Rats, mice and chickens
Hound's-tongue is very toxic to grazing animals, and perhaps their owners. Wiki says "toxic to cows and specially dangerous to pasture owners". Still trying to parse that one. Being toxic, it was of course used for medicine. I'm not sure I've ever heard a better argument for legalizing marijuana. The rats and mice treatment for madness from a 1725 herbal begins with shaving the head of the victim, and setting a live hen on it. Things go rapidly downhill from there. If they're going to put stuff up my nose, not this! Bring on the weed!
This image is a previous effort.