Which compound is commonly called milk of magnesia?

The compound commonly called milk of magnesia is magnesium hydroxide.

Magnesium hydroxide has the chemical formula Mg(OH)₂. It is a sparingly soluble white solid, and its suspension in water has a milky appearance, which explains the traditional name “milk of magnesia.” The commercial name was associated with an early patented medicinal preparation in the nineteenth century.

Because it is basic, magnesium hydroxide can neutralize acids. It has long been used as an antacid to relieve heartburn and indigestion. It can also act as a laxative because unabsorbed magnesium hydroxide draws water into the intestine. Medical products vary in strength, so labels and dosing directions matter.

Milk of magnesia is not the same compound as magnesium carbonate, although both are magnesium-containing substances. It is also different from calcium hydroxide, sometimes called slaked lime. The word “milk” describes the suspension’s appearance, not a dairy ingredient.