Black walnut

Black Walnut (Juglans nigra L.): A Comprehensive Reference

1. Identity and Botanical Classification

Botanical name: Juglans nigra L. Common names: Black walnut, eastern black walnut, American walnut. Family: Juglandaceae (walnut family).

Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, native to central and eastern North America, growing mostly in riparian zones. Black walnut is a member of the walnut family (Juglandaceae), which includes other nut-producing trees such as pecan (Carya illinoinensis), shagbark hickory (Carya ovata), butternut (Juglans cinerea), and English walnut (Juglans regia).

Native to North America, black walnut trees can grow to 100 feet or more and live for over 250 years. Spanning the entirety of the eastern half of the United States, they are among the most prized North American hardwood trees due to their beautiful, dark, fine-grained wood frequently used for high-quality furniture, cabinets, gunstocks, and paneling.

The plant part most commonly used in dietary supplements is the outer hull (pericarp), particularly the green, unripe hull harvested before full maturation. The inner bark, leaves, and nut kernels have also been used medicinally and as food. It is mainly the outer covering of the nut (the hull) that is used in herbal medicine, but the bark, whole nut, and leaves have also been used.

1.1 Common Dosage Forms and Preparations

Black walnut is commercially available in several forms:

• Alcoholic tincture: Made by macerating the green hulls in alcohol. One commercial preparation is extracted from the fresh, unripe green outer hull of Juglans nigra trees, wildcrafted by hand each summer.
• Capsules/tablets: Filled with dried, powdered hull or hull extract. Commercially available tablets or capsules usually contain around 500 mg to 1,000 mg of powdered black walnut.
• Powdered hull: Dried green hull ground to a fine powder, used in capsules or topical preparations.
• Topical preparations: Creams, salves, and ointments incorporating hull extract or powder for skin application.

The husk is the green fleshy covering over the walnut shell which contains the nut. Medicinally, the husks are used when they have turned from green to black and are beginning to break down.

2. Historical and Traditional Use

2.1 Indigenous North American Use

The black walnut was invaluable to Native Americans, who made the tree's edible nut a significant part of their diet and used many parts of the tree for medicinal applications.

Historically, the bark of black walnut was used by several Native American communities including the Cherokee, Delaware, Iroquois, and Meskwaki, in tea as a cathartic, emetic, or disease remedy agent, and chewed or applied for toothaches, snake bites, and headaches. The Comanche pulverized the leaves of black walnut for the treatment of ringworm, the Cherokee used leaves to make a green dye, and the Delaware used the leaves as an insecticide.

The Comanche created a paste from the leaves and husk of the fruit for treatment of ringworm. Black walnut was also used by the Appalachian, Cherokee, Comanche, Iroquois, and Rappahannock to treat athlete's foot, hemorrhoids, and as an insecticide.

As a food, indigenous peoples mixed the crushed nuts in breads and puddings. The nut meats were also an important ingredient in corn soups and several other traditional native dishes. The fresh nuts would be crushed and boiled to make a beverage.

The bark was used cautiously in medicine because it is poisonous. It was chewed for toothache and made into a decoction as an emetic, to get rid of bile, and as a laxative. The bark of charred twigs and old bark from the trunk was mixed with water and used as a remedy for snakebites. An inner bark infusion was used for smallpox.

A tea from the leaves was used to treat goiter and to wash sores. A poultice from the leaves and crushed hulls of the nuts was used for topical applications to remove ringworm, treat athlete's foot, and hemorrhoids. The sap of black walnut was used externally as an anti-inflammatory.

2.2 Early European and Colonial Use

Nicholas Culpepper, the 17th-century English herbalist, said walnuts that had grown older and more oily could be used to heal "wounds of the sinews, gangrenes, and carbuncles…" and that a "piece of green husk put into a hollow tooth, eases the pain."

The brownish-black dye was used by early American settlers to dye hair. Historical references suggest that Native American tribes used black walnut husks to treat various infections and skin conditions. Early European settlers in North America also recognized the medicinal value of black walnut husks.

2.3 Traditional Dermatological and Antiparasitic Use

Traditional black walnut herbal medicine is extracted from the black, tarry, sticky part in the outermost hull, and has been used for skin conditions, including eczema, pruritus, psoriasis, warts, and parasitic skin conditions. Treatment of eye irritations and styes are other traditional uses for black walnut.

