Milk-of-magnesia products are sold over-the-counter. They come in chewable-tablet, capsule, and liquid forms, and are also available in different flavors. Physicians recommend taking milk of magnesia with a full glass of juice or water to help with absorption. There are potential reactions with antibiotics, and a physician should be consulted before ingestion if an individual is pregnant or nursing. 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. In years past it was advertised with the following slogan: "Take MOM in the PM, for BM (bowel movement) in the AM." 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 effects.
According to MedlinePlus Medical Encyclopedia, milk of magnesia is useful against canker sores (aphthous ulcer) when used topically.
Milk of Magnesia is also used as a folk remedy, topically applied and massaged in (a few minutes before washing/shampooing), to relieve symptoms of seborrhea and dandruff. The mechanism for its effectiveness in this application, like the causes of seborrhea itself, are at present unknown.
The liquid form has also garnered a reputation as a mattifier/primer for oily skin. Individuals apply a thin layer to the face after cleansing to absorb oil and give the face a matte, smooth look. This trick was mentioned on the Tyra Banks Show, as well as by skincare guru Paula Begoun.
As an antacid, milk of magnesia is dosed at approximately 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.
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.
As with any other medication, some people may have adverse reactions to milk of magnesia. These can include weakness, nausea, vomiting, and diarrhea. High doses increase the likelihood of these reactions. Patients with severe chronic kidney disease are advised to avoid overconsumption of milk of magnesia. Because the kidney functions to excrete magnesium, taking too much would wear out the kidney and lead to toxic levels of magnesium in the blood. Healthy individuals should not use this type of medication continuously for longer than one week, or an excessively harsh laxative effect may result.