The extract contains cantharidin, a substance that supposedly increases sexual desire. Not only does it not do that, it may eliminate all desire permanently. When ingested, cantharidin can kill. But when used in small doses topically, it can be an effective treatment for warts.
Some 1500 species of cantharidin-producing beetles can be found around the world. They do not bite or sting but can cause a great deal of physiological mischief if they are accidentally, or in some cases, purposefully ingested. Cantharidin is synthesized by the male beetle and is given as a nuptial gift to the female during copulation for the purpose of protecting her eggs. If a predator bites into a blister beetle egg, it learns to never do it again. Cantharidin causes terribly painful blisters. But that is not all it does. In humans, it can cause a burning sensation of the urinary tract that in men can provoke an erection. This effect is responsible for cantharidin’s undeserved historical reputation as an aphrodisiac that dates back all the way to ancient Roman times. Augustus Caesar’s wife Livia supposedly slipped ground beetles into food to entice men into indiscretions for which they could be blackmailed. There are also accounts of Louis XIV’s lust for women being aided by cantharidin, but the most famous proponent of “Spanish Fly” as a sexual stimulant was the notorious Marquis de Sade who is said to have given prostitutes cantharidin-laced pastilles to “set them on fire.” It set them on fire alright, with burning abdominal pain. The Marquis and his valet were accused of poisoning the women and quickly fled to Italy. They were sentenced to death in absentia.
Cantharidin also resulted in a death sentence for an American fisherman, but in a different fashion. In 1954, for some bizarre reason, the man came to believe that blister beetle extract would attract fish. He shook some ground-up dried beetles with water in a bottle, using his thumb as a stopper. Immediately after, in the process of handling his hooks, the fisherman pricked his thumb, and as most would do, proceeded to suck on it. There was enough residual cantharidin on the thumb to kill him. It doesn’t take much. The fatal dose of cantharidin is in the range of 10-65 milligrams. According to some accounts, blister beetle extract was one of the components in Giulia Tofana’s infamous “widow-maker” poison in the 17th century, although most evidence points to the poison actually being a mixture of arsenic, lead and belladonna. Whether Spanish Fly was a component of “Aqua Tofana” is contentious but there certainly was a belief that cantharidin was being used as a poison. Indeed, attempts were made to try to identify it in the bodies of victims who were thought to have been poisoned. A piece of an internal organ from the deceased would be extracted with oil and the solution placed on the shaved skin of a rabbit to see if blistering would be produced.
Pure cantharidin was first isolated by the French chemist Pierre Robiquet in 1810 who identified it as the substance being responsible for the blistering properties of the beetle eggs. The exact molecular structure was not proposed until the mid-twentieth century and was shown to be correct when the American chemist Gilbert Stork synthesized cantharidin from simple molecules. This was only of academic interest because there is no need to synthesize cantharidin commercially. Whatever amounts are needed for dermatological use or for animal husbandry can be extracted from beetles, which are not hard to find. Why in animal husbandry? In some cases, mating needs to be encouraged. As in humans, cantharidin can produce an erection in males prompting the animal to try to resolve the situation through mating.
Because of the myth of the aphrodisiac effect, various varieties of “Spanish Fly” are available for purchase on the web. It is unlikely they actually contain any cantharidin, which of course is a good thing given the danger of the actual compound. Mostly they are herbal concoctions based on cayenne pepper extract that will produce a mild burning sensation, mostly in the wallet. There are also some “Spanish Fly” candies available for purchase. You’ll get some very ordinary candies wrapped in paper adorned with the name "Spanish Fly." In Germany, homeopathic remedies based on cantharidin are available for the treatment of urinary tract infections. They cannot cause any harm because homeopathic remedies are diluted to an extent where either none or a trivial amount of the original substance remains. Needless to say, such products cannot produce a physiological reaction and rely totally on the placebo effect. A 0.7% solution of cantharidin when applied to a wart, however, can produce a physiological effect. The compound can cause skin cells to disengage from each other resulting in the breakdown of a wart. Application must be done by a physician because the amount applied and the time the solution is left on the skin is critical.