Xiong Huang (Realgar / Arsenic Sulfide / As4S4) Pharmacology — The Mineral Arsenical Anchor of Liu Shen Wan, An Gong Niu Huang Wan, and Traditional Topical Antiparasitic Plasters

If you have spent any time inside a serious TCM pharmacy, you have seen it: a small glass jar of orange-red, crystalline powder, sometimes labeled in red ink, sometimes locked behind the counter alongside aconite and cantharides. This is Xiong Huang (雄黄) — realgar, a naturally-occurring arsenic sulfide mineral with the formula As₄S₄ (sometimes written As₂S₂). It is one of only a handful of mineral drugs that still hold flagship positions in modern TCM formulas, and it sits at the chemical heart of three of the most iconic preparations in Chinese medicine: Liu Shen Wan (六神丸), An Gong Niu Huang Wan (安宫牛黄丸), and the entire Duanwu Festival realgar-wine (雄黄酒) tradition.

It is also, unambiguously, a regulated arsenic compound. The 2020 Chinese Pharmacopoeia mandates that medicinal-grade realgar contain greater than 90% As₂S₂. China’s Regulations on Management of Toxic Medicinal Products list xiong huang in the official 28-substance “toxic Chinese medicines” category, and An Gong Niu Huang Wan — which is 10% realgar by mass — is prohibited from import into the United States and European Union because its mineral content exceeds Western pharmacopeial limits. None of this means realgar is useless or that the classical formulas are wrong. It means realgar belongs in the same conversation as digoxin and lithium: a narrow-therapeutic-window agent whose pharmacology only makes sense once you understand the chemistry and the formulation context.

For topical medicated-oil and plaster work in particular, realgar is a fascinating ingredient because it does not behave like an aqueous arsenic salt. The very property that limits its oral bioavailability — As₄S₄ is essentially insoluble in water — is the same property that makes it relatively safe when applied to intact skin in a properly compounded ointment. The danger, as we will see, lies in two places: oxidized impurities (As₂O₃, As₂O₅) introduced by sloppy processing, and broken-skin or mucosal application that bypasses the solubility barrier.

This article walks through the chemistry, the mechanism of action across the multiple indications realgar is used for, the actual absorption and toxicology data from the past three decades of Chinese case reports, the formulation logic of the three flagship products that anchor realgar in modern practice, and the safety framework any practitioner — or curious consumer — needs to evaluate a realgar-containing product.

1. Mineralogy and chemistry — what realgar actually is

Realgar is a naturally-occurring monoclinic crystal of tetra-arsenic tetrasulfide (As₄S₄). The mineral forms in low-temperature hydrothermal veins, often alongside its yellow cousin orpiment (As₂S₃) and the white arsenic oxide arsenolite (As₂O₃). In its pure mineral state, realgar is a brilliant orange-red, semi-translucent crystal that resembles a chunk of dried apricot resin — which is reflected in the Chinese name xiong (male/bright) huang (yellow).

Three distinctions matter pharmacologically:

As₄S₄ vs. As₂O₃ — Sulfide-form arsenic is dramatically less bioavailable than oxide-form arsenic. Sodium arsenite (NaAsO₂, an As³⁺ oxide) has an oral LD₅₀ in rodents of roughly 15 mg/kg; purified realgar’s LD₅₀ is several thousand mg/kg. The sulfide bond is what makes realgar a usable drug rather than a quick poison.
Total arsenic vs. soluble arsenic — Realgar’s total arsenic content is roughly 70% by mass, but its water-soluble arsenic content is only about 4.16%. Pharmacological effect, and most of the toxicology, tracks the soluble fraction.
Processed vs. raw — Traditional processing (水飞法 — water-flotation grinding) is not decorative. It separates dense As₄S₄ from lighter oxide impurities and washes off any soluble arsenite that has formed on the crystal surface. Improperly processed or heat-roasted realgar can contain dangerous As₂O₃ from oxidation; this is the single most common cause of realgar poisoning in modern case series.

When realgar is heated above 240°C in air — for example, in a careless decoction, in cooking with realgar-containing herbal mash, or in the historical practice of warming realgar wine — As₄S₄ oxidizes to As₂O₃, which is highly water-soluble and is acutely toxic. This is why the old guidance “do not heat realgar” is not folklore; it is solid solid-state chemistry.

2. Mechanism of action — five overlapping pharmacological actions

Realgar’s TCM functions are listed as 解毒杀虫 (detoxify and kill parasites), 燥湿祛痰 (dry damp and resolve phlegm), and 截疟 (cut malaria). Mapped onto modern pharmacology, this becomes five overlapping mechanisms.

