Oak

Oak (Quercus spp.) as a Dietary Supplement and Medicinal Ingredient

1. Identity and Botanical Description

Taxonomy and Nomenclature

Oak belongs to the genus Quercus, an important member of the family Fagaceae, widely distributed in temperate forests of the northern hemisphere and tropical climatic areas. The Quercus genus is commonly known as oak and comprises about 450 species of deciduous or evergreen trees spread across North Africa, Asia, Europe, and North, Central, and South America.

The principal medicinal species are Quercus robur L. (English oak, pedunculate oak), Quercus petraea (Matt.) Liebl. (sessile oak), Quercus pubescens Willd. (downy oak), and Quercus alba L. (white oak). Common synonyms and vernacular names include Chêne Anglais, Chêne Blanc, Quercus pedunculata, Quercus sessiliflora, Sessile Oak, Stave Oak, Stone Oak, Tanner's Bark, and White Oak. The official pharmacopoeial drug is designated Quercus cortex (oak bark). The oak is a large, spreading deciduous tree demonstrating extreme longevity of 800 years or more, which may grow to over 30–40 metres in its lifetime.

Parts Used and Common Preparations

It is generally the bark that is specified when oak is indicated for medicinal use; however, many sources, both traditionally and within modern herbal medicine practice, mention the use of other parts of the tree including leaves, acorns, and galls as also being beneficial.

In the modern European Pharmacopoeia, oak bark is described as the cut and dried bark of young branches and the lateral shoots of Quercus robur, Quercus petraea, and/or Quercus pubescens, harvested in spring from March to April.

Oak bark is sold as a powder, tea, pill, and liquid extract; it is available over the counter in the United States and may be labeled as white oak or different varieties of its genus Quercus, including robur, cortex sessiliflora, and pedunculata. Preparations for external use include compresses, bath additives, and ointments. While Quercus robur is the primary European medicinal species, Quercus alba (white oak) has been more frequently used in American herbalism, and many other oak species have been explored globally for their therapeutic effects both in practice and in research studies.

2. Traditional and Historical Use

European and Mediterranean Traditions

Medicinal use of oak bark dates back to ancient Greek, Roman, and European folk medicine, where it was employed for treating wounds, diarrhea, and inflammatory conditions. Oak bark, primarily sourced from Quercus robur (English oak) and Quercus alba (white oak), has been used for centuries in traditional medicine, renowned for its high tannin content and its astringent, anti-inflammatory, and antimicrobial properties.

Since medieval times, the bark of these trees has been used in European folk medicine and applied topically to burns and wounds, or orally for gastrointestinal diseases. Oak bark was applied in traditional herbal medicine as a bath supplement for the treatment of skin diseases, weeping and itching eczema, foot sweat, burns, and hemorrhoids, and internally for the therapy of mouth rot, heartburn, and diarrhea.

The common, or British Oak, for many centuries the chief forest tree of England, is intimately bound up with the history of the British Isles from Druid times. The astringent effects of the oak were well known to the Ancients, by whom different parts of the tree were used; its action is slightly tonic, strongly astringent and antiseptic, with a strong astringent bitter taste, and its qualities are extracted both by water and spirit.

Records of the bark's use in folk medicine are found in many countries to counter diarrhoea, in infusions as a gargle for sore throat, and for adding to a hot bath for sore or inflamed skin. The infusions were also used for treatment of ulcers.

Native American and Global Traditions

White oak bark has been used for centuries in traditional medicine, particularly by Native Americans and European herbalists. It was valued for its astringent properties, being applied externally to treat wounds, burns, and skin infections, and internally for diarrhea, dysentery, sore throats, and other conditions requiring its anti-inflammatory and antimicrobial effects.

Indigenous peoples in many areas of the world use oak as an antiseptic and to treat gastrointestinal tract disorders such as diarrhea and hemorrhoids. The bark of the oak has been used extensively in medicine as an antiseptic and hemostatic, to cure toothache and gastropathies, and also as a pacifying agent in inflammation and as a healing agent in burns.

In various traditions, oak has been widely used to treat gonorrhea, gastritis, asthma, pyrexia, Parkinson's disease, and hepatoprotective diseases, as well as hemorrhoids, varicose veins, diarrhea, gastric ulcers, and superficial injuries.

Oak bark was used traditionally by herbalists to treat hemorrhoids, varicose veins, diarrhea, and cancer. Tannic acid derived from oak trees also has a long history of application in tanning hides and making ink.

