Each review contains information about the ingredient’s clinical applications, formulations, dosing & administration, adverse effects, and pharmacokinetics. Learn more about our critical appraisal research or contact us for initial guidance and more information.

Quercetin

Quercetin (derived from the Latin word quercetum, meaning “oak forest”) is an abundant yellow bioactive flavonoid found in many plant-based foods. (3)(65) In Western diets, daily intakes have been estimated to range between 3 to 40 mg, though intake may be as much as 250 mg in individuals with high fruit and vegetable consumption. (2) Apples, berries, broccoli, cabbage, cauliflower, grapes, nuts, onions, bell pepper, red wine, black and green tea, and tomatoes all contain quercetin, but cooking these foods can substantially reduce their quercetin content. (10) Many popularly used botanicals, including Ginkgo biloba, St. John’s wort, milk thistle, or elderberry, also contain small amounts of quercetin. (45)

Quercetin possesses antioxidative, anti-inflammatory, anti-carcinogenic, anti-microbial, and immunomodulating properties. (2)(64)(65) Though clinical studies in humans are limited, quercetin may have applications in allergic diseases, (24) bone health, (63) cancer, (46)(61) cardiovascular disease, (18)(39) diabetes, (6)(53) neurocognitive disorders (1)(27)(59) obesity, (8) viral infection, (30) and wound healing. (43) It is possible that, to date, the clinical use of quercetin has been limited by its low bioavailability; however, as new formulations are developed, improved therapeutic benefits may be achieved. (3)

Main uses

Antioxidation
Blood pressure and endothelial function
Muscle fatigue in resistance exercise
Allergies
Pain

Formulations

Form	Characteristics
Quercetin (aglycone/anhydrous)	Supplements typically contain quercetin in its isolated aglycone or anhydrous form by removing the glucose and water molecules from quercetin that are naturally present in plants and herbs. This is the purified, lipophilic form of quercetin. (11) Providing quercetin in a lecithin formulation (Phytosome ®) increased the bioavailability of quercetin aglycone by up to 20x in humans. (49)
Quercetin dihydrate	Supplements may use quercetin dihydrate, which attaches two water molecules to isolated quercetin. By weight, this form contains ~90% quercetin. An individual would require 16.6 mg of quercetin dihydrate to achieve an equivalent bioavailability to 1 mg from red onion in humans. (54) The provision of 155 mg of quercetin glycones (from various sugar moieties) from onion skin extract was approximately 5x more bioavailable than 134 mg of quercetin aglycone from quercetin dihydrate in humans. (7)
Enzymatically modified isoquercitrin (EMIQ ®)	EMIQ increases the water solubility of quercetin. EMIQ was more bioavailable than natural quercetin glucosides in humans. (33)

Dosing & administration

Allergies
General outcomes from A-level evidence	No data currently available.
Dosing & administration	50 mg (as EMIQ) twice per day for eight weeks to Px with Japanese cedar pollinosis
Outcomes	↓ ocular symptom scores trends for other allergic symptom scores were also observed. (22)(26)
Class of evidence	C

Antioxidation
General outcomes from A-level evidence	No data currently available.
Dosing & administration	500 mg twice per day for two weeks to healthy individuals undertaking resistance exercise
Outcomes	↑ erythrocyte GSH/GSSG ratio (51%) & plasma GSH/GSSG ratio (20%) ↓ erythrocyte GSSG (30%), plasma GSSG (17%), erythrocyte SOD/GPx ratio (12.5%), erythrocyte TBARS (25-32%), plasma TBARS (13%), & erythrocyte oxidation susceptibility (10%) post-exercise compared to placebo (14)
Class of evidence	C
Dosing & administration	42 mg (as EMIQ) per day for four months added to 20 g of whey protein to athletes undertaking a resistance training program
Outcomes	↑ antioxidative status (17%) compared to whey protein alone (36)
Class of evidence	C

