Antidote
Sometimes, antidotes are poisons themselves. For example, atropine, which is found in deadly nightshade, is very poisonous. However, because of the way it reacts with the body, atropine is used as an antidote against some insecticides and certain kinds of nerve gas.  Atropine is carried on many ambulances because it has other uses.  For example, it can help restart a heart that is not beating.