Historically, other documented uses include an affinity for treatment of the oral cavity, including bleeding of the mouth, ulcerated and putrid sore throats, and as a gargle in aphthous sores; and other various documented uses include cholera, marasmus, scrofula, diabetes, and menorrhagia.

3. Key Constituents and Active Compounds

Tannins

The predominant constituents in oak are the tannins, which may be up to 20% of the active compounds contained within the bark. These polyphenolic compounds include grandin and ellagitannins, such as gallic acid, ellagic acid, vescalagin, and castalagin.

Oak bark is a source of polyphenolic secondary metabolites: hydrolysable tannins (previously known as pyrogallol tannins) and condensed tannins—proanthocyanidins. The total content of tannins in the dried substance varies from three to twenty percent, depending on the time of harvesting, the age of the branches, and the method of assay used.

The oak bark contains a highly variable amount of tannins (8–20%). The tannin content depends on the time of the harvest, the age of the branches, and on the method of assay used. Tannins are polyphenolic secondary metabolites of higher plants.

When compared, Q. alba has lower ellagitannins, whereas higher levels of gallic acid and total phenolics were found in Q. robur.

Flavonoids and Other Phenolics

Oak also contains flavonoid glycosides, including rutin and quercetin. Condensed tannins include catechin and epicatechin; phenolic compounds include polyphenols such as ellagic acid, vanillic acid, and flavonoids (flavan-3-ols and flavonols).

Roburins (Wood Extract)

Among the polyphenols present in oak wood are roburins, which belong to the class of ellagitannins and are unique to oak. These compounds are particularly relevant to the standardized wood extract Robuvit® and are distinct from the bark tannins described in traditional preparations.

Triterpenoids

Multiple biological activities including anti-inflammatory, antibacterial, hepatoprotective, antidiabetic, anticancer, gastroprotective, antioxidant, and cytotoxic activities have been ascribed to the presence of bioactive compounds such as triterpenoids, phenolic acids, and flavonoids. The cytotoxic activity of 17 triterpenoids isolated from oak heartwood of Quercus robur has been studied against human prostate cancer (PC3) and human estrogen-dependent breast adenocarcinoma (MCF-7) cell lines.

4. Established Mechanisms of Action

Astringency and Protein Precipitation

The antimicrobial effect of tannins is attributed to their ability to inactivate microbial adhesins, enzymes, and cell envelope transport proteins. The tannins' major mechanism of action is considered their ability to hydrogen bond their available polyhydroxyphenolic groups with proteins (especially proline-rich proteins), so that multivalent cross-linking leads to precipitation and aggregation effects.

Tannins are the primary constituents of oak bark and potent astringents, akin to those found in witch hazel. They bind liquids, absorb toxins, and soothe inflamed tissues. The oak tannin ellagitannin inhibits intestinal secretion, which helps resolve diarrhea.

Anti-Inflammatory Activity

Oak bark may contain up to 20% tannins depending on type and time of harvesting. Tannins act as astringents that bind to proteins in the skin to constrict body tissues, tightening pores and drying out irritated areas. The tannins in oak bark have been shown to inhibit the release of inflammatory compounds.

Research into the anti-inflammatory action of ellagitannins found in oak bark has indicated they may have a key role to play as a source of bioavailable gut microbiota metabolites shown to demonstrate anti-inflammatory activity.

Vasorelaxant / Cardiovascular Mechanism

Using electrophysiology and myography, researchers discovered that the KCNQ5 voltage-gated potassium channel mediates vascular smooth muscle relaxant effects of barks used in Native American folk medicine. Bark extracts (1%) from White Oak, among others, each strongly activated KCNQ5 expressed in Xenopus oocytes; White Oak KCNQ-dependently relaxed rat mesenteric arteries. Two compounds common to the active barks — gallic acid and tannic acid — had similarly potent and efficacious effects on both KCNQ5 activation and vascular relaxation, providing a molecular basis for smooth muscle relaxation effects of these bark extracts. This research was preclinical, performed in cell and animal models.

Antimicrobial and Anti-Quorum Sensing Activity

Its antimicrobial activity is a well-known therapeutic property of oak bark, and a novel anti-quorum sensing (QS) ability has also been described recently. The antimicrobial effect of Quercus ilex bark extracts was revealed against Brucella, Enterobacter, Escherichia, Neisseria, Pseudomonas, and Bacillus, while quite potent antibacterial effects have also been shown against Escherichia coli strains.

Certain hydrolysable tannins (ellagitannins), namely vescalagin and castalagin, have exhibited anti-quorum sensing properties, which may be relevant to controlling bacterial virulence.

