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Derived from the flowers and rhizome. The raw herb and the homeopathic arnica have been used in the treatment of bruises. In vitro studies showed that Arnica montana has antimicrobial activities. Conclusions from a systematic review point to insufficient evidence to support use of arnica. But clinical trials conducted since then suggest benefits of arnica for osteoarthritis  and reduction in postoperative swelling  and pain. However, one small study reported that arnica increased pain 24 hours after calf exercises. Another study found that topical arnica ointment significantly reduced bruising compared to placebo or low concentration vitamin K ointments.
Side effects following oral administration of the herb (but not the homeopathic preparation) include gastrointestinal distress, hypotension, tachycardia, shortness of breath, coma, and death. Topical administration is benign, but can cause contact dermatitis, irritation, and burning.