Quercetin: Benefits, Dosage, Contraindications
Other name(s)
Quercetol
Family or group:
Phytosubstances
Indications
Rating methodology
EFSA approval.
High Blood Pressure ✪✪✪✪✪
A meta-analysis of mainly small clinical studies shows that the daily intake of 500 to 1000 mg of quercetin for 4 to 10 weeks decreases systolic blood pressure by 3.04 mmHg and diastolic blood pressure by 2.63 mmHg compared to a placebo. The benefit of quercetin on blood pressure seems modest.
Posologie
Oral intake
500 - 1095 mg
10 - weeks
Quercetin reduces blood pressure in hypertensive subjects.
Acute, quercetin-induced reductions in blood pressure in hypertensive individuals are not secondary to lower plasma angiotensin-converting enzyme activity or endothelin-1: nitric oxide.
Prostatitis ✪✪✪✪✪
Preliminary clinical research shows that taking 500 mg of quercetin twice daily for one month reduces pain and improves quality of life, but does not seem to affect urinary dysfunction, in patients with chronic non-bacterial prostatitis. Combining it with bromelain and papain may enhance absorption and outcomes according to these studies. The anti-inflammatory, antioxidant, and immunomodulatory activities of quercetin could contribute to explaining these results.
Posologie
Oral intake
1000 mg
1 - month
Synergies
Quercetin's influence on exercise-induced changes in plasma cytokines and muscle and leukocyte cytokine mRNA.
Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial.
Allergies ✪✪✪✪✪
Quercetin is used in the treatment of acute and chronic allergic symptoms, such as hay fever and chronic rhinitis. The anti-inflammatory activity of quercetin and its ability to block allergic mediators justify its use in these indications. Quercetin inhibited histamine release by 46 to 96%, proportional to the dose, in a study involving 123 patients with mild to severe allergic rhinitis symptoms. Further human trials are necessary to further explore the potential of quercetin to act as an antihistamine agent in various allergic conditions.
Posologie
Oral intake
2000 mg
Respiratory Infections ✪✪✪✪✪
A clinical trial on 40 male cyclists receiving 1000 mg/day of quercetin or placebo for 2 weeks showed that active treatment led to a significant reduction in the incidence of upper respiratory tract infections over a 2-week period post-exercise. Another double-blind randomized clinical trial examined the effect of two different doses of quercetin (500 mg/day and 1000 mg/day) on the incidence and severity of respiratory tract infections in a large group of adults (n = 1002) aged 18 to 85 years. Overall, quercetin supplementation for 12 weeks did not have a significant influence on the rates of respiratory tract infections or symptoms compared to placebo. A reduction in the number of illness days as well as severity related to respiratory infections was observed in the group consuming 1000 mg of quercetin per day.
Posologie
Oral intake
500 - 1000 mg
Quercetin Reduces Illness but Not Immune Perturbations After Intensive Exercise
Effectiveness of supplementation with quercetin-type flavonols for treatment of viral lower respiratory tract infections: Systematic review and meta-analysis of preclinical studies
Asthma ✪✪✪✪✪
Preliminary clinical research conducted on patients with mild to moderate asthma shows that taking quercetin at doses of 250 mg or 500 mg per day for 30 days in conjunction with standard treatment reduces daytime and nighttime asthma symptoms, improves spirometry measurements and reduces the use of rescue inhalers compared to baseline. Quercetin has also been used as an adjunct in asthma management, often in combination with Vitamin C due to its antiallergic activity.
Posologie
Oral intake
250 - 500 mg
1 - month
Synergies
Cancer ✪✪✪✪✪
Quercetin is mainly found in fruits and vegetables, which have been shown to decrease the risk of certain cancers when consumed regularly. The anticancer effects of quercetin observed in laboratory studies suggest a possible preventive role, and epidemiological studies have suggested potential benefits in the prevention of colon, lung (in smokers), pancreatic (in smokers), prostate, and breast cancers.
