One of the essential mineral elements of the body, found mostly in bone. Deficiency leads to muscular weakness and interferes with the efficient working of the heart. Some magnesium salts are used as drugs, for example magnesium trisilicate and magnesium oxide – generally known as ‘milk of magnesia’, a mild antacid – and magnesium sulphate (‘Epsom salts’), which acts as a laxative (see LAXATIVES).