This thesis examines pharmacological therapies in the medical writings of three 1 st AD authors Aretaeus of Cappadocia, Scribonius Largus and Dioscorides of Anazarbus. It argues that many pharmacological therapies widely considered irrational by modem scholarship are perfectly rational when considered in context of medical thought at that time. The standard pharmacology of these authors, held up as a model of rationality by many historians of medicine, is used here to explain the irrational pharmacological therapies. I illustrate how irrational pharmacological therapies found in the medical writings of Aretaeus of Cappadocia, Scribonius Largus and Dioscorides of Anazarbus are logically consistent with their standard pharmacological thought.