The juice of freshly macerated unripe hulls of the black walnut (Juglans nigra) has been used for many years in folk medicine as a treatment for localized, topical fungal infections such as ringworm.

3. Key Constituents and Active Compounds

3.1 Juglone (5-Hydroxy-1,4-naphthoquinone)

Black walnut drupes contain juglone (5-hydroxy-1,4-naphthoquinone), plumbagin (yellow quinone pigments), and tannin. Juglone is the single most pharmacologically studied constituent of black walnut.

Juglone represents the simplest 1,4-naphthoquinone derivative produced naturally in plants. The phytotoxin juglone is a naphthoquinone produced by the black walnut tree (Juglans nigra) and exhibits high reactivity with oxygen and reactive oxygen species (ROS).

The greatest concentration of juglone and hydroxyjuglone (a nontoxic, colorless precursor that is converted into the toxic form juglone by sensitive plants and through oxidation) is found in the vegetative buds, leaves, stems, nut hulls, and roots of the plants.

Juglone, a phenolic compound found in walnuts, has been shown to exert both oxidant and antioxidant activities, to act as an inhibitor of Pin1 (peptidyl-prolyl cis/trans isomerase), and to modulate cell signaling. These diverse actions may confer possible health benefits of walnuts.

Juglone, belonging to the same molecular family as phylloquinone (also known as vitamin K1), is produced by plants including black walnut. Using genetic analysis, a research team from Purdue University discovered that juglone is a product of the same synthetic pathway as phylloquinone and shares the same progenitor—a molecule called DHNA.

3.2 Hydrolyzable Tannins

Black walnut has been shown to contain compounds belonging to the hydrolyzable tannin group, such as ellagitannins and gallotannins. Two gallotannins present in black walnut are 1,3,6-trigalloylglucose and penta-O-galloyl-β-D-glucose. Several compounds of the ellagitannin class, including pedunculagin, tellimagrandin I, and heterophylliin E, were tentatively identified in black walnut.

Also present are alpha-hydrojuglone (1,4,5-trihydroxynaphthalene) and its glycoside beta-hydrojuglone, along with caffeic acid, ellagic acid, hyperin, and kaempferol, and the tannins galloylglucose and ellagitannins.

Ellagitannins are hydrolyzed during digestion to release ellagic acid, which in turn can be metabolized by gut microbiota to urolithins. A total of 15 urolithins, their glucuronides, and sulfate metabolites have been identified in urine, blood, feces, breast milk, and prostate tissue. Urolithins A and B were associated with antioxidant, anti-inflammatory, cardioprotective, neuroprotective, anticarcinogenic, and anti-aging activities, both in preclinical and clinical studies.

3.3 Phenolic Acids and Flavonoids

The phenolic compounds available in black walnuts and tocopherol increase the antioxidant potential of this nut. The primary phenolic compounds available in black walnuts are tannins, ellagic acid, gallic acid, ellagitannins, and hydrocinnamic acid.

Glansreginin A, azelaic acid, quercetin, and eriodictyol-7-O-glucoside are novel antibacterial compounds identified in the kernels of black walnuts. Black walnut (Juglans nigra L.) is one of the most economically valuable hardwood species and a high-value tree for edible nut production in the United States. Although consumption of black walnut has been linked to multiple health-promoting effects (e.g., antioxidant, antimicrobial, anti-inflammatory), the bioactive compounds have not been systematically characterized.

3.4 Fatty Acids, Phytosterols, and Tocopherols

Black walnut is rich in phenolics and contains higher levels of phytosterols, unsaturated fatty acids, and tocopherols than many other commonly consumed nuts.

In black walnut, fatty acids exist in saturated (SFA), monounsaturated (MUFA), and polyunsaturated (PUFA) forms. The SFA, accounting for about 7% of the total fatty acids in black walnut oil, comprise palmitic (16:0) and stearic (18:0) acids. The unsaturated fatty acids (MUFA and PUFA) in black walnut oil are similar to those in English walnut oil, consisting of oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids, and making up 93% of the total fatty acids.

