Medicinal preparations of buckthorn bark are made from the European buckthorn shrub, also known as black dogwood (Rhamnus frangula). Buckthorn herb is mostly used to relieve constipation. I could not find any human research with buckthorn to give me any clues on what medical conditions this herb is best used for.

A species of buckthorn grown throughout northern California ( Rhamnus pursiana ), also known as cascara sagrada -- has properties similar to the European buckthorn.

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Buckthorn Research
Antifungal activity of Rubia tinctorum, Rhamnus frangula and Caloplaca cerina.
Fitoterapia. 2005 Mar;76(2):244-6. Faculty of Science, University of Kragujevac, P.O.Box 60, 34000 Kragujevac, Serbia and Montenegro
The results of a preliminary antifungal screening of the methanol extracts and the major anthraquinone aglycones, alizarin and emodin, of Rubia tinctorum and Rhamnus frangula in comparison with the antifungal activity of the anthraquinone-containing lichen Caloplaca cerina and its main secondary metabolite parietin are reported.

Buckthorn questions
Q. Can buckthorn herb be taken the same day as serrapeptase enzyme, alpha lipoic acid, or the immune booster AHCC?
  A. I don't see any reason why buckthorn can't be combined with these supplements if used in low doses.