Antiparasitic action. Arsenic, even in its less-bioavailable sulfide form, irreversibly binds sulfhydryl (-SH) groups on parasite enzymes. Worms, scabies mites, and dermatophyte fungi all have higher sulfhydryl-density redox enzymes than mammalian skin keratin, so topical realgar can clear an infestation at concentrations that are tolerated by the host. This is why realgar appears in essentially every classical Chinese formula for scabies (疥疮), tinea (癣), and parasitic skin lesions.

Antibacterial and antifungal action. Modern in-vitro work shows realgar suspensions inhibit Staphylococcus aureus, Pseudomonas aeruginosa, and several dermatophyte species. The mechanism is again sulfhydryl binding plus disruption of microbial respiration. Clinically this underwrites realgar’s use for furuncles (疔疮), carbuncles (痈), and infected eczema in classical topical formulations.

Local anti-inflammatory and tissue-modifying action. At low concentrations realgar appears to dampen TNF-α and IL-6 production in cutaneous keratinocytes and macrophages. This is part of why realgar-containing plasters work on psoriasis and atopic dermatitis — two indications that have re-emerged in recent dermatology literature out of mainland China.

Antineoplastic / pro-apoptotic action. This is the modern surprise. Since the 1990s, arsenic trioxide (As₂O₃) has been a frontline drug for acute promyelocytic leukemia (APL), with cure rates above 90%. Realgar is not As₂O₃, but oral realgar-containing formulations — particularly the Realgar-Indigo Naturalis Formulation (青黛雄黄) — have demonstrated comparable efficacy in APL clinical trials, and several Chinese hospitals now use oral realgar as a maintenance agent. The mechanism is PML-RARα fusion protein degradation, identical to IV arsenic trioxide. This is why “ancient arsenic” became “modern oncology” in less than thirty years.

Counter-irritant and vesicant adjunct effect. When compounded into liniments and plasters alongside chan su, ban mao, or capsicum, realgar contributes a slow, deep-tissue irritation that is part of how 风湿 (rheumatic) plasters generate their warming sensation — though realgar itself is not the primary vesicant in any well-known formula.

3. Skin absorption — why topical use is, in practice, low-risk

The dermal pharmacokinetics of realgar are interesting and reassuring. Three factors converge:

Insolubility. As₄S₄ does not partition into the lipid-rich stratum corneum. It sits on the skin surface as a suspended particulate, releasing only the tiny fraction of soluble arsenic that can cross.
Particle size. Properly water-floated realgar has a particle size around 5–10 μm, large enough that transepidermal absorption is minimal on intact keratinized skin.
Vehicle. Traditional realgar ointments are carried in sesame oil, beeswax, or pig fat — vehicles that further reduce mineral dispersion into the aqueous microenvironment where As₂O₃ would form.

The 30-year retrospective by Wang and colleagues (Forensic Science International, 2018) identified only seven documented cases of acute arsenic poisoning from external use of realgar across mainland China. In every case the realgar had been (a) applied to broken skin or burns, (b) applied as a thick paste over large body-surface areas in folk-medicine doses, or (c) heat-roasted before application. None involved properly compounded commercial topical products on intact skin in dose-appropriate amounts.

The implication for medicated-oil consumers is straightforward: a 5–10% realgar plaster applied to intact dry skin once or twice daily for a few days is in the same risk class as a topical NSAID. A realgar paste smeared on an open eczematous lesion every day for a month is not.

4. The three flagship formulas — where realgar actually shows up

Liu Shen Wan (六神丸 / Six Divine Pills). Realgar is one of the “six divine” ingredients alongside niu huang, she xiang, chan su, zhen zhu (pearl), and bing pian (borneol). Realgar contributes roughly 13% of formula mass and provides the antimicrobial and anti-inflammatory pillar of what is otherwise a heat-clearing, throat-opening, detoxifying preparation. Liu Shen Wan is famously used in acute pharyngitis, tonsillitis, mumps, and certain dental infections.

An Gong Niu Huang Wan (安宫牛黄丸). The premier emergency formula for high-fever delirium, stroke, and heat-stroke unconsciousness. Realgar appears at 10% of formula mass alongside an equal proportion of cinnabar (HgS, 朱砂). The combination of arsenic sulfide and mercury sulfide is precisely why AGNH is import-banned in the US and EU — but is also why animal-model toxicology shows AGNH itself is dramatically less acutely toxic than its component minerals, because the surrounding herbs (huang lian, huang qin, gardenia, curcuma) measurably suppress hepatic and renal accumulation of arsenic and mercury.

