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Milk of Magnesia or Magnesium Hydroxide, Mg(OH)₂ is a saline osmotic (hydrating) laxative. The name derives from the suspension's milky white appearance and the magnesium in its composition.

Contents 1 History 2 Pharmaceutical uses 3 Biological metabolism 4 References

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 by Charles Henry Phillips in 1880 and sold under the brand name Phillips' Milk of Magnesia for medicinal usage. Although the name may at some point have been owned by GlaxoSmithKline, USPTO registrations show "Milk of Magnesia" to be registered to Bayer [1], and "Phillips Milk of Magnesia" is registered to Sterling Drug [2]. In the UK, the non-brand (generic) name of "Milk of Magnesia" and "Phillips Milk of Magnesia" is "Cream of Magnesia" (Magnesium Hydroxide Mixture, BP).

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. It is also used as an antacid, though more modern formulations combine the antimotility effects of equal concentrations of aluminum hydroxide to avoid unwanted laxative usage.

According to MedlinePlus Medical Encyclopedia, milk of magnesia is useful against canker sores (aphthous ulcer). [3]

When the patient uptakes the milk of magnesia (orally), the suspension enters the stomach. Depending on how much was taken during the uptake, one of two possible outcomes will happen.

As an antacid, milk of magnesia is dosed at approximate 500mg to 1.5g in adults and works by simple neutralization, where the hydroxide ions from the Mg(OH)₂ combine with acidic H⁺ ions produced in the form of hydrochloric acid by parietal cells in the stomach.

As a laxative, milk of magnesia is dosed at 2-5g, and works in a number of ways. First, Mg²⁺ is poorly absorbed from the intestinal tract, so it draws water from the surrounding tissue by osmosis. Not only does this increase in water content soften the feces, it also increases the volume of feces in the intestine (intraluminal volume) which naturally stimulates intestinal motility. Furthermore, Mg²⁺ ions somehow cause the release of CCK, which causes intraluminal accumulation of water, electrolytes and intestinal motility. Although it has been stated in some sources, the hydroxide ions themselves do not play a significant role in the laxative effects of milk of magnesia, as basic solutions (i.e. solutions of hydroxide ions) are not strongly laxative, and non-basic Mg²⁺ solutions, like MgSO₄, are equally strong laxatives mole for mole (Tedesco & Di Piro, 1985; Curry, 1983).

Care should be taken with the use of milk of magnesia for either of these purposes, as it can easily cause diarrhea. [4]

As already stated only a small amount of the magnesium from milk of magnesia is usually absorbed from a person's intestine (unless the person is deficient in magnesium). However, magnesium is mainly excreted by the kidneys so longterm, daily consumption of milk of magnesia by someone suffering from renal failure could lead in theory to hypermagnesemia.

• 940 498 0909: Laxative products. In: Handbook of non-prescription drugs. 1983, 6th ed., pp. 69-92, American Pharmaceutical Association, Washington DC.
• Tedesco FJ, DiPiro JT. Laxative use in constipation. American College of Gastroenterology's Committee on FDA-Related Matters. Am J Gastroenterol. 1985 80(4):303-9.