Most Mg(OH)2 that is produced industrially, as well as the small amount that is mined, is converted to fused magnesia (MgO). Magnesia is valuable because it is both a poor electrical conductor and an excellent thermal conductor.
As an antacid, magnesium hydroxide is dosed at approximately 0.5–1.5 g 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.
Only a small amount of the magnesium from magnesium hydroxide is usually absorbed by the intestine (unless one 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 kidney failure could lead in theory to hypermagnesemia. Unabsorbed drug is excreted in feces; absorbed drug is excreted rapidly in urine.
On May 4, 1818, American inventor John Callen received a patent (No. X2952) for magnesium hydroxide. In 1829, Sir James Murray used a "condensed solution of fluid magnesia" preparation of his own design to treat the Lord Lieutenant of Ireland, the Marquis of Anglesey, of stomach pain. This was so successful (advertised in Australia and approved by the Royal College of Surgeons in 1838) that he was appointed resident physician to Anglesey and two subsequent Lords Lieutenant, and knighted. His fluid magnesia product was patented two years after his death in 1873.
Although the name may at some point have been owned by GlaxoSmithKline, USPTO registrations show "Milk of Magnesia" and "Phillips' Milk of Magnesia" have both been assigned to Bayer since 1995. 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).
Magnesium hydroxide is marketed for medical use as chewable tablets, as capsules, powder, and as liquid suspensions, sometimes flavored. These products are sold as antacids to neutralize stomach acid and relieve indigestion and heartburn. It also is a laxative to alleviate constipation. As a laxative, the osmotic force of the magnesia acts to draw fluids from the body. High doses can lead to diarrhea, and can deplete the body's supply of potassium, sometimes leading to muscle cramps.
The heat absorbed by the reaction retards the fire by delaying ignition of the associated substance. The water released dilutes combustible gases. Common uses of magnesium hydroxide as a flame retardant include additives to cable insulation (i.e. cables for high quality cars, submarines, the Airbus A380, Bugatti Veyron and the PlayStation 4, PlayStation 2, etc.), insulation plastics, roofing (e.g. London Olympic Stadium), and various flame retardant coatings. Other mineral mixtures that are used in similar fire retardant applications are natural mixtures of huntite and hydromagnesite.
Brucite may also crystallise in cement and concrete in contact with seawater. Indeed, the Mg2+ cation is the second most abundant cation in seawater, just behind Na+ and before Ca2+. Because brucite is a swelling mineral, it causes a local volumetric expansion responsible for tensile stress in concrete. This leads to the formation of cracks and fissures in concrete, accelerating its degradation in seawater.