Arnica montana is a European mountain flower with several centuries of traditional topical use, principally for bruising and swelling. It remains one of the more commonly used botanicals in products aimed at the eye area.
The active constituents
Arnica's effects are attributed largely to sesquiterpene lactones, chiefly helenalin. These have documented anti-inflammatory activity, appearing to interfere with NF-κB, a transcription factor central to inflammatory signalling.
The plant also contains flavonoids and phenolic acids contributing antioxidant activity.
The evidence, honestly
Arnica is one of those ingredients where traditional use, clinical study and homeopathic marketing have become entangled, and they should be kept separate.
There is reasonable evidence for topical arnica preparations in bruising and post-procedural swelling, though trials vary considerably in quality and in the concentrations used. Homeopathic arnica, which is diluted past the point where measurable arnica remains, is a different thing entirely and should not be conflated with it.
In a cosmetic formulation, the concentration is generally well below that used in the studies, so the contribution is supportive rather than therapeutic.
Safety
Helenalin is a recognised contact allergen, and arnica is a known sensitiser for a minority of people. Anyone with an allergy to the Asteraceae family, which includes ragweed, chamomile, marigold and chrysanthemum, may cross-react.
Arnica should not be applied to broken skin, where absorption is much greater. Oral arnica is toxic at meaningful doses.
Pregnancy
Cosmetic-concentration topical arnica is not generally flagged as a concern, but arnica has traditional uses that warrant caution and the safety data is limited. Worth confirming with your doctor.
