Quercetin is a flavonoid found in onions, apples, berries, and tea. It acts as an antioxidant, has anti-inflammatory properties, and may support cardiovascular health. It’s also one of the most studied natural antihistamines, which is why it appears in so many hayfever supplement stacks.
This guide covers what quercetin does, where you’ll find it in food, what the research says about its benefits, how much to take, and what side effects to watch for.
What Is Quercetin?
Quercetin belongs to a class of plant compounds called flavonoids — the pigments responsible for the bright colours in fruits and vegetables. In plants, quercetin regulates growth by controlling the transport of the hormone auxin. In humans, it functions primarily as an antioxidant and anti-inflammatory agent.
You consume quercetin every day if you eat a varied diet. Estimates put the average daily intake from food at 25–50mg, though this varies widely depending on how many fruits, vegetables, and teas you include.
What makes quercetin interesting for supplementation is the gap between dietary intake and the doses used in clinical research — typically 500–1,000mg per day, which is difficult to reach through food alone.
Quercetin in Food
Quercetin is found in most plant-based foods, but concentration varies significantly.
Vegetables tend to be the richest sources. Onions contain roughly 300mg/kg, making them the most quercetin-dense common food. Kale follows at around 110mg/kg, with broccoli and lettuce also containing meaningful amounts.
Fruits have lower concentrations per kilogram but you tend to eat more of them in a sitting. Apples contain about 50mg/kg — less than onions, but you’re far more likely to eat a whole apple than a whole onion.
Drinks vary widely. Red wine and grape juice contain a few milligrams per glass. White wine and beer contain almost none. Tea infusions can reach up to 25mg per cup, making tea one of the most concentrated everyday sources.
| Source | Approximate quercetin content |
|---|---|
| Onions | ~300mg/kg |
| Kale | ~110mg/kg |
| Apples | ~50mg/kg |
| Broccoli | ~30mg/kg |
| Tea (infusion) | ~25mg per cup |
| Red wine | ~5mg per glass |
Benefits of Quercetin
The strongest evidence for quercetin relates to cardiovascular health, antihistamine activity, and antioxidant protection. Other areas — antiviral effects, exercise performance, cancer risk — are less conclusive.
Cardiovascular health. Several studies have found that quercetin supplementation reduces blood pressure in both animals and humans. A 2012 review by Larson, Symons, and Jalili in Advances in Nutrition concluded that daily supplementation may improve endothelial function, the ability of blood vessel walls to dilate and constrict properly. Endothelial dysfunction is a precursor to several cardiovascular diseases.
Quercetin may also reduce platelet aggregation, which is relevant to stroke risk. However, a large Finnish study of 10,000 participants (Knekt et al., 2000) found no direct link between quercetin intake and reduced stroke incidence, though apple consumption — a major quercetin source — did show a protective association.
Antioxidant activity. Quercetin can neutralise reactive oxygen species, which damage cells and contribute to ageing and chronic disease. It may help prevent the oxidation of LDL cholesterol, a process that leads to arterial plaque formation. This antioxidant mechanism is one of the proposed explanations for the so-called “French Paradox,” the observation that French populations have relatively low rates of cardiovascular disease despite diets high in saturated fat.
Anti-inflammatory effects. Quercetin blocks certain inflammatory enzymes, though the exact mechanism isn’t fully mapped. The practical result is a measurable reduction in inflammatory markers in several human and animal studies.
Antiviral properties. Laboratory studies have shown quercetin can surround certain viruses, including herpes simplex type I, and inhibit their replication. However, most of this evidence comes from in vitro (test tube) research rather than human trials. The gap between lab results and clinical outcomes remains significant here.
Quercetin and Histamine
Quercetin is often called a “natural antihistamine,” and there is some basis for this. It appears to stabilise mast cells — the immune cells that release histamine when triggered by allergens. By reducing histamine release at the source, quercetin may help manage allergic responses including hayfever symptoms.
This is different from conventional antihistamine medications, which block histamine receptors after the histamine has already been released. Quercetin works upstream of that process.
The evidence is promising but still developing. Most human trials have used doses of 500–1,000mg daily, often combined with bromelain or vitamin C for improved absorption. If you’re considering quercetin for seasonal allergies, it may take several weeks of consistent use before you notice an effect. It’s not a fast-acting remedy like cetirizine or loratadine.
For UK hayfever sufferers, the pollen season typically runs from late March through September. Starting supplementation a few weeks before your usual symptom onset may give the compound time to build up.
Quercetin Dosage
There is no official UK or EU recommended daily intake for quercetin. The EFSA has not set a specific upper limit for supplemental quercetin, though most clinical trials have used doses between 500mg and 1,000mg per day without adverse effects.
A typical quercetin capsule contains 500mg. For general antioxidant support, 500mg daily is a common starting point. For histamine or allergy management, studies have typically used 500–1,000mg daily, split into two doses.
Quercetin is poorly absorbed on its own — estimates suggest only a small percentage of oral quercetin reaches the bloodstream unchanged. This is why many supplements combine it with bromelain or vitamin C, both of which may improve bioavailability.
Timing matters less than consistency. Take it with food if you experience any stomach discomfort, but there’s no strong evidence that timing relative to meals significantly affects absorption.
| Use case | Typical dose range | Notes |
|---|---|---|
| General antioxidant support | 500mg/day | Standard single capsule |
| Hayfever / histamine support | 500–1,000mg/day | Often split AM/PM |
| Combined with bromelain | 500mg quercetin + 100–200mg bromelain | May improve absorption |
Quercetin and Bromelain
Bromelain is an enzyme found in pineapple that has its own anti-inflammatory properties. It’s frequently paired with quercetin in supplements for two reasons: it may enhance quercetin absorption, and it adds complementary anti-inflammatory activity.
