Mistletoe

Mistletoe was a plant found on Earth. It was believed that the Host — a werewolf — either was allergic to the substance or was conditioned to believe it was. Proof of this appeared in the wolf's reactions to the mistletoe. Firstly, the monks wore mistletoe around their necks so that they were not attacked. Secondly, walls varnished with oil of mistletoe kept the wolf out of the library. Lastly, the wolf was pushed away when Lady Isobel threw mistletoe broth at it. (TV: Tooth and Claw)