Arsanilic acid, also known as aminophenyl arsenic acid or aminophenyl arsonic acid, is an organoarsenic compound, an amino derivative of phenylarsonic acid whose amine group is in the 4-position. A crystalline powder introduced medically in the late 19th century as Atoxyl, its sodium salt was used by injection in the early 20th century as the first organic arsenical drug, but was soon found prohibitively toxic. Eventually it became, and still is, used as a veterinary feed additive to promote growth and prevent or treat dysentery in poultry and swine.