Cantharidin is stored in the beetle's haemolymph and released by reflex bleeding from the leg joints. On skin it causes blistering within hours, which is why preparations of the beetle were once used to raise counter-irritant blisters in Western medicine and, in purified form, are still used to treat warts and molluscum contagiosum under supervision.
Swallowed, it is a different matter. Cantharidin irritates the gastrointestinal and urinary tracts, and the genital sensation that gave the beetle its reputation is inflammation. Lethal doses in humans are small, historically estimated at the order of tens of milligrams, and poisoning damages the kidneys. Fatal cases are documented.
The beetle's own life history is stranger than the folklore. Larvae hatch as active triungulins that climb flowers, attach to visiting solitary bees, and are carried into the nest, where they eat the bee's egg and provisions before completing development as grub-like larvae.