This essay examines three medieval pharmaceutical treatises purportedly authored by Yūhannā ibn Māsawayh (anglicized to John Mesue) and traces their immense influence on the development of pharmacy in early modem Europe and the Hispanic world. Despite the importance of these works throughout the early modern period, Mesue is relatively unknown in the history of pharmacy and medicine, and his exact identity remains unclear. This essay argues that "Mesue" was most likely a pseudonym used by an unknown author of the Latin West and that the three works were crafted to meet the demands of the developing "medical marketplace" of late thirteenth-century Europe, where the manuscripts first appeared. At the same time, however, as the Arabic reference of the pseudonym suggests, these treatises were clearly products of the medieval Islamic world, including many innovations that would provide the basis for the theory and practice of pharmacy for centuries and arguably formed part of the artisanal epistemological influence on the Scientific Revolution.