Milk of magnesia is a liquid used in medicinal applications as an antacid and a hydrating laxative. Also known as magnesium hydroxide or Mg(OH)2, the solution is taken orally. It is so named because it looks milky white and contains the naturally occurring mineral magnesium. The substance acts to work within six hours of a dose in adults and children to temporarily relieve occasional bouts of constipation. The original concentrated formula was concocted by a man named Charles Henry Phillips in 1880, and sold under the brand Phillips' Milk of Magnesia. Today, the rights to the name "milk of magnesia" appear to be owned by Bayer Corporation.

Milk of magnesia is an alkaline suspension, meaning that it undergoes a neutralizing reaction when encountering anything acidic. This makes it an effective combatant of excess stomach acid when taken internally. Too much hydrochloric acid (HCl) excreted by the parietal cells in the stomach can lead to indigestion, heartburn, and stomach ulcers. Milk of magnesia in the form of an antacid is dosed from 500 mg-1.5 mg (0.02-0.05 oz) and readily enters the stomach, where the hydroxide ions in milk of magnesia combine with the hydrogen ions in HCl to calm overactivity in the stomach.

As a laxative, milk of magnesia enters the stomach to stimulate intestinal motility, helping flush out the intestines. A dose measures 2-5 g (0.07-0.18 oz). While the hydroxide ions play less of a role in this process, the magnesium helps through the following steps:

1. Magnesium ions draw water from the surrounding body tissues into the intestinal tract by osmosis.
2. The higher quantity of water in the intestinal tract softens and increases the volume of feces, stimulating nerves in the intestines.
3. Magnesium ions also play a role in releasing the peptide hormone cholecystokinin, causing accumulation of water and electrolytes in the intestine and triggering intestinal motility.

As with any other medication, milk of magnesia does have side effects, which include weakness, nausea, vomiting, and diarrhea. 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 magnesium toxicity of the blood. Products are not recommended for chronic use, since it could produce too harsh of a laxative effect. Usage should last no longer than one week.

Milk of magnesia products are sold over the counter. They come in chewable tablet, capsule, and liquid form, and also are 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.