Arnica

Arnica (Arnica montana L.)

1. Identity: Botanical Classification, Natural Source, and Forms

Taxonomy and Nomenclature

Arnica montana L. (Asteraceae) is a perennial, herbaceous vascular plant species of commercial importance. It was given its scientific name in 1753 by Carl Linnaeus and is part of the genus Arnica, classified in the family Asteraceae. The Latin specific epithet montana refers to mountains or coming from mountains. The plant is widely known by common names including mountain arnica, leopard's bane, wolf's bane, mountain tobacco, and mountain daisy. It has two accepted subspecies: Arnica montana subsp. atlantica, native to Portugal, Spain, and France; and Arnica montana subsp. montana, the more widespread subspecies found in all parts of its range except Portugal.

The Arnica genus consists of 32 species worldwide, all belonging to the Asteraceae family. In addition to A. montana, Arnica chamissonis ssp. foliosa is used as an alternative source for the official herbal drug Arnicae flos in regulatory and pharmacopoeial contexts, particularly where A. montana supplies are limited.

Morphology and Habitat

Arnica is a perennial native to the mountainous regions of Europe and southern Russia, growing from 0.3 to 0.6 m tall. Oval-shaped, opposite leaves form a basal rosette at the soil surface. The plant has bright yellow, daisy-like flowers that, when dried, are the primary parts used medicinally. Roots and rhizomes may also be utilized. It mostly grows on alpine meadows and up to nearly 3,000 m above sea level, and in more upland regions may also be found on nutrient-poor moors and heaths. Arnica does not grow on lime soil, making it an extremely reliable bioindicator for nutrient-poor and acidic soils.

Commercial Preparations and Dosage Forms

Alcoholic (ethanolic) extracts of the flower heads — tinctures and fluid extracts — are the dominant preparation forms; oil-based extracts obtained with vegetable oils such as sunflower oil are also in use. Preparations containing arnica flowers intended for medicinal use are only for cutaneous use in semi-solid dosage forms: ointment, cream, and gel. The only acceptable dosage forms recognized in official monographs are the following topical semisolid dosage forms: creams, gels, lotions, ointments, pastes, and salves.

Arnica is used commercially in cosmetics, shampoos, hair tonics, anti-dandruff products, and bath products, and arnica oil is used in perfumery. The Homœopathic Pharmacopœia of the United States (HPUS) provides two arnica monographs for over-the-counter and prescription uses. For its use as a traditional herbal medicine, the European Directorate for the Quality of Medicines (EDQM) provides quality standards monographs for arnica flower and arnica tincture in the European Pharmacopoeia, while the European Medicines Agency (EMA) provides labeling standards monographs for prepared forms including tinctures and liquid extracts.

2. Traditional and Historical Use

European Traditions

For centuries, Arnica montana preparations have been used to treat inflammation and to promote the healing of blunt injuries, such as contusions and bruises. Extracts of A. montana flowers have a long history of use in European traditional medicine as a remedy to treat several ailments including pain, injuries, hematomas, dislocations, sprains, and bruising. A wound-healing effect was already attributed to arnica in a manual of Tabernaemontanus in 1613. It was recorded as being used for the treatment of 66 different pathological conditions, though most frequently for contusion, wounds, rheumatism, and inflammation. In early medieval texts, however, the name "Arnica" was not referred to anywhere, suggesting its formal documentation in European medicine grew progressively through the early modern period.

In traditional medicine, alcoholic preparations from flowers of Arnica montana and Arnica chamissonis ssp. foliosa were applied externally to treat hematomas, contusions, sprains, rheumatic diseases, and superficial inflammations of the skin. Flowers, roots, and rhizomes of arnica were traditionally used for the topical treatment of various ailments such as bruises, sprains, backache, rheumatic arthritis, and phlebitis. Arnica montana was specifically recognized in Alpine communities as an anti-inflammatory remedy for traumas and musculoskeletal pains.

Internal Historical Uses and Their Abandonment

While arnica was historically used internally as well as externally, the former was abandoned in the 20th century due to a certain degree of toxicity upon internal use. Modern use is therefore restricted to external application on intact skin. Historically, internal preparations — teas, tinctures taken orally — were used in some European folk traditions for sore throats, respiratory complaints, and as circulatory stimulants, but these uses are no longer considered acceptable within regulatory frameworks.

