The active ingredient in milk of magnesia is magnesium hydroxide. The liquid form also contains purified water as an inactive ingredient.
Milk of magnesia is often used as a laxative for the treatment of constipation. It is available over the counter as well as by prescription. The most commonly used forms of milk of magnesia are liquid or chewable tablets. Milk of magnesia works by bringing water into the intestines to aid in the movement in the intestines according to WebMD.
Additional uses of milk of magnesia include heartburn, indigestion and upset stomach caused by the excessive production of stomach acids. Milk of magnesia works by reducing the amount of acid in the stomach.
Milk of magnesia can take anywhere from 30 minutes to 6 hours to start working. It is intended only for temporary use. Extended or excessive use can create a dependence on laxatives, loss of too much body water, ongoing diarrhea and mineral imbalances.