarrhea and bowel cleansing, mostly used for constipation, colonoscopy preparation, etc. The onset of action time after taking mannitol is not fixed, generally between one hour to two hours to take effect, some patients may be slightly faster or slower to take effect. After the onset of action, the patient will have several bowel movements, and during the bowel movement, the patient should drink more water to replenish the body’s moisture. Dizziness, severe abdominal pain, intestinal perforation and other adverse reactions may occur during the use of the drug, need to pay attention to acute tubular necrosis of anuria patients, severe water loss, active intracranial hemorrhage, acute pulmonary edema, severe pulmonary stasis, allergy to mannitol and other conditions prohibited. Mannitol should be used under the guidance of a doctor, not blindly self-medication to avoid adverse effects.