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 it was soon found prohibitively toxic for human use. Arsanilic acid saw long use as a veterinary feed additive promoting growth and to prevent or treat dysentery in poultry and swine. In 2013, its approval by US government as an animal drug was voluntarily withdrawn by its sponsors. Still sometimes used in laboratories, arsanilic acid's legacy is principally through its influence on Paul Ehrlich in launching the chemotherapeutic approach to treating infectious diseases of humans.