Arnica montana has been used medicinally for centuries. Arnica is used in liniment and ointment preparations used forstrains, sprains, and bruises.[medical citation needed] Commercial Arnica preparations are frequently used by professional athletes. According to The Memorial Sloan-Kettering Cancer Center, “A few clinical trials suggest benefits of topical arnica forosteoarthritis; and for affecting significant reduction of bruising compared to placebo or low concentration vitamin K ointments. However, a small study reported that topical arnica actually increased pain 24 hours after calf exercises.” 
Arnica montana 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. Homeopathic preparations of Arnica 24X dilution or more contain no active ingredient are therefore ineffective and non-toxic.
Homeopathic 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 homeopathy would be inherently flawed.