Cantharidin

Cantharidin is an odorless, colorless fatty substance of the terpenoid class, which is secreted by many species of blister beetles. Its main current use in pharmacology is treating molluscum contagiosum and warts topically. It is a burn agent, poisonous in large doses. It has been historically used as an aphrodisiac (in preparations sold under the name "Spanish fly"). In its natural form, cantharidin is secreted by the male blister beetle, and given to the female as a copulatory gift during mating. Afterwards, the female beetle covers her eggs with it as a defense against predators.

Poisoning from cantharidin is a significant veterinary concern, especially in horses, but cantharidin can also be poisonous to humans if taken internally (where the source is usually 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 also been used therapeutically, for instance, for treatment of skin conditions, such as molluscum contagiosum infection of the skin.

Cantharidin is classified as an extremely hazardous substance in the United States, and is subject to strict reporting requirements by facilities that produce, store, or use it in significant quantities.