How fast does Miralax work?

When used according to label directions, Miralax produces a bowel movement within 12 to 72 hours, according to Drugs.com. Propylene glycol helps to increase the amount of water in the intestines, stimulating bowel movements and softening the stool.

Doctors sometimes prescribe Miralax with other laxatives in preparation for medical procedures, such as a colonoscopy. When used as a bowel prep, the patient takes a much larger dose of the medication and begins to experience its effects within one to four hours. According to Gastroenterology Practice Associates, the purpose of the medication is to create large amounts of watery diarrhea and cleanse the colon for the procedure.