Quercetin is a plant flavonol found in onions, apples, tea, berries and many other foods, and it is also sold as a concentrated supplement. Human trials have measured small changes in blood pressure readings and endurance capacity at 500 to 1,000 mg a day, while most research on histamine and seasonal comfort is still lab-based. Short-term use at studied doses has been well tolerated, but quercetin can change how some medications are handled by the body.
Key Takeaways
- Quercetin is a flavonol that people already eat every day; US adults average roughly 13 mg a day of total flavonols from food, far below the 500 to 1,000 mg used in supplement studies.
- Absorption depends on the form: quercetin bound to glucose (as in onions and isoquercitrin) was absorbed about twice as well as plain quercetin aglycone in an early human study, and a lecithin-based form raised blood levels in a small manufacturer-run trial.
- Pooled exercise trials found a statistically significant but trivial-to-small effect on endurance capacity, about a 2% difference versus placebo.
- Research on histamine and seasonal comfort is mostly cell and animal work, with only small human trials.
- Quercetin at up to 1,000 mg a day for up to two months has been well tolerated in studies, but long-term safety at these doses has not been established.
- Quercetin can change how some medicines are absorbed and broken down, so talk to your doctor or pharmacist first if you take any prescription medicine, or if you are pregnant or breastfeeding.
Evidence grade: Emerging. Two meta-analyses of small, short randomized trials found modest effects on blood pressure readings and endurance capacity, while most other proposed effects, including histamine and seasonal comfort, rest on lab and animal studies plus a few small human trials.
What Quercetin Is and Where It Comes From
Quercetin belongs to the flavonols, a subgroup of the flavonoid family of plant polyphenols. The NIH Office of Dietary Supplements describes it as a polyphenolic flavonol naturally present in fruits such as apples, vegetables such as onions, and beverages such as wine and, especially, tea.[1] The NIH LiverTox database adds berries, green beans, asparagus, broccoli and tomatoes to that list, and notes that quercetin also occurs in several herbs, including ginkgo, St. John's wort, milk thistle and elder.[2]
Because it is so widespread in plant foods, quercetin is a regular part of a normal diet. LiverTox estimates that a typical Western diet supplies about 4 to 40 mg a day,[2] and an analysis of US national survey data (NHANES 1999 to 2002) found that adults averaged about 13 mg a day of total flavonols, including quercetin.[1] Supplements usually provide 500 to 1,000 mg a day,[2] which is many times more than food provides. That gap matters: food studies and supplement studies are testing very different amounts.
In foods, quercetin is mostly attached to sugars, forming compounds called glycosides. Onions are rich in quercetin glucosides, while tea contains mainly quercetin rutinoside (quercetin bound to the sugar rutinose).[4] Most supplements, by contrast, contain the free form without a sugar, called the aglycone.[3] For more on how flavonoids fit alongside other antioxidant compounds in the diet, see our guide to antioxidant powerhouses.
Forms of Quercetin and How Well They Are Absorbed
Plain quercetin has poor solubility and low oral absorption, so the form you take strongly affects how much reaches the bloodstream.[5] Three human studies help explain the differences.
In a 1995 study, nine healthy volunteers who had an ileostomy (a surgical opening that lets researchers collect what passes through the small intestine) received fried onions, pure quercetin rutinoside or pure quercetin aglycone on separate days. About 52% of the quercetin from onion glucosides was absorbed, compared with 24% for the aglycone and 17% for the rutinoside. The authors concluded that attaching glucose enhances absorption.[4] A 2004 pilot study found that a quercetin glucoside supplement (150 or 300 mg) produced measurable blood levels within 30 minutes.[14]
A 2019 crossover study in 12 healthy volunteers compared a lecithin-based form (quercetin phytosome) with standard quercetin. The phytosome form produced much higher blood levels, which the authors described as up to 20 times more than usually obtained, with no notable side effects. The authors were employees of the company that makes the ingredient, and it was a single-dose study.[5]
| Form | What it is | What human research shows |
|---|---|---|
| Quercetin aglycone | The free form with no sugar attached; the most common supplement form | About 24% absorbed in a small ileostomy study, about half the rate of onion glucosides[4] |
| Quercetin glucosides, including isoquercitrin and enzymatically modified isoquercitrin (EMIQ) | Quercetin bound to glucose, as it occurs in onions; EMIQ is a modified glycoside form used in some trials | Glucose-bound quercetin was absorbed best in the ileostomy study; a glucoside supplement reached blood within 30 minutes[4][14] |
| Quercetin rutinoside (rutin) | Quercetin bound to rutinose, the main form in tea | About 17% absorbed, the lowest of the forms tested[4] |
| Quercetin phytosome | Quercetin combined with food-grade lecithin | Much higher blood levels than standard quercetin in one single-dose, manufacturer-run study of 12 adults[5] |
| Quercetin with bromelain or vitamin C | Common product pairings | The absorption studies cited here tested quercetin on its own or in lecithin form, not these combinations |
Higher blood levels are not the same as proven benefit. None of these absorption studies measured health outcomes, and a form that is better absorbed may also be more likely to interact with medications.
