Quercetin is a popular nutritional supplement in the flavonoid family. Quercetin occurs ubiquitously in foods of plant origin, such as red wine, onions, green tea, apples, berries, and Brassica vegetables (cabbage, broccoli, cauliflower, turnips).

In the manner of a drug, quercetin inhibits part of the inflammatory cascade. Quercetin is used by some Marshall Protocol (MP) patients to partially reduce intolerable immunopathologyAn unbearable or unsafe severity of bacterial die-off reaction., especially symptoms of fatigue, muscle pain and cognitive dysfunctionThe loss of intellectual functions such as reasoning; memory loss; and other neurological abilities that is severe enough to interfere with daily functioning. (i.e. brain fogThe loss of intellectual functions such as reasoning; memory loss; and other neurological abilities that is severe enough to interfere with daily functioning.).

Quercetin interacts with several medications. As with any supplement, MP patients should always ask their physician before beginning a supplement – even ones available without a prescription.

When looking to purchase a product, is it important to check the full ingredient list. Single ingredient Quercetin (without additional supplements like bromelain, vitamin C, etc.) is important for effective recovery on the Marshall Protocol.

Molecular actions of quercetin

Quercetin is a kinase inhibitor and it binds to many, if not all, of the kinases. The most important action of quercetin is the ability to inhibit I-kappaB kinase, the dimer protein which migrates to the phagocyte nucleus. When the I-kappaB kinase is inhibited, production of Nuclear Factor-kappa BA protein that stimulates the release of inflammatory cytokines in response to infection (NF-kBA protein that stimulates the release of inflammatory cytokines in response to infection) is downregulated.1) I-kappa B and NF-kB are at the heart of the Th1 inflammatory reaction.

Note that olmesartanMedication taken regularly by patients on the Marshall Protocol for its ability to activate the Vitamin D Receptor. Also known by the trade name Benicar. also acts on NF-kB, but by a different pathway. Quercetin is therefore synergistic with the NF-kappaBA protein that stimulates the release of inflammatory cytokines in response to infection-calming actions of olmesartan. It is this synergistic action combined with the anti-inflammatory effects of olmesartan which can reduce some immunopathology, especially fatigue, muscle pain and mind-fog.

Dietary plant polyphenols such as the flavonoids quercetin (QCT) and epigallocatechin-gallate act as antioxidants and as signaling molecules. We have previously shown that these polyphenols chelate zinc cations and hypothesized that these flavonoids might be also acting as zinc ionophores, transporting zinc cations through the plasma membrane. 2)

Using quercetin in conjunction with the Marshall Protocol

Patients in the initial stages of the Marshall Protocol when olmesartan (Benicar)Medication taken regularly by patients on the Marshall Protocol for its ability to activate the Vitamin D Receptor. is effecting powerful hormonal changes have reported that the addition of quercetin is either not effective or makes the symptoms worse. At later stages of the MP, many report some palliative effect from quercetin, with reduction in some immunopathology symptoms.

Quercetin may be taken routinely only if it is needed to keep symptoms tolerable. Patients can also take it only occasionally when immunopathology symptoms are worse.

Quercetin is purely palliative. It does not improve bug killing, it just makes some of the symptoms less acute. Do not use it unless it is giving you enough palliation to make up for possibly reduced bug killing.

Trevor Marshall, PhD

Quercetin can offer some reduction in symptom severity to patients on the Marshall Protocol.

Quercetin, not caffeine, is a major neuroprotective component in coffee. 3)


Patients should check with their pharmacist and/or doctor to see if they are taking medications or supplements that interfere with quercetin. This is important as quercetin slows metabolism of certain drugs, namely those that are metabolized via an enzyme which quercetin affects, P450. Olmesartan does not use the P450 metabolism, so its absorption and clearance are not affected.

These medications include:

  • fluoroquinolone antibiotics – Based on laboratory study, quercetin may bind to the DNA gyrase site on bacteria and competitively inhibit quinolone antibiotics.4)
  • warfarin (Coumadin) – Patients need to seek professional advice if they are using warfarin prior to taking quercetin.

Quercetin may interact negatively with the cyclospines, felodipine and estradiol.


Quercetin has not been safety-tested by the FDA. The FDA does not keep a file of adverse drug reactions involving quercetin. No clear toxicity has been identified.

Quercetin is not an essentially safe drug, unlike olmesartan, as quercetin has a dose-related effect on the body.

Trevor Marshall, PhD

Dosage of quercetin

At higher doses, quercetin binds kinases and has effects on the body's enzymes far beyond its desirable effect, that is, the suppression of Nuclear Factor-kappa B.

Patients interested in taking quercetin should begin by taking the doses recommended by the manufacturer, which is typically 200mg every 8 hours. One may have to divide the capsules contents to obtain the correct dose because quercetin is usually available in only two sizes: 400mg or 500mg.

It may require several days or up to a week of taking quercetin before the substance starts to have the desired effect. Using quercetin for longer periods of time is okay.

Larger than recommended doses may have unexpected effects.