Posologie
Oral intake
40 mg
Quercetin Decreases the Expression of ErbB2 and ErbB3 Proteins in HT-29 Human Colon Cancer Cells
Dietary supplementation with the anti-tumour promoter quercetin: its effects on matrix metalloproteinase gene regulation
A Prospective Study of Dietary Flavonoid Intake and Incidence of Epithelial Ovarian Cancer
Quercetin, a Potent Inhibitor Against beta-catenin/Tcf Signaling in SW480 Colon Cancer Cells
Quercetin and Resveratrol Potently Reduce Estrogen Sulfotransferase Activity in Normal Human Mammary Epithelial Cells
Dietary Flavonoid Intake and Smoking-Related Cancer Risk: A Meta-Analysis
Overexpression of c-Jun Induced by Quercetin and Resverol Inhibits the Expression and Function of the Androgen Receptor in Human Prostate Cancer Cells
Flavonols and Pancreatic Cancer Risk: The Multiethnic Cohort Study
COVID-19 ✪✪✪✪✪
Two small open-label clinical trials conducted in Pakistan with symptomatic adults who tested positive for COVID-19 and had an oxygen saturation level above 93% evaluated quercetin as a complement to standard care, which included analgesics, antipyretics, and antibiotics. One study shows that taking 200 mg three times daily for 7 days followed by 200 mg twice daily for 7 days seems to reduce the time needed to resolve symptoms and obtain a negative COVID-19 test result, but it does not prevent hospitalization, ICU admission or death, compared to standard care alone. Another clinical trial conducted in the same population shows that taking quercetin at 200 mg twice daily for 30 days reduces the risk of hospitalization by 68%, the duration of hospitalization (among those hospitalized) by about 5 days, and the need for supplemental oxygen by 93% compared to standard care alone. No patient receiving quercetin was admitted to the intensive care unit or died, whereas 10.5% of patients in the standard care group were admitted to the ICU and nearly 4% died.
Posologie
Oral intake
400 - 600 mg
Possible Therapeutic Effects of Adjuvant Quercetin Supplementation Against Early-Stage COVID-19 Infection: A Prospective, Randomized, Controlled, and Open-Label Study
The therapeutic efficacy of quercetin in combination with antiviral drugs in hospitalized COVID-19 patients: A randomized controlled trial
Polycystic Ovary Syndrome ✪✪✪✪✪
Clinical research conducted on PCOS patients shows that taking 1000 mg of quercetin daily for 12 weeks reduces testosterone and luteinizing hormone (LH) levels compared to baseline values; both improvements are significant compared to placebo. Insulin resistance also improved by 18% from baseline, which was significant compared to placebo.
Posologie
Oral
1000 mg
Effects of Quercetin on Adiponectin-Mediated Insulin Sensitivity in Polycystic Ovary Syndrome: A Randomized Placebo-Controlled Double-Blind Clinical Trial
The effects of quercetin supplementation on metabolic and hormonal parameters as well as plasma concentration and gene expression of resistin in overweight or obese women with polycystic ovary syndrome
Endometriosis ✪✪✪✪✪
In a controlled study conducted on 90 patients suffering from endometriosis, a combination of dietary supplement including 1002 mg of linoleic acid, 432 mg of alpha-linolenic acid, 200 mg of quercetin, 20 mg of nicotinamide, 400 mcg of folate, 20 mg of turmeric titrated, 19.5 mg of parthenium titrated, showed a significant reduction in symptoms at the end of the treatment on the cohort of patients treated with the combination for three months, compared to the placebo group. Symptoms included: headaches, cystitis, muscle pain, irritable colon, dysmenorrhea, chronic pelvic pain. It was found that inflammation in endometriosis leads to an increase in cytokines by monocytes and macrophages, stimulating the production of PGE2 (prostaglandin E2) in the endometriotic tissue. Thus, the reduction of PGE2 levels by certain dietary supplements could explain the clinical improvement observed in patients after treatment.
