Dragonsbane is a plant poisonous to dragons, that, when potent enough, can even kill them. Even a small dose of dragonsbane requires both an antidote and a sufficient amount of rest. At the minimum, a day to three days of rest is needed for small amounts of dragonsbane.
It was what killed the King of the Dragons before Kazul and has nearly killed Kazul and the unnamed dragon from Talking to Dragons several times. It is the wizards' main defense against dragons, as it affects them immediately.