Cantharidin is excreted by the kidney and will, during excretion, irritate the entire urinary tract. The irritation of the urethra will increase the blood flow to this region and might result in priapism . It is likely that the priapism is the origin of the use of Spanish fly as an aphrodisiac. In 1772 the infamous Marquis de Sade doctored some aniseed sweets with Spanish fly and offered this to some prostitutes who took part in a flogging orgy. A more sensible use for catharidin is for the removal of digital warts.
Priapism is a persistent abnormal erection of the penis.
It is usually a quite painful condition, not associated with any sexual desire. The name is derived from Priapus , the Greek god of male procreative power.