Mannitol usually takes effect in 0.5-1 hour after taking it, but the exact time varies somewhat depending on the individual’s constitution. Mannitol is a hypertonic agent, and the main purpose of taking Mannitol is to cleanse the intestines in preparation for a colonoscopy. It will have an effect about 0.5-1 hour after taking the medicine and symptoms of diarrhea will appear. However, the exact onset of effect will vary from person to person, and if the patient usually suffers from constipation, the onset of effect may be extended up to 4 hours. When drinking mannitol, it is recommended to drink more water at the same time, in order to facilitate the excretion of stool, but mannitol is relatively sweet, if the patient drinks fast, it may cause nausea, vomiting, and it is recommended that the patient does not drink too fast, too hard. If mannitol is taken in excess for a long period of time, it is likely to cause electrolyte disorders. It may also induce kidney disease, venous disease, and tissue dehydration. It is contraindicated in patients with hypersensitivity to the drug, severe water loss, acute tubular necrosis, acute pulmonary edema, severe pulmonary bruising or active intracranial hemorrhage. In addition, the day before taking mannitol, you should try to eat light and easy to digest semi-liquid food, such as rice porridge, and avoid eating hard and stimulating food, so as not to cause indigestion, poor excretion, and then not fully exposed to the operation field, which is not conducive to the doctor’s examination. It is recommended to use medication under the guidance of the doctor, not blindly self-medication, in order to avoid adverse effects.