Niu Huang Jie Du Pian (牛黄解毒片). A widely-used over-the-counter heat-clearing tablet for sore throat, gum inflammation, and constipation. Realgar accounts for roughly 5% of the formula. Of the three flagship realgar formulas, this is the one most commonly encountered in casual self-medication, and consequently the one with the largest documented incidence of chronic arsenic overload from prolonged misuse.

Beyond these three, realgar also appears in topical formulations such as 二味拔毒散 (Two-Ingredient Toxin-Drawing Powder, realgar + alum) for snake bites, 雄黄解毒膏 plasters for fixed skin lesions, and the antimalarial powder formulations once common in southern China.

5. The Duanwu Festival and the cultural footprint

No discussion of xiong huang is complete without the Duanwu Festival (端午节) realgar-wine tradition. On the fifth day of the fifth lunar month, families in southern China traditionally drank or dabbed onto children’s foreheads a wine infused with a tiny amount of realgar powder, intended to ward off “five poisons” — snake, scorpion, centipede, gecko, toad — as the summer parasite season began. The custom has a real pharmacological skeleton: dilute realgar topically applied to forehead and ear margins genuinely repels mosquitoes and certain mites for a few hours.

Modern Chinese public-health authorities discourage drinking realgar wine, because warmed realgar wine is precisely the As₂O₃-generating scenario described in §1. Topical dabbing on intact skin remains culturally widespread and is, by the dermal-pharmacokinetics argument above, essentially harmless at festival doses. Several Hong Kong and Taiwan pharmacies still sell pre-blended Duanwu realgar paste each May–June; these are external-use products only and are clearly labeled as such.

6. Safety framework — what actually matters

A practical checklist for any realgar-containing product:

Intact skin only. Never apply realgar formulations to broken skin, weeping eczema, surgical wounds, or burns.
Watch surface area and duration. Limit application to localized lesions, not whole-limb coverage. Do not exceed two weeks of continuous topical use without re-evaluating.
Children under 12, pregnancy, and lactation — avoid. The classical contraindication is not folklore; pediatric arsenic excretion is slower and the placenta does pass soluble arsenic.
G6PD deficiency. Realgar plasters are not a classical hemolytic trigger, but some compounded formulations also contain naphthalene-class moth-deterrents or borneol that can be. Read the label carefully if a G6PD-deficient family member will use the product.
Liver and renal disease. Both organs concentrate arsenic for excretion. Anyone with cirrhosis, chronic kidney disease, or current hepatitis flare should not use systemic (oral) realgar formulas; topical use should be brief and supervised.
Never heat. Do not warm a realgar plaster with a hot water bottle or heat pad. Do not apply realgar product to an area immediately after a hot bath or sauna. The combination of heat plus moisture plus broken stratum corneum is the worst-case absorption scenario.
Buy from a regulated source. The single most common cause of realgar poisoning is impurity, not the drug itself. Pharmacopoeial-grade realgar from a licensed Chinese-medicine pharmacy is dramatically safer than informal market product.

7. Where realgar fits in the modern medicated-oil cabinet

Realgar is not a daily-driver ingredient the way menthol, methyl salicylate, or camphor are. It is a specialist agent that anchors three flagship formulas (Liu Shen Wan, An Gong Niu Huang Wan, Niu Huang Jie Du Pian) and a handful of topical antiparasitic and dermatological compound products. For the medicated-oil consumer, realgar will most often be encountered in:

Premium throat-relief lozenge-style preparations (Liu Shen Wan and analogs)
Traditional scabies, tinea, and chronic eczema plasters from Chinese-medicine pharmacies
Snake-bite and insect-bite emergency powders in rural pharmacies across southern China and Southeast Asia
Duanwu Festival topical paste

It is the right ingredient when the indication genuinely calls for an antiparasitic, antimicrobial, mineral-based agent and the formulation has been compounded by someone who understands the As₄S₄/As₂O₃ chemistry. It is the wrong ingredient for ordinary muscle soreness, headache, or generalized rubefacient use — there are dozens of safer plant-derived options for those.

Understood for what it is, xiong huang is one of the most remarkable substances in the materia medica: a 1,500-year-old mineral drug that quietly evolved into the chemical scaffold for modern leukemia therapy, while continuing to anchor some of the most iconic emergency formulas in Chinese medicine.