Laxatives are required 10 hours before colonoscopy for bowel cleansing preparation. How soon most people start to have diarrhea after taking 2000 ml of laxatives is mainly related to the type of laxatives and the individual patient’s condition, and cannot be generalized. Commonly used clinical laxatives include compound polyethylene glycol, sodium phosphate, lactulose oral solution, magnesium sulfate, electrolytes or senna leaf infusion. The time to start diarrhea varies among patients because of individual differences and the type of laxatives taken. Most people start diarrhea 2-3 hours after taking 2000 ml of laxatives. Some patients have better gastrointestinal motility and usually start diarrhea about 30 minutes after taking laxatives. Due to the large amount of water drunk, some patients may experience mild nausea and vomiting, which are normal and need not be overly worried. If severe dizziness and weakness symptoms occur, attention needs to be paid to whether electrolyte disorders are present.