As Andreas-Holger Maehle (1999) pointed out, discrepancies between the taste of drugs, such as Peruvian bark and opium, and their effects, were used to criticize Galenic medicine in the first half of the 17th century. In this paper I will argue that, ironically, it was the study of medicinal drug properties within the tradition of Galenic medicine itself which brought to light the discrepancy inherent in Galenic drug theory between medical theory and practice, and between knowledge derived from reason and from experience. According to this theory, drug properties were supposed to be investigated and established through both taste and therapeutic experience. The relationship between these two ways of getting to know a drug, that is through reason and through experience, was examined in a number of texts written by authors connected to the University of Leiden. I will show that these texts by Rembert Dodonaeus (1517–1585), Johannes Heurnius (1543 -1601), Adrianus Spigelius (1578–1625) and Gilbert Jacchaeus (ca.1585-1628) were part of an effort to incorporate drug properties known through therapeutic experience into an elaborate system of different primary, secondary, tertiary and quaternary qualities or faculties. In his innovative textbook Institutiones medicinae or “The foundations of medicine” (1592) Heurnius made Galenic drug theory the cornerstone of his efforts to develop a methodus medendi. By the study of this rational method of treating patients, already discussed by Galen, and by developing it further, Renaissance physicians hoped to overcome the division between medical theory and practice that existed in medieval universities and thus to restore the connection between the two as envisioned by Galen. Figuring out how drug properties could be related to each other and how they worked to cure diseases became a core problem for maintaining a medical practice that was both rational and effective. However, it was precisely in trying to understand the various properties of drugs, including their taste, within the Galenic framework, that the properties of some drugs became problematic. This encourages us to consider the idea that the changes with regard to the notion of qualities and the constitution of matter occurring at this time, arose, at least partially, from discussions within Galenic medicine itself.