Magnesium Hydroxide - Biological Metabolism

Biological Metabolism

When the patient drinks the milk of magnesia, the suspension enters the stomach. Depending on how much was taken, one of two possible outcomes will occur.

As an antacid, milk of magnesia is dosed at approximately 0.5–1.5g 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 to produce water.

As a laxative, milk of magnesia is dosed at 2–5 g, 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 cause the release of cholecystokinin (CCK), which results in intraluminal accumulation of water, electrolytes, and increased 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.

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 long-term, daily consumption of milk of magnesia by someone suffering from renal failure could lead in theory to hypermagnesemia.