Milk of Magnesia

Magnesium hydroxide,(MgOH2) commonly called milk of magnesia is both an antacid and a laxative. As an antacid milk of magnesia works to reduce stomach acid and as a laxative it works to speed up the rate of excretion via the bowels. The name milk of magnesia comes from the cloudy or milky appearance of the magnesium hydroxide in water. The name dates back to Charles Henry Phillips, who first sold the product as Phillips milk of magnesia in 1880. However, the production and use of milk of magnesia dates back to 1829, where Sir James Murray treated the Lord Lieutenant of Ireland with a fluid magnesia preparation of his own creation. The fluid magnesia preparation gained him much recognition and was patented two years after his death in 1873.


Milk of magnesia is prepared by the reaction of a hydroxide (sodium, potassium or ammonium) with magnesium salts. The magnesium displaces the sodium, potassium or ammonium from the hydroxide and the magnesium from the salt is attached to the hydroxide thus forming magnesium hydroxide.

Milk of magnesia as a laxative

In high concentrations (greater than 2g) milk of magnesia is used as a laxative. It is an osmotic laxative because its mechanism of action or the way in which it produces its laxative effect is by a process called osmosis. When the milk of magnesia is present in the intestinal lumen it dissociates into Mg2? and OH?. Since the Mg2? is poorly absorbed into the body via the intestines, it remains in the lumen producing an osmotic effect. This osmotic effect pulls water from the tissues of the intestines by osmosis and disallows the absorption of water already in the lumen thus increasing the volume of water in the lumen.  In addition to increased water volume, the water softens the feces thus increasing its volume in the lumen. The combination of increased water and feces volume stimulates an increase in the intestinal motility thus increasing bowel movements more frequently, causing a laxative effect.

Milk of magnesia as an antacid

Milk of magnesia as an antacid, works via a neutralization reaction in the stomach, it reduces the acid concentration in the stomach. When the magnesium hydroxide -Mg(OH)? reaches the stomach it encounters a high concentration of acid in the form of hydrochloric acid- HCl,  Mg(OH)?  then reacts with the HCl to form water and magnesium thus neutralizing the stomach acid.
The neutralization reaction taking place in the stomach is;
Mg(OH)?  + 2 HCl ? MgCl? + 2 H?O


Milk of magnesia should not be used as an everyday treatment method, nor is it to be abused as the adverse effects can be damaging.
Magnesium is excreted by the kidneys therefore persons with kidney damage should avoid this product as it could lead to toxicity due to the build up of magnesium in the system.

When taking milk of magnesia as a laxative it is important to take potassium salt supplement because high amounts of potassium are lost in the induced diarrhea. Potassium is important in preventing muscle spasms or contractions.

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