How Quercetin Works in the Body
Like other flavonoids, quercetin has antioxidant activity in laboratory tests. LiverTox summarizes several other actions seen mainly in cell and animal research: it inhibits cyclooxygenase enzymes, reduces the production of leukotrienes and prostaglandins (signaling molecules involved in the body's inflammatory response) and inhibits the release of histamine from immune cells.[2] The NIH exercise fact sheet lists hypothesized mechanisms for exercise effects, including more mitochondria in muscle, lower oxidative stress and improved blood flow, while noting that the actual mechanisms are not known.[1]
Quercetin also interacts with the systems the body uses to process medications. In human studies it changed the activity of drug-metabolizing enzymes, and it inhibited platelet aggregation after a single dose.[12][14] These effects are the main reason quercetin needs more care than its food origin suggests.
A useful reality check comes from LiverTox: although quercetin extracts have been tried for many purposes, it notes they have not been shown to be effective in clinical trials for any medical condition.[2]
What Human Research Has Measured
Blood pressure readings
A 2016 meta-analysis pooled seven randomized, placebo-controlled trials (nine treatment arms, 587 participants). Quercetin lowered systolic pressure by an average of about 3 mm Hg and diastolic pressure by about 2.6 mm Hg compared with placebo. In trials using 500 mg a day or more, the average reductions were about 4.5 and 3 mm Hg, while trials using less than 500 mg a day showed no significant change. The authors noted that formal comparisons between dose groups were not significant and that more studies are needed to judge whether the changes are clinically meaningful.[6]
Exercise and endurance
A 2011 meta-analysis of 11 studies (254 people, median dose 1,000 mg a day, median duration 11 days) found a statistically significant benefit on VO2max and endurance performance, but the size of the effect was between trivial and small, about a 2% improvement over placebo.[7] The NIH Office of Dietary Supplements reviewed the same body of research and concluded that results were inconsistent, ranging from no benefit to a small improvement that might not matter in real-world competition, and that larger trials are needed before any recommendation can be made.[1] A six-week study in 57 ROTC cadets cited in LiverTox found fitness measures similar with quercetin and placebo.[2]
Histamine and seasonal comfort
Quercetin's ability to inhibit histamine release is the main reason it appears in seasonal products, but that evidence comes largely from laboratory studies.[2] Human data are thin. In a 2009 randomized, double-blind trial, 20 Japanese adults with cedar-pollen sensitivity took 200 mg a day of EMIQ or placebo for eight weeks during pollen season. Eye itching and total eye symptom scores were lower with EMIQ, while nasal scores, immune markers such as IgE and most cytokines did not differ significantly.[9] A trial of this size cannot establish an effect, and no large human trials have confirmed it. If you are interested in nutrients studied for normal immune function, our article on essential vitamins for immune function covers the nutrients with established reference values.
Senolytic research
Quercetin is also discussed in aging research because it was one of the first compounds identified as "senolytic," meaning it helped clear aged, non-dividing (senescent) cells in laboratory and animal models. In that research it has usually been tested together with dasatinib, a prescription drug, so the findings describe a drug combination rather than quercetin supplements on their own. A 2020 review by researchers whose institution holds patents in this field reported that early pilot trials in people suggest the approach can reduce senescent cells, and stated that it is too early for senolytics to be used outside clinical trials.[10] For a wider look at this area, see our overview of longevity supplements and our piece on diet and longevity.
Evidence summary
| Outcome | What trials found | Certainty |
|---|---|---|
| Blood pressure readings | Average drop of about 3/2.6 mm Hg across 7 small trials; larger at 500 mg a day or more[6] | Low to moderate: few, short trials with varied populations |
| Endurance capacity (VO2max, performance) | Statistically significant but trivial-to-small effect, about 2%[1][7] | Moderate for a small effect; real-world relevance unclear |
| Absorption by form | Glucose-bound forms and lecithin forms reached higher blood levels than plain aglycone[4][5][14] | Moderate for blood levels; no outcome data |
| Seasonal comfort and histamine | Lab studies show reduced histamine release; one 20-person trial found lower eye symptom scores[2][9] | Very low: mostly lab data |
| Senescent cells | Early pilot studies of a drug combination with dasatinib[10] | Very low: not applicable to quercetin alone |
Doses Used in Studies
Most supplement studies used 500 to 1,000 mg a day, which matches the range LiverTox describes for over-the-counter products.[2] Some studies of glucoside forms used lower amounts.
