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A genus of poisonous and rhamnaceous trees and shrubs commonly called buckthorn. The bark and fruit of its many species are commonly used as purgatives. The most common species are Rhamnus cathartica (purging buckthorn), Rhamnus frangula (alder buckthorn, a cathartic), and Rhamnus purshiana, the bark of which is the source of CASCARA and EMODIN. Karwinskia humboldtiana of the family Rhamnaceae, known also as tollidora, causes buckthorn polyneuropathy, an ascending paralysis resulting from the ingestion of Karwinskia fruit.