Inter-ethnic botanical borrowing is usually deemed to be based on pragmatic ef ciency. However, in the regional system we discovered between several indigenous groups from the Peruvian rainforest, the transfer of ethnomedicinal knowledge relies much more on relational factors than on any kind of strictly therapeutic efficacy. This is clearly substantiated by a detailed comparison between objective ethnobotanical measurements and indigenous self- assessments recorded by anthropologists. Such alternative motivations for ethnobotanical borrowing are probably not so exceptional. They raise some questions about the representation of plant efficiency from an indigenous point of view, and probably in some Western contexts too. They also entail direct implications for development and cooperation policies.