Cantharidin

Cantharidin, a type of terpenoid, is a poisonous chemical compound secreted by many species of blister beetle, and most notably by the Spanish fly, Lytta vesicatoria. The false blister beetles and cardinal beetles also have cantharidin.

History
Cantharidin was first isolated by Pierre Robiquet in 1810. It is an odorless and colorless solid at room temperature. It is secreted by the male blister beetle and given to the female during the mating. Afterwards the female beetle will cover its eggs with it as a defense against predators. The complete mechanism of the biosynthesis is currently unknown. If cantharidin is ingested, it irritates the urinary tract as it is excreted, causing swelling of the genitalia. This can cause a harmful condition known as priapism in men, where an erection lasts more than about four hours.

Medical uses
Diluted, it can be used to remove warts, tattoos and to treat the small papules of Molluscum contagiosum.

Medical risks
Its potential for adverse effects have led it to being included in a list of "problem drugs" used by dermatologists.

When ingested by humans, the LD50 is around 0.5 mg/kg, with a dose of as little as 10 mg being potentially fatal. This makes the use of cantharadin as an aphrodisiac highly dangerous and it is illegal to sell it for this purpose in many countries.