Regulatory and Pharmacopoeial Recognition of Traditional Use

In 2014, the Committee on Herbal Medicinal Products (HMPC) of the European Medicines Agency (EMA), based on its Final Assessment Report, issued a Community herbal monograph on "Arnica montana L., flos," in which the legal status of arnica and its preparations is defined: herbal preparations in semi-solid and liquid dosage forms for cutaneous use, produced on the basis of various defined ethanolic extracts, are defined as herbal medicines in Traditional Use, meaning that they may be marketed after simplified registration. Traditional Use registration does not require clinical safety and efficacy trials but is accepted on grounds of sufficient safety data and plausible efficacy, mainly based on the literature, given that the drug has been in use for at least 30 years, including 15 years in the EU. In Germany, Latvia, and Slovenia, certain arnica preparations also have market authorization due to "well-established use."

Several scientific monographs dealing with the use of arnica and its preparations have been issued in the last few decades, most prominently by the former German Commission E and by the European Scientific Cooperative on Phytotherapy (ESCOP).

3. Key Constituents and Active Compounds

Overview of Phytochemistry

Over 150 bioactive components have been identified in the stem and leaves, flowers, seeds, and roots of arnica. These include sesquiterpene lactones, flavonoids, volatile oils, coumarins, helenalin, carotenoids, diterpene alcohols, arnidiol, pyrrolizidine alkaloids, phenolic acids, essential oils, oligosaccharides, and lignans. A wide range of chemical compounds including sesquiterpene lactones and their short-chain carbonic acid esters, flavonoids, carotenoids, essential oils, diterpenes, arnidiol, pyrrolizidine alkaloids, coumarins, phenolic acids, lignans, and oligosaccharides are found in different parts of the plant.

Sesquiterpene Lactones (Primary Active Constituents)

The main bioactive constituents are sesquiterpene lactones (STLs) of the helenanolide type. Among these, a variety of esters of helenalin and 11α,13-dihydrohelenalin with low-molecular-weight carboxylic acids — namely acetic, isobutyric, methacrylic, methylbutyric, as well as tiglic acid — represent the main constituents, in addition to small amounts of the unesterified parent STLs. A plethora of reports exist on the pharmacological activities of these STLs, and it appears unquestioned that they represent the main active principles responsible for the herbal drug's efficacy.

Arnica flowers from Arnica montana have been used in traditional medicine to treat a large variety of different ailments. It has been shown that the known diverse effects of the flowers correlate closely with the pharmacological and toxicological profile of their constituents helenalin, 11α,13-dihydrohelenalin, and their esters, which must therefore be considered as their main active compounds.

Flowers collected from high-altitude heathlands contain principally helenalin esters, while the flowers from lower-altitude meadows contain dihydrohelenalin esters in large amounts. The sesquiterpene lactone content can be influenced by genetic factors and environmental conditions including altitude, temperature, and rainfall. Pseudoguaianolide sesquiterpenes constitute 0.2–0.8% of the flower head of Arnica montana.

Flavonoids

Arnica flower constitutes various constituents including flavonoids (0.4–0.6%), sesquiterpenes, acetylenes, hydroxycoumarines, phenyl acrylic acids, and essential oil (0.2–0.35% in flower heads and 0.2–0.5% in leaves), as well as phenolcarbonic acids such as chlorogenic acid, cynarin, and caffeic acid. A number of flavonoids are typically present, including isoquercitrin, luteolin, kaempferol, quercetin, and astragalin.

Other Constituents

The flowers also contain caffeic acid and its derivatives, and an essential oil containing fatty acids, carotenoids, and thymol derivatives, along with the coumarins umbelliferone and scopoletin. The pyrrolizidine alkaloids tussilagine and isotussilagine may pose a risk of hepatotoxicity, though their concentrations in arnica are generally regarded as low compared to other pyrrolizidine alkaloid-containing plants. Triterpene diols (arnidiol, faradiol) are increasingly recognized as important contributors to arnica's topical anti-inflammatory activity, complementing the sesquiterpene lactone mechanism through different pathways. Faradiol and arnidiol demonstrate anti-edema and anti-inflammatory effects in animal models, and their contribution is most significant in lipophilic preparations such as oils and ointments that efficiently extract these compounds.