Athletic performance
General outcomes from A-level evidence	↑ mean power output (0.74-2.82%) & VO2max (~2-3%) in endurance exercise, but these effects are likely too small to have an important difference in healthy individuals (29)(41)(57)
Dosing & administration	500 mg twice per day for one week to healthy untrained participants undergoing endurance exercise
Outcomes	↑ VO2max (3.9%), time to fatigue (13.2%), (12) & greater distance covered in a fixed time (2.9%)(34)
Class of evidence	B
Dosing & administration	500 mg twice per day for two weeks to healthy moderately active volunteers undergoing resistance exercise
Outcomes	↓ muscle weakness caused by eccentric exercise compared to placebo ↑ isometric strength (4.7%) compared to baseline before initiation of the exercise protocol (4) Note: a single dose ingested three hours before resistance exercise attenuated muscle weakness (40)
Class of evidence	C
Dosing & administration	42 mg (as EMIQ) per day for four months added to 20 g of whey protein to athletes undertaking a resistance training program
Outcomes	↑ lower limb fat-free mass and muscle mass (~3x) compared to whey protein alone (36)
Class of evidence	C

β-Thalassemia major
General outcomes from A-level evidence	No data currently available.
Dosing & administration	500 mg per day for 12 weeks to Px with β-Thalassemia major
Outcomes	Improved iron status via reduction in serum iron (3%), ferritin (5%), transferrin (7%), & transferrin saturation (7%) (51)
Class of evidence	B

Blood pressure
General outcomes from A-level evidence	↓ SBP (3.04-3.09 mmHg) & DBP (2.63-2.86 mmHg) (23)(52) ↓ SBP (1.69 mmHg) but not DBP in Px with metabolic disorders (59)
Dosing & administration	500 mg per day for a minimum of eight weeks
Outcomes	↓ SBP (4.45 mmHg) & DBP (2.98 mmHg) (52)
Class of evidence	A
Dosing & administration	54 mg (fromm onion peel extract) three times per day for six weeks to obese/overweight Px with hypertension
Outcomes	↓ SBP (3.9 mmHg) compared to placebo Note: no changes were observed in Px with prehypertension (5)
Class of evidence	B
Dosing & administration	365 mg twice per day for four weeks to Px with hypertension
Outcomes	↓ SBP (7 mmHg), DBP (5 mmHg), & mean arterial pressure (5 mmHg) compared to baseline, but no change in placebo no changes were observed in Px with prehypertension (15)
Class of evidence	C

Endothelial function
General outcomes from A-level evidence	No data currently available.
Dosing & administration	50 mg (from onion peel extract) twice per day for 12 weeks to healthy overweight/obese individuals
Outcomes	↑ flow-mediated dilation (2.7%) & endothelial progenitor cell count (61%) compared to baseline, but placebo did not change (9)
Class of evidence	B
Dosing & administration	160 mg (as isoquercitrin) per day for four weeks to Px with prehypertension
Outcomes	↓ plasma sE-selectin (marker of endothelial dysfunction, 7.4 ng/mL difference), IL1ꞵ (marker of inflammation, 0.23 pg/ml difference), & methylglyoxal (precursor to advanced glycation end products, 40.2 nmol/L difference) compared to placebo (13)(62)
Class of evidence	C

Metabolic and inflammatory profile
General outcomes from A-level evidence	↓ FBG (~1.0 mg/dL), insulin (1.57 μIU/mL), total cholesterol (large effect, SMD= 0.98), LDL-C (large effect, SMD= 0.88), CRP (0.33 mg/L, moderate effect, SMD = 0.64) in Px with metabolic disorders, (37)(31)(58) & TGs (24.54 mg/dL) and IL-6 (moderate effect, SMD= 0.69) only at higher doses (>500 mg/day) (38)(50) Note: individual analyses show no effect was observed on glucose, insulin, insulin resistance, HbA1C, TGs, total cholesterol, LDL-C, HDL-C, IL-6, or TNF-α (19)(23)(37)(38)(50)(58)
Dosing & administration	500 mg per day for a minimum of eight weeks
Outcomes	↓ FBG (~1.0 mg/dL), insulin (1.57 μIU/mL), (37) & CRP (0.34 mg/L) (31)
Class of evidence	A
Dosing & administration	500 mg per day for a minimum of four weeks
Outcomes	↓ TGs (24.54 mg/dL) (50)
Class of evidence	A
Dosing & administration	150 mg (as quercetin dihydrate) per day for 6-8 weeks to Px at high risk for cardiovascular disease with ApoE3 genotype
Outcomes	↓ waist circumference (0.63 cm), SBP (2.6-5.7 mmHg, with greater effect in hypertensive or in younger Px), TGs (11%), & atherogenic oxidized LDL (92%) compared to placebo HDL-C (2.32 mg/dl) & TNF-ɑ (0.11 pg/ml) (16)(17)(42)
Class of evidence	B
Dosing & administration	500 mg (as quercetin dihydrate) per day for four weeks to healthy Px
Outcomes	↓ uric acid (26.5 µmol/l) compared to baseline, but no change in placebo (55)
Class of evidence	C

