http://en.wikipedia.org/wiki/Milk_of_magnesia
"Available over-the-counter, milk of magnesia is primarily used in alleviating constipation, but can also be used to relieve indigestion and heartburn. When taken internally by mouth as a laxative, the osmotic force of the magnesia suspension acts to draw fluids from the body and to retain those already within the lumen of the intestine, serving to distend the bowel, thus stimulating nerves within the colon wall, inducing peristalsis and resulting in evacuation of colonic contents."