Posologie
Oral
200 mg
Synergies
Properties
Antioxidant
The antioxidant effects of quercetin likely play a role in most of its potential clinical effects. As a dietary flavonoid, quercetin has antioxidant effects like other flavonoids. Preliminary research shows that quercetin inhibits the oxidation of low-density lipoproteins and may suppress physiological reactions of heavy metal ions known to generate free radicals. Compared to curcumin, quercetin shows higher reducing potential with a total antioxidant capacity 3 times higher than that of curcumin. However, preliminary research in humans suggests that quercetin does not reduce biomarkers of oxidative stress, indicating that the antioxidant effects observed in vitro and in vivo may not translate into clinically significant benefits. Research also suggests that oxidative stress reduction can be expected only in individuals with increased oxidative stress or inflammation, which could explain the negative outcome of many clinical studies, where antioxidants were provided to healthy subjects. Moreover, when acting as an antioxidant, quercetin may be converted to substances releasing reactive oxygen species when reconverted to quercetin. It might therefore have a pro-oxidant activity, especially at high doses. It's suggested that during its protective process, quercetin depletes GSH levels and transforms into potentially toxic products (thiol-reactive quercetin metabolites). In the absence of GSH, potentially harmful oxidation products like ortho-quinone may be produced when quercetin exercises its antioxidant activity. Consequently, adequate GSH levels should be maintained when supplementing with quercetin.
Chronic quercetin ingestion and exercise-induced oxidative damage and inflammation
A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity, including lack of genotoxic/carcinogenic properties
Quercetin reduces markers of oxidative stress and inflammation in sarcoidosis
Antiviral
Quercetin demonstrates activity against retroviruses as well as herpes simplex, polio, parainfluenza, and respiratory syncytial viruses (causing respiratory infections). In vitro and in vivo evidence also suggests that quercetin has the capacity to block rhinovirus replication, responsible for the common cold. In cell cultures, quercetin reduces the virus's ability to infect, possibly by stimulating interferon production and blocking certain proteins necessary for the virus to replicate. Quercetin's antioxidant effects could also protect the lungs against the deleterious effects of free radicals during a flu infection. Research on the effect of quercetin on SARS-CoV-2 (the virus responsible for COVID-19) indicates quercetin seems capable of binding to the virus's enzymes, thereby blocking its entry into the cells. Moreover, its anti-inflammatory properties could help control and prevent the severe inflammatory response often observed in severe COVID-19 cases.
Usages associés
Antihistamine
Preliminary evidence suggests that quercetin could have an effect similar to cromolyn (an antihistamine), inhibiting the histamine release stimulated by the antigen from mast cells in patients with allergic rhinitis. Further research shows that quercetin might inhibit mast cell proliferation in vitro. Quercetin has been proposed as a useful treatment for allergic inflammatory diseases derived from mast cells, including contact dermatitis and photosensitivity, especially in formulations with effective oral absorption.
Usages associés
Immunomodulator
In vitro research has shown that quercetin induces Th1-derived cytokines (promoting cellular immunity) and inhibits Th2-derived cytokines involved in allergies, providing a theoretical basis for using quercetin as an antiallergenic substance. Conversely, animal studies have demonstrated that quercetin is capable of inhibiting Th1 differentiation and interleukin (IL)-12 signaling and appears to exert an effect on Th1-mediated immune responses by suppressing interferon-gamma and IL-2 cytokine production. Consequently, it is possible that quercetin exerts an immunomodulatory effect on these cells.
Usages associés
Anti-inflammatory
In animal and in vitro studies, quercetin has shown an ability to reduce inflammation, notably by influencing neutrophil function and inhibiting the production of certain inflammatory cytokines like tumor necrosis factor-alpha (TNF-b1) and modulating nitric oxide synthesis. However, human studies have not always confirmed these effects. A meta-analysis of 10 clinical studies revealed that quercetin did not significantly affect levels of C-reactive protein (CRP), interleukin-6 (IL-6), or TNF-b1 compared to a placebo. Slightly positive results were observed in patients with diseases having an inflammatory component, such as diabetes or metabolic syndrome.
