One of the most trustworthy sources of information for medical doctors in the United States is the Physicians Desk Reference, or PDR for short. There is now a PDR for Herbal Medicines (PDR-HM) and the following statement and excerpt comes from the third edition of this work: “The medicinal use of Arnica goes back many centuries. The essential oil used as a natural remedy in many countries all over the world comes from the flowers leaves and roots of Arnica montana and related species.”