| Context | Amount studied | Duration |
|---|---|---|
| Typical diet | About 4 to 40 mg a day from food (US adults average about 13 mg of total flavonols)[1][2] | Ongoing |
| Blood pressure trials | Effects seen mainly at 500 mg a day or more[6] | Short-term trials |
| Exercise trials | Most often 1,000 mg a day[1][7] | 1 to 8 weeks |
| Seasonal comfort trial (EMIQ) | 200 mg a day (two 100 mg capsules)[9] | 8 weeks |
| Medication interaction study | 500 mg twice a day[12] | 10 days |
| Platelet study (glucoside form) | 150 or 300 mg, single dose[14] | Single dose |
The FDA considers up to 500 mg per serving of quercetin generally recognized as safe (GRAS) as an ingredient in certain foods and beverages.[1] That is a food-ingredient determination, not a recommended supplement dose.
Timing and How to Take It
No study cited here compared morning and evening use, or taking quercetin with and without food, for any outcome. Several trials split the daily amount, for example 500 mg twice a day.[12] Because the form changes absorption so much, the same milligram amount can produce very different blood levels, so compare products by form as well as by milligrams.[4][5]
Exercise studies typically ran for 1 to 8 weeks,[1] and the seasonal trial began during pollen season and lasted eight weeks.[9] Beyond about 12 weeks at 1,000 mg a day or more, there are no adequate safety data.[3]
Stacking Notes
Quercetin is often sold with vitamin C or bromelain. The human absorption studies reviewed here did not test those combinations, so claims that they boost quercetin's effect are not supported by this evidence.[4][5] Because quercetin inhibited platelet aggregation in a human study,[14] combining it with other supplements that may affect bleeding is worth reviewing with a pharmacist. Onions, apples, berries and tea remain the way most people get quercetin, as a regular part of a normal diet.[2]
Safety and Interactions
Side effects and tolerability
Quercetin supplements are generally well tolerated. LiverTox reports abdominal discomfort, nausea and headache in some studies, usually at rates similar to placebo, and rates quercetin as an unlikely cause of clinically apparent liver injury (likelihood score E), with no published reports of liver injury attributed to it.[2] The NIH exercise fact sheet notes no side effects in athletes and untrained adults taking up to 1,000 mg a day for up to two months, but adds that longer-term use at that amount or more has not been studied.[1] A safety review from Germany's Federal Institute for Risk Assessment likewise found that adverse effects were rare and mild in human studies, and that adequate data on long-term use (more than 12 weeks) of 1,000 mg a day or more are not available.[3]
Kidneys
The same review identified, from animal studies, a possible risk that quercetin could worsen damage in kidneys that are already impaired.[3] In a 1996 phase I trial that gave quercetin by intravenous infusion at very high doses, kidney toxicity was the dose-limiting side effect.[11] Those IV doses are not comparable to oral supplements, but people with kidney problems should talk to their clinician before using quercetin.
Medication interactions
Human and animal studies show that quercetin can change how certain drugs are absorbed and broken down.[3] Talk to your doctor or pharmacist before using quercetin if you take:
- Drugs processed by liver enzymes (CYP substrates). In 12 healthy volunteers, 500 mg of quercetin twice a day for 10 days significantly raised blood levels of diclofenac, a drug cleared by the CYP2C9 enzyme, and the authors advised caution with CYP2C9 substrates.[12]
- Cyclosporine. In rats, repeated quercetin lowered cyclosporine blood levels by changing intestinal and liver drug-handling proteins.[13] Even small changes in this drug's levels can matter.
- Blood thinners (anticoagulant or antiplatelet medicines). A single 150 or 300 mg dose of a quercetin glucoside inhibited platelet aggregation in people,[14] which could add to the effect of these medicines.
- Quinolone antibiotics. In a laboratory binding study, quercetin competed with a quinolone antibiotic for binding at the bacterial enzyme these antibiotics target, though more weakly than the antibiotics themselves.[8] Whether this matters in people is unknown.
Pregnancy and breastfeeding
Quercetin from food is a normal part of the diet,[2] but none of the safety reviews cited here establishes that supplemental doses are safe during pregnancy or breastfeeding. Long-term safety at supplement doses is not established even in healthy adults.[1][3] If you are pregnant, planning a pregnancy or breastfeeding, rely on food sources unless your clinician advises otherwise.