Antioxidant Activity

All bark extracts of Quercus rubra studied exhibited antioxidant, anti-glucosidase, and anti-tyrosinase activity. Tannins, a diverse class of polyphenolic compounds, are widely present in plant-based foods and beverages, contributing significantly to numerous health benefits. Known for their antioxidant, anti-inflammatory, and cardioprotective properties, tannins are associated with a reduced risk of chronic diseases such as cardiovascular disease, cancer, and diabetes.

Mitochondrial and Urolithin Pathways

The standardized French Quercus robur wood extract Robuvit® has been investigated in 1,172 subjects in over 20 published clinical trials. The results of the clinical studies are consistent with reported effects of urolithins regarding increased mitophagy, pointing to enhanced energy capacity. The Robuvit® metabolite urolithin A, B, and C levels and the number of urolithin producers were found to be increased after intake of the extract.

5. Scientific Evidence by Area of Use

5.1 Gastrointestinal: Diarrhea

Regulatory standing: The available data are sufficient to include the traditional use of specified preparations of oak bark in a Community herbal monograph (EMA/HMPC). Oak bark fulfils the requirement of therapeutic use for at least 30 years, including 15 years within the European Union, as per Directive 2004/24/EC.

Based upon long-standing use, oak bark can be used internally for the symptomatic treatment of mild diarrhea and externally for mild inflammation of the skin and mucous membranes. Oak bark is also used externally to relieve itching and burning in hemorrhoids.

The HMPC monograph classifies oak bark for traditional use: symptomatic therapy of mild diarrhea (internal); mild inflammation of the oral mucosa and skin; and burning and itching in hemorrhoidal disorders (external).

Evidence strength: While indigenous cultures have long valued oak bark for its potential anti-inflammatory, antibacterial, and antioxidant effects, scientific studies validating these claims remain sparse. Recommendations from Germany's Commission E support its use for diarrhea and skin issues, yet there is a lack of robust clinical evidence confirming these benefits. The therapeutic use classification by the EMA's HMPC is based on traditional use rather than demonstrated clinical efficacy from controlled trials.

5.2 Topical: Inflammatory Skin Conditions and Mucosae

The German Commission E monograph endorses oak bark externally for inflammatory skin diseases and mild inflammation of the mouth and throat, genital and anal area; baths with at least 10% tannins are indicated for inflammatory skin diseases, weeping eczema, intertrigo, itching, hyperhidrosis, wound treatment, and post-treatment, as well as for draining infected skin processes.

Some people apply oak bark directly to the skin in a compress or add it to bath water for pain and swelling (inflammation) of the skin, mouth, throat, genitals, and anal region; and for red itchy skin due to cold exposure (chilblains).

The Commission E monograph cautions against using it externally for more than 2–3 weeks or topically on large areas of extensive skin damage.

Evidence strength: Evidence for topical applications is primarily based on traditional use, established pharmacological rationale (astringency, anti-inflammatory properties of tannins), and regulatory acceptance under the traditional herbal medicinal product framework. Robust randomized controlled trials in human subjects are lacking.

5.3 Fatigue and Energy: Robuvit® (French Oak Wood Extract)

A registry study evaluated the effects of supplementation with Robuvit® (French Quercus robur extract) capsules in subjects with Chronic Fatigue Syndrome (CFS) associated with increased oxidative stress. Robuvit® is a wood extract from Quercus robur (Horphag Research) used to improve liver dysfunction and chronic fatigue.

The registry study included 38 CFS subjects and 42 comparable controls, with no dropouts in the 4 weeks of follow-up; subjects were evaluated for a further period of 6 months. This preliminary registry indicated that supplementation with Robuvit® improved CFS in otherwise healthy subjects with no presence of clinical disease or risk conditions. The effects may be partially mediated by a clear reduction of plasma free radicals and oxidative stress.

Robuvit®, a registered product containing 300 mg of standardised Q. robur extract, was seen to significantly improve levels of fatigue during convalescence from acute illness when compared to the control group. Other clinical trials have shown beneficial effects in cases of chronic fatigue syndrome, liver failure, and in the treatment of lymphoedema. These effects may be potentially attributed to the rejuvenation of mitochondrial function evidenced following Robuvit® supplementation.

Three clinical trials using the same Quercus robur extract demonstrated that supplementation at a dose of 300 mg/day resulted in an overall improvement in fatigue symptoms and a significant reduction in oxidative stress.

In one registry, mood tests showed a significant general improvement in 13 out of 16 items of the Brief Mood Introspection Scale (BMIS) in the active treatment group; oxidative stress levels decreased significantly after 4 and 8 weeks; fatigue and insomnia scores were also significantly better in supplemented subjects (P<0.05); no changes were observed in controls.

