The term "Milk of Magnesia" was first used to describe a white aqueous, mildly alkaline suspension of magnesium hydroxide formulated at about 8%w/v. Milk of magnesia is primarily used to alleviate 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. Magnesium supplements have also been shown to reduce platelet aggregation by inhibiting in the influx of calcium, a crucial component of platelet aggregation.
Furosemide (Lasix) may promote excretion of magnesium. It increases excretion of water by interfering with the chloride-binding cotransport system, which in turn inhibits sodium and chloride reabsorption in the ascending loop of Henle and distal renal tubule.
For these reasons, oral replacement should be given in the asymptomatic patient, preferably with a sustained-release preparation, given the ability of magnesium to induce diarrhea. Bioavailability of oral preparations is assumed to be 33% in the absence of intestinal malabsorption. Several preparations are available: Mag-Ox 400, containing magnesium oxide; Slow-Mag, containing magnesium chloride; and Mag-Tab, containing magnesium lactate. These preparations provide 5-7 mEq (2.5-3.5 mmol or 60-84 mg) of magnesium per tablet. Six to 8 tablets should be taken daily in divided doses for severe magnesium depletion. Two to 4 tablets may be sufficient for mild, asymptomatic disease. Mag-Ox 400 contains 242 mg (20 mEq) of elemental magnesium, but absorption is less efficacious.