Divisions and Precisions

This is quite possibly the strangest thing I have ever asked in my journal...

...but does anyone know of any herbs or plants that cause impotence in males, or at least have traditionally been thought to cause it? I know of a bunch that are supposed to help cure it, but not cause it. My web research has turned up naught, mostly because as soon as you include 'impotence' in a search you get all sorts of drek. Did you know that "over-masturbation" causes male pattern baldness? Neither did I!

Anyway, any herbalists out there have any ideas? Or you folklore people? Suffice to say, someone's a wee bit pissed at one of the male characters in my novel... ;)