Laxatives are usually required 4-6 hours in advance if the colonoscopy is done the next morning. If the patient has persistent constipation, laxatives should be taken the night before the colonoscopy. It is important to empty the bowel before the colonoscopy so that the test can be performed better. For normal people, it takes 4-6 hours for the bowel to be completely emptied after taking laxatives. If the patient has constipation, the time for bowel emptying will be relatively longer, and may consider taking laxatives the night before the colonoscopy in order to better empty the bowel for the examination. Because of individual differences, it is advisable to follow your doctor’s instructions on when to take the laxative.