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Compound Monograph

Quercetin

Quercetin — one of the most widely distributed dietary flavonols, found across a large number of medicinal plants, with antioxidant, anti-inflammatory, antiviral, and antiproliferative activity.

Work in progress

Classification

Quercetin is a flavonol (flavonoid), part of the phenolics class. Antioxidant compounds built around one or more phenol rings — the flavonoids, tannins, phenolic acids, coumarins, and pigments behind much of a plant's protective chemistry.

Phenolics Flavonoids

Where Does It Come From? (57)

Quercetin is a naturally occurring flavonol (flavonoid), found in Bearberry, Buchu, Bupleurum and 54 other sources. It is well tolerated orally (low toxicity).

Acacia Acacia spp. African Dream Herb Entada rheedii Amaranth Amaranthus blitum AppleApples Arjuna Terminalia arjuna Arnica Arnica montana Bay Bean Canavalia rosea Bearberry Arctostaphylos uva-ursi Blue Lotus Flower Nymphaea caerulea Buchu Agathosma betulina Buckwheat Bupleurum Bupleurum falcatum Calamus Acorus calamus Camu-camu Myrciaria dubia Capers Catnip Nepeta cataria Chanca Piedra Phyllanthus niruri Chrysanthemum Chrysanthemum morifolium Coltsfoot Tussilago farfara Cornflower Centaurea cyanus Couchgrass Elytrigia repens Fennel Foeniculum vulgare Feverfew Tanacetum parthenium Fish Mint Houttuynia cordata Ginkgo Goldenrod Solidago virgaurea Gotu Kola Centella asiatica Graviola Annona muricata Guamura Cecropia mexicana Gynostemma Gynostemma pentaphyllum Hawthorn Crataegus oxyacantha Hibiscus Hibiscus rosa-sinensis Hops Humulus lupulus Horse Chestnut Aesculus hippocastanum Juniper Juniperus communis Marshmallow Althaea officinalis Mexican Tarragon Tagetes lucida Milk Thistle Silybum marianum Motherwort Leonurus cardiaca OnionOnions Passionflower Passiflora incarnata Pau D'Arco Tabebuia impetiginosa Raspberry Leaf Rubus idaeus Sarsaparilla Smilax spp. Shatavari Asparagus racemosus St. John's Wort Hypericum perforatum Stevia Stevia rebaudiana Stinging Nettle Urtica dioica Tea Camellia sinensis Tribulus Tribulus terrestris Willow Salix alba Wireweed Sida acuta Wormwood Artemisia absinthium Yarrow Achillea millefolium Yerba Maté Ilex paraguariensis

Research & Evidence

Quercetin is one of the most common flavonols in the plant kingdom and turns up in the leaf, flower, and bark chemistry of a large share of the herbs in this database. On most herb pages it appears within the broader flavonoid fraction rather than as a singled-out active, but several monographs do attribute specific actions to it.

  • Antiviral — quercetin (alongside the closely related glycoside rutin, and likely other flavonoids) has been reported as active against a range of viruses including HSV-1, HIV-1, HIV-2, poliovirus type 1, parainfluenza virus, and respiratory syncytial virus 1Reference 1Ashour et al. · 2014Anti-infective and cytotoxic properties of Bupleurum marginatumView study →. In Bupleurum, quercetin and rutin are regarded as the main antiviral agents; the proposed mechanism is that these flavonoids inhibit viral polymerase activity and bind viral nucleic acid or capsid proteins, reducing viral replication 1Reference 1Ashour et al. · 2014Anti-infective and cytotoxic properties of Bupleurum marginatumView study →.
  • Anti-psoriatic / antiproliferative — quercetin isolated from the rhizome of Smilax china (sarsaparilla) has been shown to produce significant orthokeratosis along with anti-inflammatory and antiproliferative activity, which is put forward as part of the basis for the use of Smilax species against psoriasis 2Reference 2Vijayalakshmi A et al. · 2012Screening of flavonoid “quercetin” from the rhizome of Smilax china Linn. for anti-psoriatic activity.
  • Anti-inflammatory — quercetin is one of the flavonoids of Tanacetum (feverfew), a group whose anti-inflammatory properties have been characterised 3Reference 3Williams CA et al. · 1999The flavonoids of Tanacetum parthenium and T. vulgare and their anti-inflammatory properties. It is also part of the phenolic/flavonoid fraction credited with the broader anti-inflammatory and skin-supporting effects described for several of these herbs.
  • Cardiovascular (flavonoid fraction) — in hawthorn, quercetin is one of the leaf and flower flavonoids, and isolated flavonoids and flavonoid mixtures from this group have been shown to inhibit cyclic-AMP phosphodiesterase activity in heart tissue 4Reference 4Petkov et al. · 1981AnimalInhibitory effect of some flavonoids and flavonoid mixtures on cyclic AMP phosphodiesterase activity of rat heart, one of the mechanisms associated with hawthorn’s cardiotonic effect.
  • Antioxidant / antibacterial contribution — quercetin is consistently described as part of the antioxidant flavonoid content of plants such as bearberry, tea, stinging nettle, stevia, and yerba maté, where it contributes to the overall free-radical-scavenging and, in some cases, antibacterial activity of the whole-plant extract. In bearberry it is named among the flavonoids that contribute to the herb’s clinical effects 5Reference 5American Herbal Pharmacopeia · 2015ReviewUva Ursi (Arctostaphylos uva-ursi): A Review of Clinical TherapeuticsView study →.

