In the Western tradition of learned medicine, it is Galen who looms largest as model medical thinker and authority. His works were in use into the seventeenth century, so what he had to say about poisons was crucial. This chapter shows that Galen had extensive experience of poisons and antidotes and wrote about them both at length. It also shows that among ruling families and elsewhere in Roman imperial times, poisons and suspected poisons were widely used against one’s enemies. Antidotes were used to accustom the body in advance to poisonous substances, especially the famous mithridatium (mithridate), in order to evade the attempts of poisoners. Dosage was all important. Epidemics, the much-feared appearances of a deadly disease across a whole community, were particularly difficult to account for and to treat in a medical system like that of Galen, based as it was on the individual person and his or her particular humoral balance. King explores how Galen effectively adopted the theory of how poisons work in order to explain also how the whole humoral body works, beginning ‘to think about poison as a model for disease and, indeed, for healing’.