The time it takes to have a bowel movement after drinking mannitol varies from person to person, usually half an hour to an hour will start the bowel movement, and it is usually finished in about 4 hours. Mannitol is a relatively common and effective laxative medication that has the ability to help with bowel movements and lubricate the bowel, and is often used in surgical bowel preparations as well as in cases of constipation. Generally you need to drink more water after drinking mannitol, which is a hypertonic state in the intestines that eventually leads to diarrhea. Adequate excretion is indicated when the feces resembles a clear watery consistency, a process that usually takes about 4 hours. If constipation using mannitol diarrhea, should pay attention to develop good bowel habits, can not rely too much on mannitol. Usually should pay attention to eat more fruits and vegetables, supplemental water. Take the medication as prescribed by your doctor.