Polyethylene glycol (PEG) is an osmotic laxative. It acts by absorbing more moisture into the stool, increasing the fecal volume, and inducing a laxative effect. The typical onset of action for Miralax is 1―2 days for constipation and 1―3 days for fecal impaction. It is contraindicated in severe inflammatory intestinal conditions and intestinal obstruction.

Reference

Twycross R, Wilcock A, Dean M. Canadian Palliative Care Formulary, 1st Canadian ed. Nottingham, UK: Palliativedrugs.com; 2010.