The field of medical ethnobotany has historically contributed to the advancement of modern pharmaceutical and biomedical science through bringing discoveries from the field into the laboratory. In ethnopharmacology, a sub-field of ethnobotany, there is a concerning lack of ethnographic methods reported in the literature. The ethnographic approach is essential for detailing traditional methods of preparation and administration of plant medicines, yet pharmaceutical researchers overemphasize western epistemologies of medicinal discovery and production. In the present work, we propose an ethnopharmaceutical survey as a model to investigate the culturally recognized standards necessary for the formulation of herbal medicines. Fieldwork based on participant observation and semi-structured interviews examined the modes of preparation employed by traditional healers in the Amazonian region of San Martín, Peru. The authors draw on anthropological insight into plural epistemological encounters and propose an ethnopharmaceutical approach that takes seriously the Amazonian methods and perceptions for the preparation of traditional plant medicines.