Cantharidin

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Cantharidin is a poisonous chemical compound found in the Spanish fly. It's IUPAC name is benzofuran-1,3-dione.

It is one of the world’s most famous aphrodisiacs. In Roman times Emperor Nero’s wife would slip it into food, hoping to inspire her guests to some indiscretion with which she could later blackmail them. In 1772 the notorious Marquis de Sade gave aniseed sweets laced with cantharidin to some prostitutes, hoping to make them lusty, but instead it poisoned them.

It is considered an aphrodisiac because male blister beetles give it to the females as a mating gift, but actually the female uses this poison to protect her eggs from predators. If cantharidin is ingested by human males, as it is excreted by the kidneys, it irritates the urinary tract causing spectacular swelling, called priapism, but this is quite painful and does not result in arousal.

Dilute it can be used to remove warts and tattoos. However, when ingested 10mg is usually a fatal dose.