This circumboreal and montane (sub-alpine) genus occurs mostly in the temperate regions of western North America, while two are native to Eurasia (A. angustifolia and A. montana).Several species, such as Arnica montana and Arnica chamissonis, contain helenalin, a sesquiterpene lactone that is a major ingredient in anti-inflammatory preparations (used mostly for bruises).
Arnica montana has been used medicinally for centuries, however there are no scientific studies that prove the medical effectiveness. The roots contain derivatives of thymol,which are used as fungicides and preservatives. Arnica is currently used in liniment and ointment preparations used for strains, sprains, and bruises. Commercial arnica preparations are frequently used by professional athletes.The thymol derivatives concentrated in the plants roots have been clinically shown to be effective vasodilators of subcutaneous blood capillaries. A study of wound-healing after surgery to treat varicose veins found a trend towards a beneficial effect of reduction of pain and hematoma following surgery.
Arnica contains the toxin helenalin, which can be poisonous if large amounts of the plant are eaten, and contact with the plant can also cause skin irritation.If enough of the material is ingested, the toxin helenalin produces severe gastroenteritis, and internal bleeding of the digestive tract. Homoeopathic preparations of Arnica 24X dilution or more are not toxic as negligible amounts of Arnica remains.
Homoeopathic preparations of Arnica are widely marketed and used. In the UK, the Medicines and Healthcare products Regulatory Agency has registered the product for sprains and bruising under the National Rules for Homoeopathic Products (2006). These rules allow claims of efficacy for these conditions to be made on the packaging in the absence of similar evidence to that required for conventional medicines under the Medicines Act 1968 and 1971.A systematic review of clinical trials showed that homeopathic arnica was no more effective than a placebo.In some quarters, the fact that homeopathic Arnica has been the subject of published clinical trials at all has drawn criticism grounded on the allegation that the basic premise of the high dilutions used in homoeopathy would be inherently flawed.With respect to the range of homoeopathic Arnica creams available on the market, these are generally formulated using the mother tincture rather than a dilution and therefore contain measurable quantities of the medicinally active substance.