Black and Persian walnuts have differing fatty acid, antioxidant, and amino acid profiles, according to USDA food and nutrition database statistics. According to recent studies, black walnuts contain protein, minerals, flavonoids, gallotannins, fatty acids, phytosterols, and phenolic compounds, although compared to Persian walnut, black walnut contains the same amount of phenolic compounds.

4. Mechanisms of Action

4.1 Antimicrobial and Antifungal Mechanisms

Juglone occurs in all parts of the Juglandaceae family and is found extensively in the black walnut plants. It possesses antifungal, antimalarial, antibacterial, and antiviral properties besides exhibiting cytotoxic effects.

It has been proposed that the biological activity of the walnut hulls is due to the presence of the simple naphthoquinone, juglone (5-hydroxy-1,4-naphthoquinone), which has been isolated from the unripe hulls by sublimation.

4.2 Antiparasitic Mechanisms

Researchers have identified juglone, a naphthoquinone found in black walnut, that generates reactive oxygen species and can inhibit key mitochondrial enzymes in parasites in laboratory and animal experiments, and some preclinical papers report reductions in parasite numbers or immunopathology in those models.

Tannins in black walnut also contribute to its antiparasitic effect by precipitating proteins in parasite tissues and creating an astringent environment in the gut.

4.3 Anti-inflammatory Mechanisms

A study on black walnut revealed that black walnut extracts have the capacity to suppress the production of six pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8, IL-10, and MCP-1) in PMA-differentiated, LPS-induced human pro-monocytic cells (U937). The authors suggest that the inhibitory effects are linked to the presence of several phenolics in black walnut, including gallic acid, quercetin, quercetin glycosides, 3-methylquercetin, and ellagitannins.

4.4 Anticancer Mechanisms (Preclinical)

In terms of its toxicity mechanisms, a number of studies have established that juglone causes cell death, disrupts the cell cycle, modifies DNA (especially in rapidly dividing cells), inhibits mRNA synthesis, alkylates basic protein thiol or amine groups, and reduces tumor suppressor levels.

Being a quinone molecule, juglone could act as a redox cycling agent and produce reactive oxygen species. Such pro-oxidant properties of juglone may confer health effects, such as by killing cancer cells.

Juglone is an inhibitor of Pin1 (peptidyl-prolyl cis/trans isomerase) that could regulate phosphorylation of Tau, implicating potential effects of juglone in Alzheimer's disease. Juglone also activates mitogen-activated protein kinases that could promote cell survival, thereby protecting against conditions such as cardiac injury.

5. Scientific Evidence by Area of Use

5.1 Antimicrobial and Antifungal Activity

In vitro / laboratory evidence:

Since the fresh juice of unripe walnut hulls is utilized for the treatment of ringworm, and since it has been speculated that the activity is due to the presence of juglone, researchers determined the efficacy of juglone compared to standard commercially available antifungal agents. The comparative efficacy was determined by evaluation of the minimum inhibitory concentration (MIC) values of juglone and the standard antifungal agents clotrimazole, triacetin, tolnaftate, griseofulvin, zinc undecylenate, and selenium sulfide, as well as two investigational antifungal antibiotics, liriodenine and liriodenine methiodide, for two dermatophytes, Trichophyton mentagrophytes and Microsporum gypseum. MIC values for juglone showed it to have moderate antifungal activity and to be as effective as certain commercially available antifungal agents such as zinc undecylenate and selenium sulfide.

Six antibacterial bioactive compounds responsible for antimicrobial activity were successfully identified in black walnut kernels. Glansreginin A, azelaic acid, quercetin, and eriodictyol-7-O-glucoside are novel antibacterial compounds identified in the kernels of black walnuts.

Evidence strength: The antifungal and antibacterial activity of juglone and black walnut extracts is supported by in vitro studies with MIC determinations. No controlled human clinical trials have been conducted to confirm antimicrobial efficacy for specific infections. Where small studies or older reports are cited for antifungal or antibacterial effects, they do not substitute for modern, species-specific clinical trials.

