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 In 2013, its approval by US government as an animal drug was voluntarily withdrawn by its sponsors.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 antimicrobial chemotherapy approach to treating infectious diseases of humans. Chemistry Synthesis was first reported in 1863 by Antoine Béchamp and became the basis of the Bechamp reaction. The process involves the reaction of aniline and arsenic acid via an electrophilic aromatic substitution reaction. C6H5NH2 + H3AsO4 → H2O3AsC6H4NH2 + H2O Arsanilic acid occurs as a zwitterion, H3N+C6H4AsO3H−, yet is typically represented with the non-zwitterionic formula H2NC6H4AsO3H2. History Roots and synthesis Since at least 2000 BC, arsenic and inorganic arsenical compounds were both medicine and poison. In the 19th century, inorganic arsenicals became the preeminent medicines, for instance Fowler's solution, against diverse diseases. In 1859, in France, while developing aniline dyes, Antoine Béchamp synthesized a chemical that he identified, if incorrectly, as arsenic acid anilide. Also biologist, physician, and pharmacist, Béchamp reported it 40 to 50 times less toxic as a drug than arsenic acid, and named it Atoxyl, the first organic arsenical drug. Medical influence In 1905, in Britain, H W Thomas and A Breinl reported successful treatment of trypanosomiasis in animals by Atoxyl, and recommended high doses, given continuously, for human trypanosomiasis (sleeping sickness). By 1907, more successful and less toxic than inorganic arsenicals, Atoxyl was expected to greatly aid expansion of British colonization of Africa and stem loss of cattle in Africa and India. (So socioeconomically valuable was colonial medicine that in 1922, German company Bayer offered to reveal the formula of Bayer 205—developed in 1917 and showing success on sleeping sickness in British and Belgian Africa—to the British government for return of German colonies lost via World War I.) Soon, however, Robert Koch found through an Atoxyl trial in German