Evidence strength: Clinical evidence for Robuvit® is derived almost exclusively from open-label registry studies (non-blinded, non-randomized), many by overlapping author groups with potential financial conflicts of interest. These are preliminary in nature and do not yet meet the standard of high-quality randomized controlled trial (RCT) evidence. Independent replication is limited.

5.4 Antimicrobial Properties

The expressed activity of Quercus infectoria ethanolic extracts has been demonstrated against the clinically relevant entero-haemorrhagic E. coli O157:H7 strain. The antimicrobial activity of Q. robur bark extracts has been described against Staphylococcus aureus, Enterobacter aerogenes, and Candida albicans.

Red oak bark extracts exhibited antioxidant, anti-glucosidase, and anti-tyrosinase activity, and antibacterial potential was observed especially against S. aureus, C. parapsilopsis, and C. krusei; inhibition of biofilm formation was observed only for MRSA. These findings show that red oak bark might be an important source of bioactive compounds with antioxidant and antimicrobial properties.

Evidence strength: Antimicrobial evidence is in vitro only. No controlled human clinical trials have evaluated oak bark as an antimicrobial agent in infectious disease settings.

5.5 Anti-Allergic Activity

Tannins extracted with hot water from Q. robur bark showed an inhibitory action against basophils and mast cell degranulation, decreasing the production of pro-inflammatory and allergic mediators in vitro. Infusions and kombucha beverages prepared with different oak leaves have been shown to decrease IL-6 secretion in cell models.

Evidence strength: Anti-allergic evidence is currently limited to in vitro cell-based models. Human clinical data are absent.

5.6 Oak Galls: Antioxidant, Antimicrobial, and Metabolic Effects

A systematic review of studies published between 2018 and 2023 (screening PubMed and Embase) focused on oak galls as a source of health-promoting phytochemicals. Oak galls are rich in active compounds, mostly gallotannins and phenolic acids. Due to these secondary metabolites, the reviewed studies demonstrated a wide range of biological activities, including antioxidant and anti-inflammatory actions, antimicrobial properties, tissue-protective effects, and antitumor, anti-aging, and hypoglycemic potential.

Evidence strength: Gall research is predominantly in vitro and in vivo (animal) in nature. Human clinical evidence is minimal.

5.7 Cardiovascular and Metabolic Effects

A study investigated the effects of a mixture of ellagitannins derived from oak bark (Quercus petraea L.) on cardiovascular, metabolic, and liver health in both spontaneously hypertensive rats and rats on a high-fat diet. The study found that tannin extracts improved cardiovascular, metabolic, and hepatic function, suggesting that ellagitannins from oak bark may enhance beneficial effects in those models.

Evidence strength: Cardiovascular and metabolic data are from animal models only. No controlled human trials have been conducted specifically for these endpoints using oak bark as the test intervention.

6. Body Systems and Health Areas Associated with Oak

• Gastrointestinal system: Oak bark is used as a tea for diarrhea, colds, fever, cough, and bronchitis; for stimulating appetite; and for improving digestion.
• Skin and mucous membranes: Some people apply oak bark directly to the skin in a compress or add it to bath water for pain and swelling (inflammation) of the skin, mouth, throat, genitals, and anal region, and for red itchy skin due to cold exposure (chilblains).
• Anorectal: A traditional herbal medicinal use is for symptomatic relief of itching and burning associated with haemorrhoids after serious conditions have been excluded.
• Energy and fatigue: In several clinical studies, Robuvit® supplementation was shown to induce better sleep, improve sport performance, lower symptoms of chronic fatigue syndrome, enhance burnout symptoms, and improve mood and erectile function, as well as enhance postsurgery and disease recovery.
• Cardiovascular: Preclinical data suggest vasorelaxant effects via KCNQ5 channel activation by oak tannins; no human cardiovascular trials are available.
• Immune and allergic response: In vitro evidence suggests inhibition of mast cell degranulation and cytokine release.
• Antimicrobial: In vitro activity against a spectrum of bacteria, fungi, and quorum-sensing pathogens.
• Hepatic (liver): Robuvit® has been studied in the management of functional, temporary hepatic damage in a registry pilot study.