Usages associés
Cardiovascular
The cardioprotective effects of quercetin may be related to its vasorelaxant, anti-inflammatory, and antioxidant properties and the inhibition of vascular smooth muscle cell proliferation and migration, as demonstrated by animal and in vitro models. In humans, quercetin inhibits platelet aggregation and signaling as well as thrombus formation with a dose of 150 or 300 mg of quercetin glucoside. A small randomized controlled trial involving 12 healthy men testing the effects of oral administration of 200 mg of quercetin, epicatechin, or epigallocatechin gallate on nitric oxide, endothelin-1, and oxidative stress showed that quercetin increased nitric oxide status and reduced endothelin-1 concentrations, suggesting potential to enhance endothelial function. Flavonoids like quercetin also inhibit LDL oxidation, a key event in atherosclerosis plaque genesis. Although researchers believe quercetin might protect against cardiovascular diseases, short-term supplementation does not seem to affect cholesterol levels, and its effects on blood pressure are mixed.
Usages associés
Anticancer
Quercetin might reduce cancer risk by inactivating malignant precursors or inhibiting carcinogenesis. Epidemiological studies have shown that a high dietary intake of quercetin and related flavonols could reduce the risk of pancreatic cancer, particularly in male smokers, and also reduce lung cancer risk. Preliminary studies suggest that quercetin might have inhibitory effects on various cancer types, including breast cancer, leukemia, colon, ovaries, oral squamous cells, endometrium, stomach, and lung cancer.
Usages associés
Hypoglycemic
Animal and laboratory research has shown that quercetin improves glucose metabolism. Diabetic rats receiving quercetin (15 mg/kg/day) for 4 weeks also experienced a reduction in blood glucose and an increase in plasma insulin, calcium, and magnesium. In human research, taking quercetin increased levels of adiponectin, a hormone involved in blood sugar control. Quercetin also decreases resistin, an adipokine involved in inducing insulin resistance.
The effects of quercetin supplementation on metabolic and hormonal parameters as well as plasma concentration and gene expression of resistin in overweight or obese women with polycystic ovary syndrome
Safety dosage
Adult from 18 year(s): 200 mg - 1000 mg
Quercetin has been used safely at doses of up to 500 mg twice a day for a duration of up to 12 weeks.
Interactions
Médicaments
Antidiabetic: moderate interaction
Research results in humans suggest that a combination of quercetin, myricetin, and chlorogenic acid reduces fasting glucose levels in patients with type 2 diabetes, even those on antidiabetic medication. While the effect of quercetin alone is unknown, in theory, the concurrent use of quercetin and antidiabetics could increase the risk of hypoglycemia. Dose adjustment may be necessary.
Antihypertensive: moderate interaction
Quercetin may slightly lower blood pressure in people with mild hypertension. Theoretically, combining quercetin with other herbs and supplements with hypotensive effects could increase the risk of hypotension.
Cyclosporine: moderate interaction
A small study in healthy volunteers shows that pretreatment with quercetin increases plasma levels and prolongs the half-life of a single dose of cyclosporine, possibly due to the inhibition of p-glycoprotein or cytochrome P450 3A4 (CYP3A4) metabolism of cyclosporine.
Cytochrome P450 2C9: moderate interaction
A small clinical study in healthy volunteers shows that taking 500 mg of quercetin twice a day for 10 days before taking diclofenac increases plasma levels of diclofenac by 75% and prolongs its half-life by 32.5%. Diclofenac is a substrate of cytochrome P450 2C9 (CYP2C9). Animal research also shows that pretreatment with quercetin increases plasma levels and prolongs the half-life of losartan, a substrate of CYP2C9. Additionally, laboratory research shows that quercetin inhibits CYP2C9. Theoretically, using quercetin with drugs metabolized by CYP2C9 could result in reduced drug elimination, increased serum drug levels, and increased effects. Some CYP2C9 substrates include celecoxib, diclofenac (Voltaren), fluvastatin, glipizide, ibuprofen (Advil), irbesartan, losartan, phenytoin, piroxicam, tamoxifen, tolbutamide, torsemide, and warfarin.