Polycystic ovary syndrome (PCOS)
General outcomes from A-level evidence	May improve hyperandrogenemia, ovarian histomorphology, folliculogenesis, and luteinisation processes, as well as insulin resistance and chronic inflammation (58)
Dosing & administration	500 mg twice per day for 12 weeks to women with PCOS
Outcomes	↓ FBG (1.2-1.21 mg/dl), insulin (1.5-1.6 μU/ml), insulin resistance (7-18%), resistin (0.81 ng/ml), weight (0.33-0.34 kg), BMI (0.13-0.20 kg/m2), waist circumference (0.5 cm), testosterone (0.06-0.9 ng/ml), & LH (0.11-0.13 mIU/ml) as differences from baseline, but no change in placebo adiponectin (0.82 ng/ml), high molecular weight adiponectin (0.38 ng/ml), & SHBG (0.31-0.32 nmol/l) as differences from baseline, but no change in placebo (28)(48) Note: upregulation of adiponectin receptors (ADIPOR1 & 2) and AMPK were also observed (47)
Class of evidence	B

Prostatitis
General outcomes from A-level evidence	No data currently available.
Dosing & administration	500 mg twice per day for four weeks to Px with chronic prostatitis
Outcomes	↓ NIH symptom score (38%) compared to baseline, but placebo did not change ↑ percentage of Px (47% difference) with at least a 25% improvement in symptoms versus placebo Note: improvements were primarily driven by improvement in pain score and QoL, but little change in urinary score (56)
Class of evidence	C

Rheumatoid arthritis
General outcomes from A-level evidence	No data currently available.
Dosing & administration	500 mg per day for eight weeks to Px with rheumatoid arthritis as adjunct therapy
Outcomes	↓ morning pain (47%), post-activity pain (29%), disease activity 28 score (15%), health assessment questionnaire score (49%), & hs-TNFα (19%) compared to standard therapies with placebo (25)
Class of evidence	B

Upper respiratory tract infections (URTI)
General outcomes from A-level evidence	No data currently available.
Dosing & administration	500 mg twice per day for five weeks to healthy Px undergoing training endurance training protocol
Outcomes	↓ incidence of URTI (89%) post-exercise compared to placebo (34)
Class of evidence	C

Adverse effects

Quercetin is considered to be well-tolerated, and mild adverse events are rarely reported. However, published safety information from studies that provide doses higher than 1,000 mg for longer than 12 weeks is not widely available. (2) Many analyses do not show a difference in adverse effects between quercetin and placebo groups or indicate that no adverse events were reported in individual trials. (23)(38)(52)(57)

Pharmacokinetics

Absorption

If ingested in plant-based food, quercetin glycosides are hydrolyzed by β-glucosidases to the lipophilic quercetin aglycone form before passive intestinal transport. (2)(3)
Quercetin glycosides that are not hydrolyzed may also be uptaken by the sodium/glucose cotransporter-1. (2)
Overall, the absorption of quercetin is considered to be low but highly variable between individuals. Absorption may improve in the presence of the sugar moiety, non-digestible fiber, dietary fat, when quercetin aglycone is consumed as part of a food component (vs. purified), or in various nano-formulations. (20)

Distribution

In animal models, the highest levels were found in the liver, small intestine, lungs, testes, and kidneys. (2)

Metabolism

Regardless of the ingested form, quercetin is conjugated by phase II enzymes (glucuronidated, sulfated, or methylated) prior to circulation in plasma. (2)(20)

Excretion

Quercetin is eliminated via the feces and urine, mainly as 3-hydroxyphenylacetic acid, benzoic acid, and hippuric acid. (20)
Quercetin’s half-life is between 3.5 to 7.5 hours (32)(49)