Arnica is a perennial plant in the Asteraceae family; it is native to Europe and North America. There are multiple species of arnica, but the most commonly used in commercially sold products is Arnica montana, which typically grows at high altitudes (up to 3,000 m altitude).^[2][3]

Arnica is mainly applied topically as a gel or a cream, but it can also be found in oral homeopathic preparations, albeit in extremely diluted forms.^[2][2]

While arnica has a long history of use in traditional medicine and homeopathy for diverse conditions (e.g., pain, contusion, wounds, bruising, rheumatism, arthritis in general, and inflammation), the scientific evidence supporting its efficacy remains limited.^[4][2]

In regard to post-operative pain, one meta-analysis assessing homeopathic arnica’s effect on pain, bleeding, swelling, and discomfort after minor surgeries (e.g., carpal tunnel surgery, oral-facial, or dental surgery) found no significant effect compared to placebo or other interventions.^[5]

Arnica is also used topically to treat skin bruising. While the evidence may not align with the marketing claims, one RCT demonstrated that a topical application of arnica was more effective than petrolatum ointment or a mixture of 1% vitamin K and 0.3% retinol in reducing laser-induced skin bruising.^[6] However, larger studies are essential to validate and establish the consistency of this effect.

Regarding arnica’s effectiveness in relieving osteoarthritis symptoms, positive outcomes have been observed, particularly for hand and knee osteoarthritis.^[1][7] For more detailed information, please refer to the question “Is arnica effective in relieving symptoms of osteoarthritis?”.

The oral use of arnica is currently limited to homeopathic preparations, due to a certain degree of toxicity being observed in past studies when arnica was taken orally in higher dosages.^[3]

Notably, one randomized controlled trial (RCT) reported side effects such as dry mouth, headaches, drowsiness, and lethargy with the use of homeopathic arnica tablets,^[8] though causality could not be definitively established.

Although infrequent, topical applications of arnica may lead to skin irritation or contact dermatitis, emphasizing the importance of conducting a patch test before applying it to larger areas.^[2][1] Moreover, arnica should not be applied to broken skin.

Finally, due to a lack of data, arnica is not recommended during pregnancy or breastfeeding.^[9]

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.^[3][7]

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.^[2]