Diclofenac: moderate interaction
A small clinical study in healthy volunteers shows that taking 500 mg of quercetin twice a day for 10 days before taking diclofenac increases plasma levels of diclofenac by 75% and prolongs its half-life by 32.5%. Concurrent use of quercetin and diclofenac could increase the effects and side effects of diclofenac.
Diltiazem: moderate interaction
In vivo pretreatment with quercetin resulted in increased bioavailability of diltiazem, a calcium channel blocker, which could result from P-glycoprotein and CYP 3A4 inhibition. Use with caution; dose adjustments may be necessary.
Midazolam: moderate interaction
A small clinical study shows that quercetin may increase the metabolism of midazolam.
P-GLYCOPROTEIN substrate: moderate interaction
There is preliminary evidence that quercetin inhibits the P-glycoprotein efflux pump in the gastrointestinal tract. This inhibition could increase the bioavailability and serum levels of drugs transported by the pump, such as paclitaxel, diltiazem, losartan, cyclosporine, saquinavir, and digoxin. Use quercetin with caution in combination with P-glycoprotein substrates. Drugs transported by the pump include certain chemotherapy agents (etoposide, vinblastine, vincristine, vindesine), antifungals (ketoconazole, itraconazole), protease inhibitors (amprenavir, indinavir, nelfinavir), H2 antagonists (cimetidine, ranitidine), verapamil, corticosteroids, erythromycin, fexofenadine, loperamide, quinidine, and others.
Pravastatin: moderate interaction
In vitro evidence shows that quercetin may inhibit OATP 1B1-mediated pravastatin uptake. Additionally, preliminary evidence shows that taking quercetin with pravastatin increases the maximum concentration of pravastatin by 24%, prolongs the half-life of pravastatin by 14%, and decreases the apparent clearance of pravastatin by 18% in healthy men, suggesting that quercetin slightly inhibits pravastatin uptake into liver cells. Theoretically, concomitant use of quercetin and pravastatin could increase the therapeutic effects and side effects of pravastatin.
Quinolones: moderate interaction
Theoretically, quercetin could competitively inhibit quinolone antibiotics by binding to the DNA gyrase site on bacteria. Quinolones (fluoroquinolones) include ciprofloxacin, gemifloxacin, levofloxacin, moxifloxacin, and others.
Pioglitazone: moderate interaction
Quercetin may increase the bioavailability of pioglitazone. Due to the potential for toxicity, careful monitoring of liver and heart functions is necessary.
Plantes ou autres actifs
Hypotensive: mild interaction
Quercetin may lower blood pressure in people with mild hypertension. Theoretically, combining quercetin with other herbs and supplements with hypotensive effects could increase the risk of hypotension.
Quercetin: mild interaction
Quercetin is an iron chelator and therefore should not be ingested at the same time as iron supplements or iron-rich foods to avoid reduced absorption. Space out by 2 hours.
Precautions
Pregnant women: use with caution
Quercetin is widely available in the diet and is considered safe when consumed in food amounts (4 to 40 mg per day). The safety of higher oral doses is unclear, as some research indicates that quercetin crosses the placenta and accumulates in the fetus, and relatively high consumption of quercetin (200 to 1800 mg/day) may have adverse effects.
Breastfeeding women: use with caution
No data available, use with caution at dietary doses.
Thyroid disorders: avoid
Quercetin may have some antithyroid properties, according to a study conducted by Giuliani C et al. (2008). It should be used cautiously in cases of thyroid diseases.
