View photos of the medicinal plant Rhamnus betulifolia [Frangula betulifolia] (Birchleaf buckthorn), profiled in Medicinal Plants of the Western Mountain States.

A passage from Medicinal Plants of the Western Mountain States:

Like its better–known relatives Cascara sagrada (Rhamnus purshiana) and Eurasian buckthorn (R. cathartica), Buckthorn is an intestinal stimulant, but unlike other anthraquinone–containing laxatives such as Aloe and Senna, it is less apt to cause irritation and dependency.