Who should talk to a clinician first
Check with your healthcare provider before starting quercetin if you take any prescription medicine, have kidney problems, are scheduled for surgery, are pregnant or breastfeeding, or plan to use more than 1,000 mg a day or continue beyond a few months.
Frequently Asked Questions
Which foods provide the most quercetin?
Onions, apples, berries, tea, broccoli, asparagus, green beans, tomatoes and red wine are among the food sources named by NIH resources.[1][2] Onions are the best-studied source, and their quercetin is mostly in the glucose-bound form that was absorbed best in human testing.[4]
How much quercetin is in a normal diet compared with a supplement?
Food typically supplies about 4 to 40 mg a day, with US adults averaging about 13 mg of total flavonols.[1][2] Supplement studies usually used 500 to 1,000 mg a day,[2] so a supplement provides many times what food does.
Has quercetin been studied for seasonal comfort?
Yes, but mostly in the lab, where it reduced histamine release from immune cells.[2] One small 2009 trial in 20 adults found lower eye-itching scores with 200 mg a day of EMIQ during pollen season, without significant changes in nasal scores or most immune markers.[9] A varied diet rich in fruits and vegetables naturally supplies quercetin along with other nutrients.[2]
Which form of quercetin is absorbed best?
In human studies, glucose-bound forms such as those in onions and isoquercitrin-type supplements were absorbed better than plain quercetin aglycone, and a lecithin-based phytosome form raised blood levels in a small, manufacturer-run study.[4][5][14] Better absorption has not been shown to translate into better outcomes.
Is quercetin safe to take every day?
Studies of up to 1,000 mg a day for up to about two months found it well tolerated, and LiverTox has not linked it to liver injury.[1][2] Data on longer use at high doses are lacking, and quercetin can interact with some medications, so check with your doctor or pharmacist first.[3]
Is quercetin a longevity supplement?
Quercetin is part of early research on senescent cells, but that work mostly paired it with the prescription drug dasatinib, and researchers say it is too early for use outside clinical trials.[10] There is no human evidence that quercetin supplements alone affect aging.
Sources
- NIH Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals (Quercetin section). Updated 2024.
- LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Quercetin. National Institute of Diabetes and Digestive and Kidney Diseases. Updated 2020.
- Andres S, Pevny S, Ziegenhagen R, et al. Safety Aspects of the Use of Quercetin as a Dietary Supplement. Molecular Nutrition & Food Research. 2018.
- Hollman PC, de Vries JH, van Leeuwen SD, Mengelers MJ, Katan MB. Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers. American Journal of Clinical Nutrition. 1995.
- Riva A, Ronchi M, Petrangolini G, Bosisio S, Allegrini P. Improved Oral Absorption of Quercetin from Quercetin Phytosome, a New Delivery System Based on Food Grade Lecithin. European Journal of Drug Metabolism and Pharmacokinetics. 2019.
- Serban MC, Sahebkar A, Zanchetti A, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association. 2016.
- Kressler J, Millard-Stafford M, Warren GL. Quercetin and endurance exercise capacity: a systematic review and meta-analysis. Medicine & Science in Sports & Exercise. 2011.
- Hilliard JJ, Krause HM, Bernstein JI, et al. A comparison of active site binding of 4-quinolones and novel flavone gyrase inhibitors to DNA gyrase. Advances in Experimental Medicine and Biology. 1995.
- Kawai M, Hirano T, Arimitsu J, et al. Effect of enzymatically modified isoquercitrin, a flavonoid, on symptoms of Japanese cedar pollinosis: a randomized double-blind placebo-controlled trial. International Archives of Allergy and Immunology. 2009.
- Kirkland JL, Tchkonia T. Senolytic drugs: from discovery to translation. Journal of Internal Medicine. 2020.
- Ferry DR, Smith A, Malkhandi J, et al. Phase I clinical trial of the flavonoid quercetin: pharmacokinetics and evidence for in vivo tyrosine kinase inhibition. Clinical Cancer Research. 1996.
- Bedada SK, Neerati P. Evaluation of the effect of quercetin treatment on CYP2C9 enzyme activity of diclofenac in healthy human volunteers. Phytotherapy Research. 2018.
- Liu Y, Luo X, Yang C, et al. Impact of quercetin-induced changes in drug-metabolizing enzyme and transporter expression on the pharmacokinetics of cyclosporine in rats. Molecular Medicine Reports. 2016.
- Hubbard GP, Wolffram S, Lovegrove JA, Gibbins JM. Ingestion of quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in humans. Journal of Thrombosis and Haemostasis. 2004.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