5.2 Antiparasitic Activity

Preclinical evidence:

Juglone has shown toxicity to various parasites, including roundworms (nematodes), tapeworms, and protozoa, by disrupting their cellular metabolism and causing oxidative damage. Lab studies on juglone and walnut hull extracts have inhibited growth of parasites like Giardia, Entamoeba, and several nematode species. Animal models (mostly poultry and livestock) have shown reduced parasite burdens and egg counts when walnut hull extracts were added to feed, with some improvement in gut health.

A 2022 study showed juglone achieved a 63% reduction in Schistosoma worm burden in animal models.

Black walnut green hulls (not the nut) contain juglone, hydrojuglone, and tannins. Juglone disrupts microbial respiratory chains and shows antiparasitic activity in Trypanosoma screening models. A 2015 study confirmed hydrojuglone glucoside as the dominant antitrypanosomal compound in J. nigra.

Human clinical evidence:

Most available evidence for black walnut's antiparasitic effects comes from in vitro studies or animal models, and well-controlled human clinical trials demonstrating safety and efficacy against intestinal parasites are lacking. Where small studies or older reports are cited (for antifungal or antibacterial effects), they do not substitute for modern, species-specific clinical trials showing that dietary walnuts prevent or clear parasitic infections in people.

Evidence strength: Preclinical only. The antiparasitic evidence is entirely from in vitro and animal studies. No peer-reviewed human RCTs have demonstrated clinical efficacy.

5.3 Anti-inflammatory Activity

Laboratory evidence:

Consumption of black walnuts has been linked to many health benefits (e.g., anti-inflammatory) stemming from its phytochemical composition and medicinal properties, but these effects have not been systematically studied or characterized. In one study, potential anti-inflammatory compounds found in kernel extracts of 10 black walnut cultivars were putatively identified using a metabolomic profiling analysis, revealing differences in potential anti-inflammatory capacities among examined cultivars. Five cultivars were examined for activities in the human promonocytic cell line U-937 by evaluating the effects of the extracts on the expression of six human inflammatory cytokines/chemokines using a bead-based, flow cytometric multiplex assay.

One report found that ellagic acid in walnut extracts may exert an antiatherogenic action, suggesting beneficial effects of walnut consumption on cardioprotection. Ellagic acid is also well known for its antioxidant capacity. The antioxidant ability of ellagic acid has attracted tremendous attention, giving rise to most of its potential health-promoting properties, including anti-inflammatory and antidiabetic effects.

Evidence strength: In vitro only for black walnut (J. nigra) specifically. The anti-inflammatory evidence is from cell culture experiments. Human clinical trials specifically examining black walnut hull or extract for inflammatory outcomes are lacking.

5.4 Anticancer Activity

Preclinical evidence:

Research indicates that black walnut extracts may have anticancer activity against multiple cancer cell types, including breast, skin, lung, bladder, stomach, cervical, and blood cancers.

One study analyzed the effect of juglone on the viability and proliferation of melanoma cells of C-32 (amelanotic melanoma) and COLO 829 (melanotic melanoma) cell lines in vitro, and on mRNA expression of genes encoding the proapoptotic BAX protein, caspase 3, and the antiapoptotic BCL2 protein. The results showed a dose-dependent effect of juglone on the viability, proliferation, and death induction in C-32 and COLO 829 melanoma cells and in HFF-1 normal dermal fibroblasts in in vitro cultures, but melanoma cells were more sensitive.

In cell culture studies, juglone-induced apoptosis and necrosis were demonstrated by oligonucleosomal ladder formation, microscopic analysis, an increase in the hypodiploid fraction, and an increased percentage of AnnexinV(+)/PI(+) cells detected by flow cytometry. A significant concentration-dependent decrease in glutathione levels and increase in dichlorofluorescein fluorescence after juglone treatment confirmed the ability of juglone to generate intracellular reactive oxygen species. The cytotoxic effect of juglone can be attributed to mechanisms including the induction of oxidative stress, cell membrane damage, and a clastogenic action leading to cell death by both apoptosis and necrosis.

Tumor progression in mice is inhibited by juglone, which triggers oxidative stress that leads to apoptosis and cell cycle detention, the suppression of hypoxia-inducible factor-1 alpha, and separation of glycolytic metabolism. Therefore, additional studies are warranted to examine the clinical potential of juglone in human cancers.

