Pharmacopoeias are, like any kind of cultural production, living and dynamic objects that constantly change and reinvent themselves through continuous contacts and exchanges between different social groups. Medicinal plants are “biocultural objects” (Pordié, 2002) because of the richness and the complexity of relationships that societies develop with them. In French Guiana, especially in its cross-border areas, cultural interactions are common. Plants and associated knowledge circulate among communities, contributing to a permanent renewal and a continuous hybridization of herbal medicines. These material and immaterial flows form the cultural reality of cross-border areas on the Guiana Shield. This paper focuses on the border between French Guiana and Brazil, which is made tangible by the Oyapock River. This border is a relatively recent political construction as it was only established in 1900 after a Swiss arbitration ended a long diplomatic conflict between France and Brazil. Even if a process of “rebordering” (Moullé, 2017) has been observed lately – despite the opening of the Oyapock bridge – the two sides of the River are part of the same dynamic cultural space since both share a relative remoteness from their respective centre. As well, constant social, economic and cultural circulations take place between the French town of Saint-Georges-de-l’Oyapock and the Brazilian town of Oiapoque. In this context, ethnobotanical circulations – namely, exchanges of medicinal plants and associated knowledge – are a fair illustration of the shared cultural dimension between people. Based on interviews with 23 inhabitants of the French town of Saint-Georges-de-l’Oyapock, the paper offers an exploratory description of ethnobotanical flows in the context of the cross-border area. It lists the medicinal plants used in Saint-Georges and analyses the diffusion of associated knowledge between the different communities. Results show that 92% of the 139 identified plants are used by several communities, and 40% of them are used among four to five different social groups. While most of the used plants are exotic species, their use is quite transversal to the socio-cultural components of Saint-Georges population. Nevertheless, the analysis shows that most of the knowledge transmission about medicinal plants happens inside the same community and via inter-generational means. The use of the same species and medicinal knowledge by different groups is therefore the result of a long-term process. Currently, it seems that early-established communities are more likely to share their knowledge with other groups than newly arrived groups, who integrate local uses to their own pharmacopoeias but seldom share their knowledge. Even though inter-community exchanges are discreet, they still contribute to vivid dynamics of local pharmacopoeias, illustrated by current ethnobotanical “interculturalisation” (Tareau, 2019), relocation and juxtapositions of phytotherapeutical practices, as well as new practices based on combinations between treatments from different origins. The cross-border area is therefore a place of cultural hybridization, in a context of intensive circulations of people and goods between two socio-political systems (Map 1). Practices of medical pluralism explain some of the to-and-fro moves of people on the border: while Brazilians come to Saint-Georges’ dispensary for health treatment, French Amerindians and Creoles visit Terras Indigenas villages in Brazil for consulting pajés . The pajés may also come to Saint-Georges to visit their family and treat sick persons. At the regional scale, circulations for access to care contribute to the delineation of a large health circulation area on the border. The latter also acts as an attraction factor for flows of processed medicinal plants, which find in Oiapoque an interface for entering the Guianese health market (see Map 1). Several factors explain this regional export capacity of Brazilian herbal goods: the presence of Brazilian diaspora and the diffusion of its pharmacopoeia in French Guiana along with the marketing strength of Brazilian herbal pharmacies and the presence of herbalists in Oaipoque, although they are not allowed in France. All these contribute to the development of flows of dried medicinal plants and processed herbal items from inner and Southern Brazil to French Guiana, and therefore to the relentless evolution of local pharmacopoeias.