Quercetin

We evaluated 104 human studies on Quercetin, including 20 randomised controlled trials and 11 meta-analyses or systematic reviews; the controlled studies include 1,228 participants in total. As of 10 October 2026.

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1. What Is Quercetin? Occurrence, Origin and Forms

Quercetin is a plant pigment belonging to the flavonols, a subgroup of the flavonoids. It occurs in many plants, including onions, apples, capers, buckwheat and tea. It is used as a component of plant-based foods and herbal preparations, and the isolated substance is offered in supplement form.

In plants quercetin occurs mainly bound to sugars as glycosides, with high amounts in the outer layers of leaves and fruits. In research it is studied as an isolated substance or in defined preparations.

Quercetin is obtained by extraction from plant material; the sugar part can be removed or exchanged to give the aglycone or specific glycosides. Studies use different forms, including the aglycone, glycosides such as isoquercetin and rutin, and complexes with phospholipids.

Forms and Preparations Studied

• Quercetin aglycone, often as the dihydrate
• Isoquercetin (quercetin-3-O-glucoside)
• Rutin (quercetin-3-O-rutinoside)
• Quercetin phytosome, a complex with phospholipids
• Enzymatically modified isoquercitrin (EMIQ)

Not covered on this page:

• Other flavonols such as kaempferol, fisetin and myricetin
• Taxifolin, also called dihydroquercetin
• Quercetin-rich foods as such, for example onions and apples
• Quercetin metabolites in blood, for example quercetin-3-O-glucuronide

2. What Does Quercetin Do? Results by Area

How to read the table: counted are human studies whose results concern the area. “High” means at least one meta-analysis and two controlled studies showing improvement, or at least three controlled studies of which three quarters show improvement, with at least 300 participants in total. “Moderate” means at least two controlled studies showing improvement, as a majority, or one meta-analysis alongside controlled studies. “Low” means one controlled study showing improvement. Where an area has no controlled study, the strength of evidence is at most “low” (result “reviews only” or “observational data only”). Everything else is “unclear”. A study counts as showing improvement when at least half of its outcomes in this area improved. Reviews of combination products do not count as meta-analyses; results that only describe side effects or shares of users do not count as an effect. The same study is counted only once.

3. What the Studies Show in Detail

Quercetin, Heart and Circulation: Human Studies

A meta-analysis of placebo-controlled trials found lower systolic and diastolic blood pressure with quercetin, significant only at higher doses 6. A large meta-analysis on micronutrients counts quercetin among the substances with medium- to high-quality evidence for lower cardiovascular risk factors 1. At high altitude, blood pressure in healthy men was lower with quercetin than with placebo 14. In male smokers, it fell with quercetin-rich onion skin extract compared with baseline, and HDL cholesterol rose more than with placebo 30. In men before a bypass operation, quercetin improved vascular relaxation in the removed artery tissue, but not in women 15. Atrial fibrillation after the operation occurred less often with quercetin than with placebo 15. In sickle cell disease, isoquercetin lowered blood coagulation and platelet aggregation compared with baseline; for the primary endpoint P-selectin, it did not differ from placebo 19. There are results without a difference: for blood pressure and endothelial function, no advantage was discernible in a study with quercetin from onion skin extract added to a test meal 24, and an isoquercetin mixture with sodium nitrite changed neither flow-mediated vasodilation nor biomarkers of endothelial dysfunction 16. The findings come predominantly from small, short studies in healthy or already ill adults; the overall reliability is high, but transferability to long-term use and different quercetin forms remains open.

Further studies in this area: 57 61 72 74 78 83 90

Quercetin, Inflammation and Immune System: Human Studies

In this area, studies on inflammation and the immune system predominantly show no advantage. No difference from placebo was found for vaccine responses 18, IL-6 and sVCAM-1 30, hs-CRP after a test meal 24, and soluble P-selectin 19. Compared with baseline, isoquercetin in sickle cell disease inhibited tissue factor gene expression of mononuclear cells and protein disulfide isomerase reductase in plasma 19. The strength of evidence is medium; various forms such as aglycone, isoquercetin and quercetin glycoside were tested. A meta-analysis on dietary polyphenols in rheumatoid arthritis, including studies with quercetin alone, found for the polyphenols overall lower disease activity as well as lower CRP and a lower erythrocyte sedimentation rate 2. In a randomized controlled trial in COVID-19, CRP and ferritin fell more with a mixture of quercetin, vitamin C and bromelain than with routine treatment alone; intubations and deaths were not less frequent 13. A randomized controlled trial on isoquercetin plus sodium nitrite in chronic kidney disease before dialysis found no difference in inflammatory biomarkers 16. A randomized controlled trial on the mixture NS-5 of resveratrol, pterostilbene, quercetin, δ-tocotrienol and nicotinic acid found lower nitric oxide and CRP values compared with baseline 29. Mixture preparations do not allow any statement about quercetin alone.

