Topical gel products of Arnica are applied to the skin for pain and swelling associated with bruises, aches, sprains, and arthritis. It is also applied to the skin for insect bites, muscle and cartilage pain, chapped lips, and acne. It is also taken by mouth for sore mouth and throat, insect bites, painful and swollen veins near the surface of the skin (superficial phlebitis), sore gums after removal of wisdom teeth, and for causing abortions (1).
In foods, Arnica is a flavor ingredient in beverages, frozen dairy desserts, candy, baked goods, gelatins, and puddings. In manufacturing, Arnica is used in hair tonics and anti-dandruff preparations. The oil is used in perfumes and cosmetics (1).
Digestion problems: Arnica can irritate the digestive system. Don’t take it if you have irritable bowel syndrome (IBS), ulcers, Crohn's disease, or other stomach or intestinal conditions.
Fast heart rate: Arnica might increase your heart rate. Don’t take arnica if you have a fast heart rate.
High blood pressure: Arnica might increase blood pressure. Don’t take arnica if you have high blood pressure.
Surgery: Arnica might cause extra bleeding during and after surgery. Stop using it at least 2 weeks before a scheduled surgery.
2. HerbsGuide.org, http://herbsguide.org/arnica.html