The physician Giovan Battista Codronchi (1547–1628) is a key figure of sixteenth-century medicine. A study of his main work De morbis veneficis ac veneficiis (1595) and his letters sent to the Congregation of the Index in Rome (1597) can teach us much about the interrelation between medicine and religion in Counter-Reformation Italy. Using Codronchi as a prism, this article uncovers a complex picture in which themes such as the production of demonological texts at the height of the European witch-hunt, the related debate about the roles of physicians and exorcists, and the influence of physicians on the development of the Index of Forbidden Books are interrelated.