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Milk of Magnesia

Milk of magnesia, also known as magnesium hydroxide, is a simple magnesium salt. It dissolves in water into positively charged magnesium ions and negatively charged hydroxide ions. A hydroxide ion is made of one oxygen atom and one hydrogen atom. Milk of magnesia acts as an antacid to combat heartburn, acid indigestion, and upset stomach. Milk of magnesia acts as an osmotic laxative to return regularity in constipated patients.

Milk of Magnesia As an Antacid
The fluids in the stomach are naturally acidic to help digest food and kill pathogens. Sometimes, the stomach acid becomes too acidic or it spreads beyond the stomach. In both cases, the acid causes damage to tissues. Antacids reduce this damage by making stomach acid less acidic. Acidity depends on the concentration of dissolved hydrogen ions. Liquids with a higher concentration of dissolved hydrogen ions are more acidic, while liquids with a lower concentration of dissolved hydrogen ions are less acidic. The hydroxide ion in milk of magnesia works to bring down the concentration of hydrogen ions in stomach acid. The dissolved hydrogen ion has a negative charge which attracts the positively charged hydrogen ion. These two ions combine to make a molecule of water. When milk of magnesia reduces the acidity of stomach acid, it is a classic example of a neutralization reaction.

Milk of Magnesia As an Osmotic Laxative
The magnesium ions and hydroxide ions in milk of magnesia are solutes that dissolve in water. When they dissolve in the digestive system, they raise the concentration of solutes. Water molecules always travel from an area with a higher concentration of solutes to an area with a lower concentration of solutes. This movement is called osmosis when it happens across a semipermeable membrane such as the lining of the intestine. Osmotic laxatives such as milk of magnesia bring water from the surrounding tissues through the lining of the intestine and into the stool. This added water makes the stool softer. Softer stool is easier to evacuate from the bowels.

How Milk of Magnesia Works


The hydroxide ions neutralize hydrogen ions in stomach acid.

Osmotic Laxative

Magnesium ions and hydroxide ions draw water into stool through osmosis.

H2Go is a hyperosmotic laxative that also contains a unique form of  magnesium oxide. Like other magnesium products, H2Go brings water from nearby tissues into the stool. Its unique chemical structure allows it to bind relatively large amounts of water to the waste inside the colon even though it has relatively few magnesium ions. As a hyperosmotic, the effective intake of H2Go is a small fraction of the level of other magnesium products.

Milk of magnesia is a simple ionic compound. It is a combination of negatively charged hydroxide ions and positively charged magnesium ions. The negatively charged hydroxide ions neutralize stomach acid by binding to hydrogen ions in the acid. Both the magnesium ions and the hydroxide ions from milk of magnesia act as osmotic laxatives in the large intestine.


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2. Prescription for Nutritional Healing, 3rd Edition by Phyllis Balch and James Balch

3. Aluminum Hydroxide and Magnesium Hydroxide. MedlinePlus

4. Neutralization Reaction - Acids + Base. Virtual Chembook
Elmhurst College