Two phenolic compounds found in black walnut, penta-O-galloyl-β-d-glucose and quercetin 3-β-d-glucoside, exhibited antiproliferative activities against both the tumorigenic alveolar epithelial cells (A549) and non-tumorigenic lung fibroblast cells (MRC-5). The antioxidant activity of black walnut is likely driven not only by penta-O-galloyl-β-d-glucose but also by a combination of multiple phenolic compounds. These findings suggested that black walnut extracts possibly possess anticancer activities, and that penta-O-galloyl-β-d-glucose could be a potential bioactive agent for the cosmetic and pharmaceutical industries.

Juglone holds particular importance in cancer therapy due to its multifaceted mechanisms of action, including its ability to inhibit cancer cell proliferation, promote apoptosis, induce autophagy, suppress angiogenesis, and impede cancer cell migration and invasion across various cancer types, including pancreatic, gastric, cervical, and prostate cancers.

Evidence strength: Preclinical only. All anticancer evidence is from in vitro cell line and animal studies. Juglone's clinical applications are still limited because of its poor solubility in aqueous solutions, poor absorption, and overall cytotoxicity to normal cells. No human clinical trials exist for black walnut as an anticancer therapy.

5.5 Cardiovascular and Lipid-Related Effects

Relevant evidence from walnut consumption research:

Similarities in contents of unsaturated fatty acids and tocopherols between black walnut and English walnut suggest that black walnut consumption may produce beneficial effects on cardiovascular disease risks.

Several studies have shown that walnut consumption has a positive effect on a variety of cardiovascular risk factors. Walnut bioactive ingredients, including omega-3 fatty acids, antioxidants, fiber, and polyphenols, have been demonstrated to improve lipid profiles, blood pressure, endothelial function, inflammation, oxidative stress, and thrombosis. These processes all contribute to the possible cardioprotective properties of walnuts. Epidemiological and clinical research indicates that daily walnut consumption can reduce the risk of CVDs like coronary heart disease and stroke.

A randomized controlled trial showed that a diet including walnuts significantly lowered serum total cholesterol and LDL cholesterol levels compared to a control diet and a diet including fatty fish. Several studies have demonstrated the positive impact of walnut consumption on cardiovascular health. This reduction in cholesterol levels is crucial for lowering the risk of coronary heart disease.

Evidence strength: Moderate to good for walnuts in general; limited specifically for J. nigra hull supplements. The cardiovascular evidence base is largely built on studies using English walnut (Juglans regia) kernels in dietary interventions, not black walnut hull extracts used as supplements. Black walnut contains greater levels of total phytosterols, unsaturated fatty acids, and tocopherols compared to many other nuts. Dietary intervention of these constituents has been associated with a great number of disease-preventive properties.

5.6 Neuroprotective Effects

Black walnuts are rich in ellagitannins, which are a type of polyphenol. Ellagitannins have anti-inflammatory and free-radical-fighting properties in the body. According to a 2024 research review, these compounds help prevent the onset and progression of neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis.

Notably, juglone is an inhibitor of Pin1 (peptidyl-prolyl cis/trans isomerase) that could regulate phosphorylation of Tau, implicating potential effects of juglone in Alzheimer's disease.

Evidence strength: Very preliminary. Neuroprotective effects attributed to black walnut constituents are based on mechanisms identified in laboratory studies. No controlled clinical trials in human subjects have evaluated black walnut for neurological outcomes.

6. Body Systems and Health Areas

Based on the research literature, black walnut and its constituents have been studied in association with the following body systems and health areas:

• Gastrointestinal system: Antiparasitic use (traditional and preclinical); laxative action of the inner bark (traditional); astringent effects of tannins.
• Integumentary system (skin): Topical antifungal (ringworm, athlete's foot); traditional use for eczema, psoriasis, and warts.
• Cardiovascular system: Lipid profile improvement associated with walnut fatty acid and polyphenol content.
• Immune system: Cytokine suppression demonstrated in vitro; antioxidant capacity of polyphenols.
• Oncology (preclinical only): Juglone-mediated apoptosis in various cancer cell lines.
• Nervous system: Juglone's Pin1 inhibition and ellagitannin-derived urolithin activity in preclinical neuroprotection models.

