The stuff what they give you to clear you out works osmotically: usually "Klean-prep" or "Movi-prep". Never been so "Klean" - or empty - in my life - yes I had both ends inspected all in the one day too!
Ingredients: Macrogol 3350, Sodium Sulphate, Sodium Bicarbonate, Sodium Chloride and Potassium Chloride, Aspartame and vanilla flavour.