The Brazilian wandering spider appeared in the Guinness World Records 2010 as the world's most venomous spider. The neurotoxin causes loss of muscle control and asphyxiation, and the bite causes intense pain and inflammation.
However, the venom can also cause priapism, a medical condition in which the penis or clitoris does not return to its flaccid state. These erections can last for many hours and even cause impotence! Still, scientists have broken down the components of the venoms and are studying one of them as treatment for erectile dysfunction!
It seems right now they haven't been able to get the treatment to achieve the erectile function without causing loss of muscle control... which would render the treatment pointless. Still, they're hard at work trying to figure this out.