Pharmacopoeial Quality Standards

The European Pharmacopoeia requires the dried flower heads (Arnicae flos) to contain a minimum of 0.4% total sesquiterpene lactones, calculated as helenalin (C₁₅H₁₈O₄), to verify authenticity and potency.

4. Mechanisms of Action

NF-κB Inhibition

The most thoroughly characterized mechanism of action involves inhibition of the transcription factor NF-κB, a central mediator of inflammatory gene expression. The active ingredients mediating the pharmacological effect are mainly sesquiterpene lactones such as helenalin, 11α,13-dihydrohelenalin, chamissonolid, and their ester derivatives. While these compounds affect various cellular processes, current data do not fully explain all aspects of how sesquiterpene lactones exert their anti-inflammatory effect. Helenalin, and to a much lesser degree 11α,13-dihydrohelenalin and chamissonolid, inhibit activation of transcription factor NF-κB.

Helenalin inhibits NF-κB activation in response to four different stimuli in T-cells, B-cells, and epithelial cells, and abrogates κB-driven gene expression. This inhibition is selective, as the activity of four other transcription factors — Oct-1, TBP, Sp1, and STAT 5 — was not affected.

Evidence shows that helenalin selectively alkylates the p65 subunit of NF-κB. This sesquiterpene lactone is the first anti-inflammatory agent shown to exert its effect by directly modifying NF-κB. These data suggest a molecular mechanism for the anti-inflammatory effect of sesquiterpene lactones that differs from that of other nonsteroidal anti-inflammatory drugs (NSAIDs) such as indomethacin and acetylsalicylic acid.

NF-κB regulates the transcription of various inflammatory cytokines such as interleukin-1, -2, -6, and -8 and TNF-α, as well as genes encoding cyclooxygenase-II, nitric oxide synthase, immunoreceptors, cell adhesion molecules, hematopoietic growth factors, and growth factor receptors.

NFAT Pathway Modulation

Sesquiterpene lactones are considered arnica's main active compounds. They attack inflammatory processes at a very central point by inhibiting the transcription factors NF-κB and NF-AT at micromolar concentrations. Both transcription factors regulate the transcription of genes encoding for many inflammatory mediators. These insights on molecular mode of action represent an important contribution to understanding the anti-inflammatory activity of arnica preparations.

Transcriptomic analysis has revealed that arnica extracts target key T cell receptor (TCR) signaling pathways. Mechanistically, the hydroethanolic root extract selectively inhibited NF-κB DNA binding, while the aqueous fermented extract predominantly suppressed NFAT-dependent gene expression. The hydroethanolic whole plant extract exerted a moderate effect on both pathways.

T-Cell Immunomodulation

Arnica extracts have been identified as promising modulators of human TCR signaling and support potential in regulating T cell-driven inflammatory responses, with implications for muscle healing and chronic inflammatory diseases. Helenalin also expresses anti-inflammatory and immunosuppressive effects. It can induce apoptosis and inhibit CD-4+ T cell proliferation through stabilizing p53 and increasing reactive oxygen species (ROS).

Polysaccharide-Mediated Immunomodulation

One source indicates that the polysaccharides, arabino-3,6-galactan-protein and fucogalactoxyloglucan, in the arnica flowers significantly promote phagocytosis and the production of TNF-α in immune cells.

5. Scientific Evidence by Area of Use

5a. Bruising and Ecchymosis — Topical Phytotherapeutic Preparations

A rater-blinded randomized controlled trial tested the ability of a topical 20% arnica formulation to enhance the resolution of laser-induced bruising, involving healthy volunteers aged between 21 and 65. In this study, four bruises of 7 mm diameter each were created on the bilateral upper inner arms using a 595 nm pulsed-dye laser. Patients were divided into groups each randomly receiving 5% vitamin K, 1% vitamin K, 0.3% retinol, 20% arnica, or white petrolatum. The 20% arnica formulation was found to accelerate bruise resolution more than the control and the low-concentration vitamin K preparations, representing one of the more methodologically rigorous topical studies in this area.

A few clinical trials suggest that topical arnica significantly reduces bruising compared with placebo or low-concentration vitamin K ointments. However, a small study reported that topical arnica actually increased pain 24 hours after calf exercises, illustrating that results in this area are not uniformly positive.

