This essay examines three medieval pharmaceutical treatises purportedly authored by Yu h anna ibn Ma sawayh (anglicized to John Mesue) and traces their immense influence on the development of pharmacy in early modern 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 unkno