Cantharidin

Cantharidin is an odorless, colorless terpene secreted by many species of blister beetles, including broadly in genus Epicauta, and in species Lytta vesicatoria (Spanish fly). False blister beetles, cardinal beetles, and soldier beetles also produce cantharidin. Poisoning from the substance is a significant veterinary concern, especially in horses from the Epicauta species, but it can also be poisonous to humans if taken internally (where the origin is most often experimental self-exposure). Externally, cantharidin is a potent vesicant (blistering agent), exposure to which can cause severe chemical burns. Properly dosed and applied, the same properties have been used for effective topical medications for some conditions, such as treating patients with Molluscum contagiosum infection of the skin.
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.