Cantharidin is a toxic secretion of several species of blister beetle, most notably the "Spanish Fly", after which the chemical is often named.
Although cantharidin has medical uses (such as the removal of warts), it is frequently abused for its purported properties as an aphrodesiac. However, a dose of as little as 10mg can be fatal. It damages the digestive tract and the urinary tract (the later causes a burning sensation which is often mistaken for something similar to sexual response). It can also damage the Kidneys, cause blood in the urine and abdominal pain. More rarely, and heightening it's undeserved reputation, it can cause a lengthy (but painful) erection.
In The Fix, the patient is poisoned by cantharidin by her boyfriend because of her infidelities.