A 2014 review found that A. montana was ineffective at concentrations of 10% or less for pain, swelling, and bruises, suggesting that preparation strength may be a critical variable in outcome.

5b. Osteoarthritis — Topical Preparations

Arnica has been studied for the treatment of knee osteoarthritis in an open, multicenter trial conducted by Knuesel and colleagues, which demonstrated statistically significant reductions in pain, stiffness, and function subscales from the overall WOMAC score. However, without a control group, it is difficult to discern efficacy, as placebo effect can be strong in areas of pain management.

A pivotal head-to-head comparison was conducted by Widrig and colleagues. The effects of ibuprofen 5% and arnica (50 g tincture/100 g, DER 1:20) as gel preparations in patients with radiologically confirmed and symptomatically active osteoarthritis of interphalangeal joints of the hands were evaluated in a randomized, double-blind study in 204 patients, to ascertain differences in pain relief and hand function after 21 days of treatment. There were no differences between the two groups in pain and hand function improvements, or in any secondary endpoints evaluated. Adverse events were reported by six patients (6.1%) on ibuprofen and by five patients (4.8%) on arnica.

The 2013 Cochrane review on topical herbal therapies for osteoarthritis assessed this evidence: mean pain in the ibuprofen group was 44.2 points on a 100-point scale; treatment with arnica gel reduced the pain by 4 points after three weeks (mean difference −3.8 points, 95% CI −10.1 to 2.5), an absolute reduction of 4% (10% reduction to 3% increase). Hand function was 7.5 points on a 30-point scale in the ibuprofen-treated group; treatment with arnica gel reduced function by 0.4 points (MD −0.4, 95% CI −1.75 to 0.95), an absolute improvement of 1% (6% improvement to 3% decline). Total adverse events were higher in the arnica gel group (13% compared to 8% in the ibuprofen group): relative risk 1.65 (95% CI 0.72 to 3.76). The confidence intervals spanning zero indicate that non-inferiority to ibuprofen cannot be confirmed from this single trial with certainty.

Patients with hand osteoarthritis treated with arnica extract gel for three weeks exhibited a moderate reduction in their pain score. The arnica fresh plant gel showed similar activity to the gel extract in patients with knee osteoarthritis, with pain scores significantly reduced through treatment for three and six weeks.

Evidence strength: Moderate for topical phytotherapeutic arnica gel in osteoarthritis of the hand as a non-inferior alternative to ibuprofen gel, based on a single well-designed RCT of 204 patients. Evidence for the knee is preliminary (open-label, no control arm). Overall, the Cochrane review characterized evidence in this area as limited and calling for further trials.

5c. Post-Surgical Recovery — Phytotherapeutic and Homeopathic Preparations

Studies on postoperative benefits from the use of arnica are mixed. It reduced swelling in patients following knee surgery, reduced pain following tonsillectomy, and may reduce bruising from rhinoplasty, but was not helpful after blepharoplasty.

In rhinoplasty, a randomized controlled trial compared arnica to corticosteroids: 48 primary rhinoplasty patients were randomized into three groups — one receiving 10 mg of dexamethasone intraoperatively followed by a 6-day oral tapering dose of methylprednisone; one receiving arnica three times a day for 4 days; and a control group. Three blinded panelists rated the extent of ecchymosis, intensity of ecchymosis, and severity of edema. Another rhinoplasty study of 48 patients found that arnica did not reduce bruising, and a 29-patient facelift trial (RCT, double-blind) found neither physicians nor patients could detect a difference between arnica and placebo in post-facelift bruising.

A recent publication summarized the effects of arnica in 20 placebo-controlled trials and authors stated that homeopathic arnica "seems to have a mitigating effect on ecchymosis, most notably following rhinoplasty and facelifts/facial procedures." A meta-analysis of homeopathic arnica for postoperative recovery suggests a small effect size for preventing excess bruising and related sequelae. Rigorous research is needed to determine the clinical benefits of homeopathic arnica.

Some 30 studies have been conducted where homeopathic arnica has been applied before or after surgery to improve wound healing, stop bleeding and swelling, and reduce pain. Studies of arnica for the prevention of muscle soreness after sporting activities such as marathon running are negative, and an early systematic review was skeptical about the effectiveness of arnica.

