Arnica

Free to every reader. The safety section of every monograph is open to everyone, signed in or not. In an emergency call 000. For poisoning, call the Poisons Information Centre on 13 11 26.

15. Safety

Red flags first. Arnica taken by mouth is a poisoning, and in pregnancy it has been followed by miscarriage: one woman in the second month of pregnancy miscarried a few days after drinking an infusion of 20 g of flowers, another within 24 hours of three tablespoons of home-made tincture, and multi-organ failure followed ingestion taken to end a pregnancy (case reports, as summarised by Bone and Mills, pp. 377–378) [8]. Anyone who has swallowed an arnica preparation (other than a homoeopathic dilution) needs emergency care; in Australia the Poisons Information Centre is on 13 11 26.

Contact allergy (the main adverse effect). The HMPC puts the frequency of allergic skin reactions at about 1 in 100 users, from 1280 patients in marketing-authorisation safety data, with no serious reactions [4]. Reactions are itching, redness and eczema, occasionally vesicles [2,4]. In patch testing of eczema patients, 4.3 per cent (150 of 3480) reacted to arnica tincture, many of them probably cross-reactions (Willuhn, as summarised in ref 13). In Danish Compositae-allergic patients, 23 per cent reacted to arnica (Paulsen, as summarised by Bone and Mills, p. 485) [8]. Across the trials, allergic reactions were the usual adverse event: 5 treatment-related events in the arnica arm of the hand-osteoarthritis trial [4], 2 allergic withdrawals out of 100 in the venous trial [4], 1 of 79 in the knee study [4]. Helenalin esters (α-methylene-γ-lactone plus cyclopentenone) are the main haptens; dihydrohelenalin esters also sensitise, and thymol esters may contribute [13]. In eight arnica-allergic patients tested with isolated lactones, two reacted to a dihydrohelenalin ester and none to a helenalin ester, as summarised by Schmidt [13]; AIP comment: the DH chemotype cannot be assumed to be less allergenic. Case reports include occupational eczema in growers and pharmacy workers, facial eczema after touching the plant, and a woman with enlarging necrotic lesions of the face and leg, fever and malaise after a 1.5 per cent arnica cream, diagnosed as Sweet’s syndrome; the authors attributed the lesions to pathergy to arnica, and Bone and Mills record that the syndrome was leukaemia-related [4,8]. Reported rates vary widely: a 1992 review found arnica allergy in 11–75 per cent of patients at dermatology clinics, while later studies found up to 1.14 per cent (Bone and Mills, p. 378) [8]. Causality assessment (WHO-UMC), AIP comment: the trial reactions are probable (temporal link, dechallenge by withdrawal, known mechanism); the Sweet’s syndrome case is possible (a plausible trigger, but an underlying leukaemia explains the syndrome as well).

Irritation. Undiluted tincture on skin, prolonged use, or use on damaged skin and leg ulcers can cause oedematous dermatitis, blistering and superficial necrosis [5]. In a reconstructed human epidermis model and in rat dermal toxicity tests, an Ph. Eur. tincture (0.05 per cent lactones) was neither corrosive nor irritant [18].

Toxicity by mouth. Arnica is poisonous if swallowed: gastroenteritis, disturbed heart rate, palpitations, breathlessness, muscle paralysis and death are recorded [5,6]. WHO cites a fatal poisoning after 70 g of tincture [5]; Barnes records serious, non-fatal symptoms after 30 mL of a 20 per cent tincture [6]. A 19-year-old man who drank tea made from arnica leaves and flowers by mistake developed myalgia, rigors, fever, tachycardia, hypotension and raised creatinine and liver enzymes, and recovered after six days of treatment (as summarised by Bone and Mills, p. 378) [8]. A mouth rinse used undiluted caused severe mucosal injury [5]. Oral LD₅₀ of helenalin: mice 150, rats 125, rabbits 90, hamsters 85 mg/kg [5]. The HMPC gives an oral LD₅₀ of an unspecified extract above 5 g/kg in rats and 123 mg/kg in mice [4].

Genotoxicity, reproduction. An unspecified extract was mutagenic in the Ames test (TA98, TA100), attributed to flavonols; helenalin itself was not mutagenic up to 30 µg/mL [4,5]. Helenalin damaged chromosomes in Chinese hamster ovary cells in vitro; the relevance in vivo is not established (Bone and Mills, p. 377) [8]. Adequate tests of reproductive toxicity, genotoxicity and carcinogenicity have not been done; the HMPC gave the inadequate genotoxicity data as its reason for not supporting an EU list entry [2,4]. Helenalin at 6–20 mg/kg intraperitoneally was not teratogenic in mice [5]; oral extracts stimulated the uterus in rats and guinea pigs [5].

Contraindications. Known allergy to arnica or other Asteraceae [2,5,9]. Broken or abraded skin, open wounds, leg ulcers [2,5,9]. Eyes and mouth [9]. Oral use of herbal preparations [5,10].

Interactions.

No pharmacokinetic interaction study exists. The HMPC lists no interactions [2]; Bone and Mills list none known (p. 377) [8]. Stockley’s Drug Interactions (2008, library copy) has no arnica entry (text search, September 2026). AIP comment.

Pregnancy and lactation. No safety data for topical use; the HMPC does not recommend use in pregnancy [2]; WHO contraindicates it in pregnant and nursing women, citing emmenagogue use and uterine stimulation in animals [5]; Barnes advises avoidance in pregnancy and breastfeeding [6]; MediHerb lists "external use only" for both [9]. Bone and Mills class oral use in pregnancy as high risk (their Category X), expect no harm from topical use, and advise against internal use while breastfeeding and against applying it near the nipple (p. 377) [8]. Oral use in pregnancy has been followed by miscarriage (Red flags, above) [8]. Grade: regulatory and reference-text advice, with case reports for oral use; no study of topical use in pregnancy or lactation.

Children and older people. Not recommended under 12 years for lack of data [2]; keep out of reach of children [5]. One ankle-sprain trial enrolled children from 6 years, without separate safety results [34]. No specific data in older people; trials enrolled adults up to 79 years [4].

Overdose. No topical overdose is reported [2]. Oral ingestion is a poisoning (above).

TGA. Checked at Medicine review, September 2026, and reported as found. The TGA website search and safety-alert listing for "arnica" returned a bot-check page or timed out, so no alert list was retrieved. The Database of Adverse Event Notifications redirected to a Microsoft Power Apps portal behind a terms-of-use page, which AIP’s tools cannot query, so it was not searched. Nothing in this monograph depends on either search, and nothing here should be read as "the TGA holds no report": AIP does not know that. A manual DAEN and safety-alert search is listed in section 18. Arnica is not in the Poisons Standard [11], and listed medicines for use other than on unbroken skin are capped at 1 mg dry flower equivalent a day [10].

Referral line. Anyone with a skin reaction, a wound, a pregnancy, a child to treat or an anticoagulant prescription should consult a degree-qualified herbalist with competencies and adequate training in pharmacognosy before using arnica. Swallowing any arnica preparation other than a homoeopathic dilution is a poisoning and needs emergency care (Poisons Information Centre, 13 11 26).