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 and it also has interesting omega-7 fatty acids. As of 2016, 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.

Buckthorn Research
Antifungal activity of Rubia tinctorum, Rhamnus frangula and Caloplaca cerina.
Fitoterapia. 2005. Faculty of Science, University of Kragujevac, 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.

Interactions with dietary supplements
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.

Sea Buckthorn Use for dry eye
J Nutr. 2010. Oral sea buckthorn oil attenuates tear film osmolarity and symptoms in individuals with dry eye.  Department of Biochemistry and Food Chemistry, University of Turku, Turku, Finland.AbstractDry eye is a common condition that can severely impair the quality of life. We aimed to find out whether oral sea buckthorn (SB) oil, containing (n-3) and (n-6) fatty acids and antioxidants, affects dry eye. In this double-blind, randomized, parallel trial, 20- to 75-y-old women and men experiencing dry eye symptoms consumed 2 g of SB or placebo oil daily for 3 mo from fall to winter. One hundred participants were recruited and 86 completed the study. Clinical dry eye tests and symptom follow-ups were performed. Tear film hyperosmolarity is a focal factor in dry eye. There was a general increase in the osmolarity from baseline to the end of the intervention. Compared with the placebo group, the increase was significantly less in the SB group when all participants were included [intention to treat (ITT)] and when only participants consuming the study products for at least 80% of the intervention days were included [per protocol (PP). The maximum intensities of redness and burning tended to be lower in the SB group. In the ITT participants, the group difference was significant for redness but not for burning. In the PP participants, the group difference was significant for burning but not for redness. In conclusion, SB oil attenuated the increase in tear film osmolarity during the cold season and positively affected the dry eye symptoms.

Common buckthorn (Rhamnus cathartica L.) is one of the most abundant and ecologically harmful non-native plants in forests of the Upper Midwest United States.