http://www.webmd.com/vitamins-and-supplements/arnica
"Some lab experiments suggest that arnica might kill bacteria and ease swelling. But the effects in people aren’t clear.
Studies of topical arnica gels and ointments for bruising and arthritis symptoms -- like pain and swelling -- have had unclear results. So far, research suggests that arnica skin treatments don’t ease muscle pain.
If eaten, the actual herb is toxic and can be fatal. However, some oral supplements contain highly diluted arnica. These are considered homeopathic treatments. These low-dose arnica tablets are safe to use and have been studied for blood clots, diarrhea, diabetic eye damage, and swelling and pain after surgery. More research needs to be done to establish effectiveness for those problems. A study of children with cancer, however, found that homeopathic low-dose arnica may help reduce mouth ulcers related to chemotherapy.
Because of the risks of pure arnica, the FDA classifies it as an unsafe herb. Doctors who practice complementary medicine generally advise against using arnica in any form other than in a highly diluted homeopathic form"