Arnica is toxic if taken internally except when diluted into homeopathic preparations. Homeopathic treatment is usually individualized to correspond specifically to the patient's symptoms. Typical homeopathic dosing uses either 5C or 30C potency tablets sublingually (under the tongue) three times a day. Doses can be taken for 24 hours or up to six months, although a qualified healthcare practitioner, including a pharmacist, should be consulted before making decisions about dosing.
Other forms of arnica dosing include tinctures taken by mouth, or ointments and fresh plant gel applied on the skin. There is not enough scientific evidence to give specific doses or times for these forms.
There is no proven safe or effective dose of arnica in children.
Avoid in individuals with a known allergy or hypersensitivity to arnica or any member of the Asteraceae or Compositae families. Possible cross-sensitivity can occur in those allergic to the Asteraceae or Compositae family (Achillea millefolium, Ambrosia species, Anthemis cotula asters, calendula, chamomile, chrysanthemum, dahlia, daisy, dandelion, dog fennel chicory, Matricaria chamomilla, mugwort, marigold, May weed, sunflower, tansy, and yarrow).
Arnica is likely safe when used short-term in oral or sublingual (under tongue) homeopathic doses. It is possibly safe when applied topically/externally to unbroken skin for short-term use. Arnica is likely unsafe when taken by mouth in doses higher than homeopathic dilutions. It may also be unsafe when used topically (on the skin) long-term. Using full strength tinctures on hypersensitive or broken skin is also not recommended.
Ingestion of arnica extracts has been known to increase heartbeat and increase bleeding time.
Allergic reactions may occur when taking arnica in full strength preparations or when handling the plant. Reactions including Sweet's syndrome, facial eczema, oral lesions (mouth wounds), itchy erythema (reddening of the skin) of the legs, trunk (torso), and face, and dermatitis.
Taking Arnica montana-containing extracts by mouth has caused severe gastroenteritis (inflammation of the stomach), including gastrointestinal problems due to mucosal irritation nervousness, nausea, and vomiting.
Arnica may also cause muscle weakness, collapse, and death. High doses may impair urine flow and damage the kidneys and liver. There is also the potential for organ damage, coma, and death with the internal use of arnica.
Internal use of arnica is not recommended in pregnant women due to the potential for uterine stimulation and toxicity. Avoid if breastfeeding.