This is a representative summary rather than an exhaustive one. The references below are the entries cited on the source-herb pages that genuinely pertain to quercetin, and this section will grow as more compound-specific research is added.

Monoamine oxidase (MAO) inhibition

Quercetin is one of the better-characterised flavonoid MAO inhibitors: it acts as a moderate, MAO-A-preferring inhibitor with a reported IC50 around 11 µM against MAO-A 6Reference 6Herraiz et al. · 2018Monoamine oxidase-A inhibition and associated antioxidant activity in plant extracts with potential antidepressant actionsView study →. This is thought to be part of the mechanism behind the mood effects of quercetin-rich plants such as St. John’s Wort, though it’s far weaker than the β-carboline inhibitors. See the natural MAO inhibitors guide for the full comparison.

Toxicity & Safety

Quercetin is a common dietary flavonol consumed routinely in foods such as onions, apples, and tea, and it is well tolerated in the amounts supplied by the herbs that contain it. The source monographs do not record specific toxicity for quercetin itself; cautions on those pages relate to the whole herbs (for example the volatile-oil content of buchu or the pyrrolizidine considerations of other plants) rather than to the flavonoid. As with any concentrated flavonoid supplement, high isolated doses are a different matter from dietary intake, and people who are pregnant, breastfeeding, or taking prescription medication should seek professional guidance before using concentrated extracts.

References

  1. Ashour, M. L, El-Readi, M. Z, Hamoud, R., Eid, S. Y, El Ahmady, S. H., Nibret, E, Wink, M. (2014). Anti-infective and cytotoxic properties of Bupleurum marginatum. Chinese Medicine, 9(1), 4. doi:10.1186/1749-8546-9-4
  2. Vijayalakshmi A, Ravichandiran V, Malarkodi Velraj, Nirmala S, Jayakumari S. (2012). Screening of flavonoid “quercetin” from the rhizome of Smilax china Linn. for anti-psoriatic activity. Asian Pacific Journal of Tropical Biomedicine. 2(4). 269-275.
  3. Williams CA, Harborne JB, Geiger H, Hoult JR. (1999). The flavonoids of Tanacetum parthenium and T. vulgare and their anti-inflammatory properties. Phytochemistry. 51:417–23.
  4. Petkov, E., Nikolov, N., & Uzunov, P. (1981). Inhibitory effect of some flavonoids and flavonoid mixtures on cyclic AMP phosphodiesterase activity of rat heart. Planta Medica, 43(10), 183-186.
  5. American Herbal Pharmacopeia. (2015). Uva Ursi (Arctostaphylos uva-ursi): A Review of Clinical Therapeutics. Alternative and Complementary Therapies, 21(4), 180-181. doi:10.1089/act.2015.29013.uva
  6. Herraiz, T., & Guillén, H. (2018). Monoamine oxidase-A inhibition and associated antioxidant activity in plant extracts with potential antidepressant actions. BioMed Research International, 2018, 4810394. https://pubmed.ncbi.nlm.nih.gov/29568754/