Evidence strength: Mixed and inconsistent. Individual trials show conflicting results depending on the procedure, preparation type, and formulation concentration. Homeopathic preparations specifically, by virtue of their extreme dilution, contain no detectable arnica constituents and are reviewed separately from phytotherapeutic preparations. A small pooled effect size for post-surgical bruising has been reported but methodological limitations and heterogeneity of preparations make firm conclusions difficult.

5d. Delayed-Onset Muscle Soreness (DOMS)

Studies of arnica for the prevention of muscle soreness after sporting activities such as marathon running are negative. This area represents one of the more definitively assessed uses, where controlled trials have generally not demonstrated benefit.

5e. Dental and Oral Surgery

Homeopathic arnica 30X tablets were tested for efficacy on post-surgical sequelae following extraction of impacted mandibular third molars. A case-control pilot study involving 23 patients divided into treatment and control groups, who received four tablets 1 hour before the procedure and four tablets four times per day starting 1 hour after the procedure and for the three following days, showed the treatment induced a significant reduction in pain, bleeding, bruising, and edema. However, the small sample size and pilot design of this study limit interpretation.

The homeopathic formulation 200C was also tested for its effects on post-extraction pain management. A triple-blind randomized controlled trial recruited 44 children aged between 8 and 12 years requiring two clinical sessions of tooth extraction in two different quadrants of the oral cavity.

5f. Anti-inflammatory, Antimicrobial, and Antioxidant Activities

Extracts and compounds from A. montana have exhibited several pharmacological activities in laboratory settings: anti-inflammatory, anticancer, antioxidant, antimicrobial, antiplatelet, and immunomodulatory activities. In vitro studies show that arnica has antimicrobial and anti-inflammatory properties. These findings are primarily preclinical and do not yet translate to robust human clinical evidence in these additional areas.

5g. Overall Assessment Across Areas

A PRISMA-compliant systematic review identified 42 studies dealing with the effects on pain and inflammatory signs due to both traumatic injuries and inflammatory conditions of arnica phytotherapeutic and homeopathic formulations. Some of these works reported in vitro and in vivo studies, while most were related to clinical trials. Even if some studies report negative outcomes, a relevant number of evidence points support the use of arnica formulations for the treatment of pain, bruises, and swelling that occur after traumatic injuries related to sport and surgical interventions as well as with arthritis and other inflammatory conditions.

The weight of evidence is most supportive for topical phytotherapeutic preparations (at higher concentrations of active extract) applied to intact skin for musculoskeletal and traumatic injury. Evidence for homeopathic preparations — which contain no pharmacologically detectable arnica constituents — is weak and generally not distinguishable from placebo in higher-quality trials.

6. Body Systems and Health Areas

• Musculoskeletal system: Bruises, sprains, contusions, muscle soreness, rheumatic joint pain, and osteoarthritis. This is the primary area of traditional and clinical use.
• Integumentary system (skin): Traditional medicinal use includes topical application for inflammations resulting from insect bites and for treatment of small boils (furuncles).
• Vascular/circulatory system: Additional external uses include surface phlebitis and varicose veins.
• Immune system: The anti-inflammatory, antimicrobial, antioxidant, and immunomodulatory activities of arnica's chemical compounds have been investigated in different models.
• Post-operative recovery: Reduction of bruising, edema, and pain following surgical procedures, particularly facial and orthopedic surgery.

7. Dosage Forms and Dosages Reported in Studies

Preparations containing arnica flowers are only for cutaneous use in semi-solid dosage forms (ointment, cream, gel). The frequency of application varies between 2 to 3, 3 to 4, and 2 to 4 times daily. Only for a few products is a restriction of the duration of use reported (10 days).

In the Widrig osteoarthritis trial: arnica was formulated as 50 g tincture/100 g (DER 1:20) in a gel preparation and applied for 21 days.

An open multicenter trial investigated the safety and efficacy of an arnica fresh plant gel applied twice daily in 26 men and 53 women with mild to moderate osteoarthritis of the knee.

In the rhinoplasty trial: one arm received arnica three times a day for 4 days following surgery.

In the third molar extraction pilot study: patients received four tablets 1 hour before the procedure and four tablets four times per day starting 1 hour after the procedure and for the three following days.

Arnica tincture must be prepared with diluted alcohol with an extract ratio of 1:5 or 1:10. Semisolid dosage forms contain 1–15% of oil of arnica.

