The active principle of cantharides, or Spanish fly (the dried bodies of a blister beetle, Cantharis vesicatoria). A toxic and irritant chemical, cantharidin causes blistering of the skin and was formerly used in veterinary medicine as a counterirritant and vesicant. If swallowed it causes nausea, vomiting, and inflammation of the urinary tract, the latter giving rise to its reputation as an aphrodisiac. It is very dangerous and may cause death.