Arnica contains several different compounds, but the main bioactive constituents are sesquiterpene lactones (STLs) of the helenanolide type (e.g., helenalin, 11-alpha, 13-dihydrohelenanin, and their esters). These compounds mainly work by inhibiting the activation of transcription of nuclear factor kappa B (NF-kB), a key mediator in the inflammatory process.[1][2]
Other mechanisms of action are still not fully understood. In vitro and animal studies suggest that arnica might also have anti-microbial, antioxidant, and immunomodulatory activity. However, the current body of evidence from human (clinical) studies is insufficient.[3]