Milk of magnesia being poured. Magnesium hydroxide is an inorganic compound with the chemical formula Mg(OH)2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite. Magnesium hydroxide is a common component of antacids and laxatives; it interferes with the absorption of folic acid and iron.