Further studies in this area: 33 43 44 45 50 57 61 63 64 69 72 73 75 77 80 81 82 87

Quercetin and Cellular Ageing: Human Studies

At high altitude, the quercetin group had lower values of the lipid peroxidation marker malondialdehyde and retained its glutathione 14. In a placebo-controlled crossover study with men, quercetin reduced oxidative DNA damage after intensive interval training 32. In men with coronary heart disease, quercetin reversed senescence and inflammation patterns in vascular cells 15. A randomized controlled trial on the mixture of isoquercetin and sodium nitrite found no difference from placebo for oxidative stress 16. With the mixture NS-5 of resveratrol, pterostilbene, quercetin, δ-tocotrienol and nicotinic acid, the total antioxidant status increased compared with baseline 29. The strength of evidence is moderate; the studies are mostly small and short, some not randomized, and the results apply only to the form and preparation studied in each case, while for mixed preparations no statement about quercetin alone is possible.

Quercetin, Absorption and Metabolism: Human Studies

For the absorption of quercetin, small controlled studies tested formulations intended to improve absorption. A randomized controlled trial found better bioavailability for quercetin aglycone in a hybrid hydrogel formulation with fenugreek galactomannans than for unformulated quercetin 26; another randomized controlled trial showed better oral absorption for a quercetin phytosome with phospholipids from food lecithin than for unformulated quercetin 27. The results apply in each case only to the formulation studied; direct controlled comparisons of aglycone, isoquercetin, rutin and enzymatically modified isoquercetin are still pending.

Further studies in this area: 90 91

Quercetin, Physical Performance and Recovery: Human Studies

In older adults in strength training, maximum strength increased more with quercetin glycosides than with placebo, accompanied by higher discharge rates of motor units 21. After one week, the recruitment threshold of motor units fell with quercetin glycosides compared with baseline, but not with placebo 22. In a non-randomized controlled study, contractile strength after light or moderate exercise declined more with quercetin glycosides than with placebo 31. The studies are small and tested quercetin glycosides; further sports studies concerned mixtures, for example with mango leaf extract or green tea extract, and do not allow any statement about quercetin alone.

Quercetin, Blood Sugar and Metabolism: Human Studies

A randomized controlled trial with quercetin aglycone from onion skin extract with a test meal showed no difference in triglycerides, cholesterol, insulin, HDL and glucose in overweight to obese adults with hypertension 24. A meta-analysis on nutraceuticals such as vitamin D, inositol and quercetin in polycystic ovary syndrome found overall lower fasting insulin, most pronounced with vitamin D 7. The strength of evidence is low, and the finding applies to a single administration with a test meal; studies on longer intake and other forms are still pending.

Further studies in this area: 76 84

Quercetin, Allergies and Histamine: Human Studies

For allergies and histamine, the statements on quercetin have so far been based on review articles. A review on natural substances in allergic rhinitis and asthma attributes to quercetin an inhibition of the release of alarmins and the maturation of dendritic cells as well as mast cell stabilization 55. A review on quercetin and its lecithin formulation mentions small clinical studies with quercetin alone that reported fewer symptoms and a better quality of life 62. Two further review articles report fewer rhinitis symptoms with quercetin, partly in better absorbable formulations 66, as well as fewer inflammatory cells 71. Larger controlled studies with a clearly named quercetin form are still pending; several of the reported effects come from mixed preparations or preclinical models.

Quercetin: Further Outcomes in Studies

A randomized controlled trial with quercetin bars lowered the frequency and severity of acute mountain sickness in healthy men during ascent to high altitude compared with placebo and improved hemoglobin and hematocrit 14. A randomized controlled trial with quercetin aglycone found an improvement in social responsiveness within the group in adults with autism spectrum disorder, but no difference between the groups 17. The findings apply to the form studied in each case; because of small participant numbers and short duration, larger replications are still pending.

Further studies in this area: 14 17 18 33 34 35 36 39 40 41 42 46 47 48 49 50 52 53 54 56 58 59 60 63 67 68 79 81 85 86 87 88 89 94 95

4. How Is Quercetin Dosed? Preparation, Dose and Duration in Studies

The studies predominantly used quercetin aglycone, sometimes as a dihydrate, as well as isoquercetin, quercetin glycosides and quercetin phytosome. Some investigations tested mixed preparations, for example with vitamin C and bromelain, with sodium nitrite, or with resveratrol and other substances. The doses and dosage forms are shown in the table.

5. Is Quercetin Safe? Side Effects in Studies

The studies predominantly report good tolerability of quercetin glycosides and isoquercetin; serious adverse events were not attributed to the preparation 1819. In one study with muscadine grape skin extract, which contains ellagic acid, quercetin and resveratrol, occasional mild gastrointestinal complaints such as bloating and soft stool occurred 48.

6. What Remains Open: Limits of the Evidence

• Many studies are small and short; data on longer-term use are still pending.
• The results apply only to the form and preparation studied, not to quercetin in general.
• Controlled comparisons with forms such as rutin and enzymatically modified isoquercetin are still pending.
• Several findings come from mixed preparations and do not allow any statement about quercetin alone.
• For allergies and histamine, the statements are based on review articles; larger studies specifically on allergic rhinitis are still pending.

7. Bottom Line: What Is Established About Quercetin

Quercetin is a flavonol that has been studied in various forms. The most consistent finding is a slight lowering effect on blood pressure, especially at higher doses 6. In older adults doing strength training, strength increased more with quercetin glycosides than with placebo 21.

Next, larger and longer studies with a clearly named form are needed, for example on blood pressure and muscle strength in old age.