This article examines the role of testing and innovation in sixteenthcentury Italian pharmacy. I argue that apothecaries were less concerned with testing drugs for efficacy or creating novel products than with reactivating an older Mediterranean pharmacological tradition and studying the materials on which it relied. Their practice was not driven by radical experimentation but by a "culture of tweaking"—of minute operational changes to existing recipes and accommodation of their textual variants—which was rooted in the guild economy fostering incremental over radical innovation and in a humanist reevaluation of past autorities. Workshop practice was also increasingly driven by a new ideal of staying true to nature fostered by the period's botanical renaissance. This led to an emphasis on ingredients over processes in the shop, and found clearest expression in the elaboration of a taxonomic "language of truth" that helped apothecaries discern between authentic and inauthentic materia medica and harness their sincerity in lieu of testing effectiveness.