No studies in animals or humans have been performed to prove the safety or effectiveness of arnica montana for cancer treatment as of 2015, according to the American Cancer Society. Only one 1994 study found that certain chemicals in arnica killed lung and colon cancer cells in lab trays.
Most claims about arnica's effectiveness for reported uses are not supported by scientific evidence, as stated by the American Cancer Society. Cancer patients may experience serious health consequences by delaying standard cancer treatment and instead relying on treatments such as arnica. The herb can be poisonous if consumed orally, and a number of severe allergic reactions have been reported as well as at least one death. The herb may also irritate the stomach if taken internally, potentially causing symptoms such as vomiting, internal bleeding and diarrhea. Arnica may cause excessive bleeding in patients who are taking blood-thinning medications, and it may reduce the effectiveness of medications used for high blood pressure.
A number of studies have been conducted to measure arnica's effectiveness in treating pain and bruising in patients after a carpal tunnel surgery, laser surgery and knee surgery, as confirmed by the American Cancer Society. In all cases, the herb performed no better than the placebo treatments.