7. Dosage Forms and Reported Dosages

The following dosages are those reported in herbal reference sources and study contexts, not general recommendations:

• Decoction (tea, internal): 1 g of dried herb per cupful of water is suggested three times daily, unless otherwise prescribed (per Commission E). For diarrhea, a cup of oak bark tea is drunk 3 times a day.
• Tincture: A typical dosage would be 20–40 ml per week, or 1–2 ml three times daily.
• Standardized oral extract (Robuvit®): Supplementation at a dose of 300 mg/day was the regimen in three clinical trials demonstrating improvement in fatigue symptoms and reduction of oxidative stress.
• Bath preparation: One preparation described adds 5–10 tablespoons of oak bark to 2 liters of boiling water, steeped for 15 minutes, strained, and added to a basin of warm water for a sitz bath lasting 10–15 minutes once daily for pelvic or anal inflammation relief.
• Duration (internal): Per the EMA Community herbal monograph, oak bark is not to be used for more than 3 days internally.
• Duration (topical): Oak bark is considered possibly safe for most people when applied directly to unbroken skin for up to 2–3 weeks; when applied to damaged skin or when taken for longer than 2–3 weeks, it is considered likely unsafe.

8. Safety Considerations and Interactions

General Safety Profile

There are no reports of adverse effects of oak bark from EU Member States in the context of the EMA assessment. Oak bark is considered safe for most people when taken for up to 3–4 days for diarrhea. Oak bark can cause serious side effects such as stomach and intestinal problems, and kidney and liver damage if misused.

Tannin-Related Considerations

There are no reported interactions with prescribed or over-the-counter medicines and supplements per herbal reference literature; however, the high tannin content of oak may result in reduced absorption of nutrients and some prescribed drugs, therefore it is advised that any remedies containing oak are administered apart from meals and other medicines. Herbs that are high in tannins, such as oak, should be avoided in cases involving constipation and iron-deficient anaemia.

There have been no reports of oak bark interacting with other drugs or supplements. However, it is advised not to take oak bark with iron supplements, as tannins may interfere with iron absorption.

Herbs containing high amounts of tannins may interfere with the absorption of ephedrine or pseudoephedrine taken by mouth; herbs containing high levels of tannins include green tea, black tea, uva ursi, black walnut, red raspberry, oak (Quercus spp.), and witch hazel.

Contraindications

Allergic reactions in patients allergic to Fagaceae should be considered. There is not enough information on the safety of oak bark preparations among pregnant and breastfeeding women, and oak bark should not be used by these populations. There are concerns that oak bark is unsafe for individuals with impaired kidney or liver function; as such, it should be avoided in these groups.

At this time, many sources consider that there is insufficient evidence to determine whether oak is safe to use internally during pregnancy and breastfeeding. However, there are no cautions stated regarding its topical use for conditions that may be associated with pregnancy, including varicose veins or haemorrhoids.

Prolonged Internal Use

Although useful for acute, short-term conditions, oak should not be used internally at high doses longer term. Use of any herb rich in tannins should be selective and for restricted amounts of time due to the effect on gut absorption of nutrients and minerals including iron, particularly given that any condition featuring regular blood loss as a symptom is associated with an increased risk of iron-deficiency anaemia.

Tolerability of Robuvit® (Wood Extract)

Tolerability to supplementation with Robuvit® was overall good, and no safety concerns were raised in the registry studies conducted.

9. Regulatory and Pharmacopoeial Status

The quality of oak bark (Quercus cortex) is specified in the European Pharmacopoeia (Ph. Eur.). The HMPC has classified oak bark as a traditional herbal medicinal product. The EMA HMPC indications are: (1) traditional herbal medicinal product for symptomatic treatment of mild diarrhoea; (2) traditional herbal medicinal product for symptomatic treatment of minor inflammation of the oral mucosa or skin. The German Commission E monograph endorses oak bark externally for inflammatory skin diseases and mild inflammation of the mouth and throat, genital and anal area, and internally for non-specific, acute diarrheal diseases.

ESCOP has not formally processed oak bark into a monograph. In the United States, oak bark is marketed as a dietary supplement and is not subject to pre-market approval by the FDA for efficacy.

References

• EMA/HMPC – Community Herbal Monograph on Quercus robur L., Quercus petraea, Quercus pubescens, cortex (European Medicines Agency)
• EMA – Final Assessment Report on Quercus robur L., Quercus petraea, Quercus pubescens, cortex
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• Belcaro G, et al. Improvement in mood, oxidative stress, fatigue, and insomnia following supplementary management with Robuvit®. PubMed PMID: 29938480
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• Lorenz P, et al. Constituents from oak bark (Quercus robur L.) inhibit degranulation and allergic mediator release from basophils and mast cells in vitro. J Ethnopharmacology. 2016;194:642-650.
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