Poison
Sometimes, poisons have an antidote. The antidote of a poison will slow or reverse its effects. The antidote may itself be a poison. As an example, atropine can be used as an antidote against certain nerve gases, like tabun or sarin, or against certain insecticides. It is also used as a medication.  In high doses, atropine is a poison. Yet, atropine is a core medicine in the World Health Organization's "essential drugs list".