Magnesium hydroxide, Mg(OH)2, otherwise known as milk of magnesia, is commonly used as an antacid or a laxative. The mineral form of magnesium hydroxide is known as brucite. Magnesium hydroxide interferes with the absorption of folic acid and iron. The diarrhea caused by magnesium hydroxide carries away much of the body's supply of potassium, and failure to take extra potassium will lead to muscle cramps.