This chapter examines the dispensation and treatment of simple and compound medicines in the Kingdom of Majorca from 1400 to 1550, focusing on the role of guaiacum in the local reception of Galenic pharmacology. It argues that guaiacum was widely regarded as an effective remedy for syphilis, a new disease requiring a new drug, and was endorsed by leading physicians. Crucially, its therapeutic use did not challenge Galenic medicine but was instead fully integrated into humoral principles. This study relies extensively on the analysis of records from the Arxiu del Regne de Mallorca, one of the best-preserved notary archives in Europe. These documents provide unprecedented insights into the operations of Majorcan apothecaries and the reception of new materia medica from the Americas. They also offer valuable evidence on the circulation of scientific knowledge among health professionals in the Western Mediterranean and the Iberian Peninsula, shedding new light on how early modern pharmacology adapted to emerging diseases while maintaining continuity with established medical traditions.