7. Dosage Forms and Reported Dosages

No standardized, clinically validated dosage for black walnut hull extracts has been established in peer-reviewed literature. The following dosages appear in available sources, reported here descriptively rather than as recommendations:

• Capsules/tablets: Commercially available tablets or capsules usually contain around 500 mg to 1,000 mg of powdered black walnut. The oral doses are generally taken three times a day but no longer than six weeks because of the high tannin content of black walnut.
• Liquid tincture (general range): One product's directions specify adults take 2 ml (81 drops) of black walnut hulls 3 times a day in a small amount of water. Another commercial preparation reports a 2 ml serving contains 2,000 mg of black walnut (Juglans nigra L.) hull extract (1:1 dry herb equivalent).
• Maximum duration noted: Not for long-term use of more than 6 weeks, followed by a 2-week break.
• Extraction rate: One commercial product's extraction rate is 560 mg fresh herb per 0.7 ml. Other ingredients include certified organic cane alcohol (40–50%), certified organic vegetable glycerin, certified organic vinegar, and distilled water.

The compounds most often implicated in antiparasitic activity — juglone and concentrated tannins — are concentrated in the green hull and bark of the black walnut tree, not the edible nut kernel itself. This distinction is pharmacologically relevant when considering supplement preparations.

8. Safety Considerations and Interactions

8.1 Juglone Toxicity and Cytotoxicity

Despite its evidenced beneficial herbicidal, antibacterial, antiviral, antifungal, and antioxidant effects, the application of juglone (5-hydroxy-1,4-naphthoquinone) is limited due to its low water solubility and allelopathic and toxic effects.

A number of studies have established that juglone causes cell death, disrupts the cell cycle, modifies DNA (especially in rapidly dividing cells), inhibits mRNA synthesis, alkylates basic protein thiol or amine groups, and reduces tumor suppressor levels.

One botanical supplier notes on their product labeling: Prolonged use is not advised due to the presence of significant quantities of juglone, a known mutagen in animals.

8.2 Tannin-Related Risks

These parts of the plant contain chemicals called tannins. Taking too much tannin can cause stomach upset and kidney and liver damage.

There are preclinical data and rare human reports suggesting potential hepatotoxicity or oxidative stress at high doses; tannins in the hulls may also contribute to stomach irritation.

8.3 Skin Application Risks

When applied to the skin, black walnut is possibly unsafe. It contains a chemical called juglone that can irritate the skin.

8.4 Bark Safety

When taken by mouth, the fruit (nut) of black walnut is commonly consumed in foods. But the bark is possibly unsafe. Taking the bark daily might increase the risk for tongue or lip cancer. Bark should be used cautiously in medicine, because it is poisonous.

8.5 Pregnancy and Lactation

The fruit (nut) of black walnut is commonly consumed in foods. But the bark is possibly unsafe during pregnancy. There isn't enough reliable information to know if black walnut leaf or shell is safe to use when pregnant or breast-feeding.

8.6 Allergy Risk

People with peanut allergies are more likely to be allergic to nuts called "tree nuts." Black walnut is a tree nut. Individuals with known tree nut allergies should exercise caution.

8.7 Animal Toxicity

Black walnut poses toxicity risks to animals. Moldy walnuts and decaying nut hulls can produce tremorgenic mycotoxins in dogs, leading to tremors, seizures, and neurological symptoms. Ingesting walnut wood shavings can cause incoordination. Exposure to black walnut shavings in horse bedding can lead to serious health issues such as laminitis, colic, and respiratory distress, and even small amounts of contaminated bedding can be harmful.

8.8 Drug Interactions and Duration Limits

Most data on black walnut's mechanisms comes from in vitro and animal studies — human pharmacokinetics and efficacy need more RCTs. Contemporary herbalism views anthelmintics as adjunctive at best, with NCCIH noting limited high-quality evidence for most.

Both the tannin content and juglone's known pharmacological activity at the enzyme and signaling level suggest the potential for interactions with drugs metabolized by similar pathways. There isn't enough reliable information to know if the leaf or the shell of the nut (hull) are safe to use as medicine. These parts of the plant contain chemicals called tannins. Taking too much tannin can cause stomach upset and kidney and liver damage.

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