If you feel you absolutely must use larger doses, then you are in unchartered territory. I always recommend increasing the olmesartan (Benicar) frequency rather than increase quercetin. In my opinion, there is less risk that way and the risk has been closely studied by the FDA.

Trevor Marshall, PhD

Quercetin, not caffeine, is a major neuroprotective component in coffee. 5)

Avoid quercetin as a part of a combination product

MP patients are advised not to buy quercetin as a part of a combination product, one which may contain other supplements such as bromelain. Bromelain has powerful, unknown effects in the body. Only use pure quercetin with no other supplements added including some which could be harmful to progress. If the only product a patient can find contains vitamin C, that is acceptable, provided the amount of added vitamin C does not exceed 1000 mg per day. The addition of routine stabilizers is okay.

The Loranthus genus has been demonstrated to be used in the treatment of wide range of diseases e.g. diabetes, inflammations and cancers. 6)

Buying quercetin

Many pharmacies sell pure formulations of quercetin. Patients can also order quercetin online.

Shipping to the United States

This search on Froogle lists a number of sources that would be willing to ship quercetin to the U.S., if not to other countries.

Shipping to other countries

Ask regarding the method of shipping. Patients have had trouble with shipments sent by courier which incurred an unavoidable very high extra charge.

===== Notes and comments =====

Bane, Thanks for this. I am forwarding it to everybody who might be interested.

At 11:58 AM 9/8/2010, you wrote: Quercetin and Ferulic Acid Aggravate Renal Carcinoma in Long-Term Diabetic Victims https://pubs.acs.org/stoken/presspac/presspac/full/10.1021/jf101580j

J Nutr Sci Vitaminol (Tokyo). 2010;56(5):326-30. Quercetin Enhances VDR Activity, Leading to Stimulation of Its Target Gene Expression in Caco-2 Cells.

Inoue J, Choi JM, Yoshidomi T, Yashiro T, Sato R.

Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo.


Vitamin D receptor (VDR) is a nuclear receptor that regulates the expression of genes involved in calcium homeostasis. Activation of VDR is thought to be a promising drug target for osteoporosis. Using a VDR-driven luciferase expression assay for screening a naturally occurring food component, we identified quercetin as a VDR activator. Quercetin also activated the GAL4 DNA-binding domain fused to the VDR ligand-binding domain. Moreover, it was confirmed that quercetin increases the mRNA level of TRPV6, which is a VDR target gene, in Caco-2 cells. These results indicate that quercetin enhances VDR activity through the alteration of cofactor recruitment, thereby stimulating its target genes while providing a new function for quercetin as the VDR activator.

PMID: 21228504 [PubMed - in process]

===== References =====

García-Mediavilla V, Crespo I, Collado PS, Esteller A, Sánchez-Campos S, Tuñón MJ, González-Gallego J. The anti-inflammatory flavones quercetin and kaempferol cause inhibition of inducible nitric oxide synthase, cyclooxygenase-2 and reactive C-protein, and down-regulation of the nuclear factor kappaB pathway in Chang Liver cells. Eur J Pharmacol. 2007 Feb 28;557(2-3):221-9. doi: 10.1016/j.ejphar.2006.11.014. Epub 2006 Nov 15.
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Dabbagh-Bazarbachi H, Clergeaud G, Quesada IM, Ortiz M, O'Sullivan CK, Fernández-Larrea JB. Zinc ionophore activity of quercetin and epigallocatechin-gallate: from Hepa 1-6 cells to a liposome model. J Agric Food Chem. 2014 Aug 13;62(32):8085-93. doi: 10.1021/jf5014633. Epub 2014 Jul 31.
[PMID: 25050823] [DOI: 10.1021/jf5014633]
3) , 5)
Lee M, McGeer EG, McGeer PL. Quercetin, not caffeine, is a major neuroprotective component in coffee. Neurobiol Aging. 2016 Oct;46:113-23. doi: 10.1016/j.neurobiolaging.2016.06.015. Epub 2016 Jul 5.
[PMID: 27479153] [DOI: 10.1016/j.neurobiolaging.2016.06.015]
Hilliard JJ, Krause HM, Bernstein JI, Fernandez JA, Nguyen V, Ohemeng KA, Barrett JF. A comparison of active site binding of 4-quinolones and novel flavone gyrase inhibitors to DNA gyrase. Adv Exp Med Biol. 1995;390:59-69. doi: 10.1007/978-1-4757-9203-4_5.
[PMID: 8718602] [DOI: 10.1007/978-1-4757-9203-4_5]
Noman OM, Mothana RA, Al-Rehaily AJ, Al Qahtani AS, Nasr FA, Khaled JM, Alajmi MF, Al-Said MS. Phytochemical analysis and anti-diabetic, anti-inflammatory and antioxidant activities of Loranthus acaciae Zucc. Grown in Saudi Arabia. Saudi Pharm J. 2019 Jul;27(5):724-730. doi: 10.1016/j.jsps.2019.04.008. Epub 2019 Apr 20.
[PMID: 31297028] [PMCID: 6598209] [DOI: 10.1016/j.jsps.2019.04.008]
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