The application instruction for semisolid forms is to apply thinly and evenly to the affected area up to 3 to 4 times per day, rubbing and/or massaging into the skin until the preparation disappears.

8. Safety Considerations and Known Interactions

Oral Toxicity

Oral administration of Arnica montana in moderate amounts can cause gastrointestinal upset, nausea, vomiting, and abdominal pain. Arnica montana extracts can cause skin rash and urticaria and should not be taken by mouth. The plant is poisonous, and ingestion can cause gastroenteritis, dyspnea, cardiac arrest, and death. The flowers and roots of the plant have caused vomiting, drowsiness, and coma when eaten by children.

At high doses, nervousness, altered pulse, and muscular weakness have been reported. Difficulty breathing may precede cardiac arrest. Deaths have occurred. Oral formulations pose a greater risk of toxicity and cause more adverse effects than topical formulations.

The Alpha-Methylene Lactone Moiety: Dual Pharmacology and Toxicology

The same reactive alpha-methylene-gamma-butyrolactone moiety that confers anti-inflammatory potency also underlies the toxicity of helenalin when taken internally (cytotoxic to gastric mucosa, hepatotoxic, and cardiotoxic at sufficient doses) and the allergic contact dermatitis risk in sensitized individuals. This dual nature — potent topical therapeutic but dangerous internal toxin — is the defining pharmacological characteristic of arnica.

Contact Dermatitis and Allergic Reactions

Numerous cases of allergy and contact dermatitis related to arnica have been described; sesquiterpene lactones — helenalin, helenalin acetate, and helenalin methacrylate — are the primary sources of contact dermatitis. Contact dermatitis can develop, resulting in a rash, itching, and dry skin. Allergic reactions are common with topical products and are cross-reactive with plants in the Compositae and Asteraceae families. Arnica is not recommended for prolonged usage or for persons with sensitivity to members of the Asteraceae family, such as daisies, ragweed, and chrysanthemums.

In the Knuesel knee osteoarthritis trial, adverse events deemed to be related to the study drug were all local reactions including itching, rash, pruritus, and dry skin — all of which were mild in severity.

Hepatotoxicity

In multiple small, rather short-term clinical trials of different preparations of arnica montana extracts, adverse side effects were described as uncommon and local, with no mention of either hepatotoxicity or ALT elevations. A review of 778 spontaneous reports of adverse reactions to herbals in the Swedish Registry found two cases of contact dermatitis but no instances of liver or other organ injury attributed to Arnica montana. A systematic compilation of all publications on the hepatotoxicity of specific herbals that identified 185 publications on 60 different herbs, herbal drugs, and supplements did not list or mention Arnica montana.

Application-Site Restrictions

Arnica preparations should not be applied to wounds or damaged skin, should not be bandaged, and should not be applied with external heat such as an electric heating pad, as this may result in excessive skin irritation or skin burn.

Regulatory Safety Classification

The U.S. Food and Drug Administration (FDA) classifies Arnica montana as an unsafe herb for oral consumption owing to its potential toxicity, prohibiting its use in ingested products or on broken skin where systemic absorption could occur. The EMA monograph explicitly states that oral or internal use is contraindicated due to the risk of serious toxicity from sesquiterpene lactones like helenalin.

9. Conservation Status

A. montana is a rare plant under strict protection and is included in the IUCN Red List of Threatened Species and in the Red Data Books and Red Data Lists of many European countries. For example, A. montana is classified as "critically endangered" on the national red list of Luxembourg; as "vulnerable" on the red lists of Bosnia and Herzegovina, Croatia, and Germany; and as "extinct" on the national red list of Hungary.

The populations of 31% of the plants assessed in the IUCN European Red List of Medicinal Plants study were considered to be declining, including the well-known arnica. Arnica is becoming rarer, particularly in the north of its distribution, largely due to increasingly intensive agriculture and commercial wildcrafting. Commercial collection in the wild, habitat loss, abandonment of land, reforestation, and ongoing climate change threaten the conservation of the species throughout its natural distribution.

The botanical drug is still partly collected in different European mountain regions. Despite the loss of habitats, A. montana is mainly harvested from the wild. Cultivation programs have been developed in several countries as a response, with the cultivar "Arbo" representing a commercially significant cultivated form.

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