The combination is particularly common in supplements marketed for allergy and sinus support. Some supplements also add vitamin C, which acts as an additional antioxidant and may further support quercetin absorption.
There are no known adverse interactions between these three compounds, though quercetin can react with some medications and antibiotics. Check with your GP before combining any new supplement with prescription medication.
Side Effects of Quercetin
At typical supplemental doses (500–1,000mg/day), quercetin is well-tolerated by most adults. Its poor absorption actually works in its favour here — it’s difficult to reach toxic levels through oral supplementation.
Common and uncommon side effects at standard doses are rare. Some people report mild headaches or stomach discomfort, particularly when taking quercetin on an empty stomach. Taking it with food usually resolves this.
At very high doses (intravenous, roughly 100 times the normal oral dose), a study on cancer patients reported pain at the injection site, nausea, vomiting, and temporary signs of mild kidney stress. All symptoms resolved quickly. This is not relevant to normal oral supplementation.
Cancer risk: The evidence is mixed and often misrepresented. One rat study found brain tumours in male rats given quercetin, but only males were affected, suggesting either a hormonal link or statistical anomaly. Other studies have found quercetin reduces cancer risk, and population-level studies have found no negative association. The weight of current evidence leans towards quercetin being safe, but more research is needed.
Pregnancy: A laboratory study (Strick et al., 2000) raised a theoretical concern that quercetin could affect a gene linked to infant leukaemia. Rat studies on foetal development found no negative effects from dietary quercetin. However, as a precaution, pregnant women or those planning pregnancy should consult their GP before supplementing.
Drug interactions: Quercetin can inhibit certain enzymes that metabolise prescription drugs, potentially altering their effectiveness. If you take blood thinners, the anti-platelet effect of quercetin could increase bleeding risk. Always check with your GP or pharmacist before combining quercetin with prescription medication.
Who Should Check with Their GP First
- Blood-thinning medication. Warfarin, aspirin, or other anticoagulants may interact with quercetin’s anti-platelet effects.
- Antibiotics. Particularly fluoroquinolones or tetracyclines, which quercetin can interfere with.
- Pregnancy. Or those planning pregnancy, as a precaution based on limited laboratory data.
- Kidney disease. High-dose studies have shown transient kidney stress, though this is unlikely at normal oral doses.
FAQ
Q: What are the main health benefits of taking a quercetin supplement?
A: The strongest evidence supports three areas: cardiovascular health (reduced blood pressure and improved blood vessel function), antioxidant protection against cell damage, and antihistamine activity that may help with allergies and hayfever. Evidence for antiviral and anti-cancer effects exists but is less conclusive, with most positive results coming from laboratory studies rather than human trials.
Q: What is the recommended daily dosage for quercetin?
A: There is no official UK or EU recommended intake. Clinical trials typically use 500–1,000mg daily. A single 500mg capsule is a common starting point for general use. For hayfever or histamine support, 500–1,000mg split into two daily doses is more typical. Quercetin is poorly absorbed on its own, so look for formulas that include bromelain or vitamin C.
Q: Are there any common side effects of quercetin supplements?
A: At standard oral doses of 500–1,000mg per day, side effects are rare. Some people experience mild headaches or stomach discomfort, which usually resolves by taking the supplement with food. Quercetin can interact with blood thinners and certain antibiotics, so check with your GP if you take prescription medication. Pregnant women should consult their doctor before supplementing.
Q: Is quercetin a natural antihistamine?
A: Yes, quercetin appears to stabilise mast cells, reducing the release of histamine at the source. This differs from conventional antihistamines, which block histamine after it has been released. The effect is not immediate — most studies show benefits after several weeks of consistent supplementation at 500–1,000mg daily.
Q: What foods are highest in quercetin?
A: Onions are the richest common source at roughly 300mg/kg, followed by kale (~110mg/kg), broccoli, and apples (~50mg/kg). Tea infusions can contain up to 25mg per cup. Red wine provides a few milligrams per glass. The average varied diet provides 25–50mg of quercetin per day.
Q: Can I take quercetin with bromelain?
A: Yes. Bromelain, an enzyme from pineapple, is commonly paired with quercetin because it may improve absorption and adds its own anti-inflammatory benefits. There are no known adverse interactions. A typical combined dose is 500mg quercetin with 100–200mg bromelain.
References
- Larson AJ, Symons JD, Jalili T. (2012). Therapeutic potential of quercetin to decrease blood pressure: review of efficacy and mechanisms. Advances in Nutrition, 3(1), 39–46.
- Knekt P, et al. (2000). Quercetin intake and the incidence of cerebrovascular disease. European Journal of Clinical Nutrition, 54, 415–17.
- Strick R, et al. (2000). Dietary bioflavonoids induce cleavage in the MLL gene and may contribute to infant leukemia. Proceedings of the National Academy of Sciences, 97(9), 4790–95.
- Harwood M, et al. (2007). A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity. Food and Chemical Toxicology, 45(11), 2179–2205.
- Formica JV, Regelson W. (1995). Review of the biology of quercetin and related bioflavonoids. Food and Chemical Toxicology, 33(12), 1061–1080.
- Ferrières J. (2004). The French paradox: lessons for other countries. Heart, 90(1), 107–111.
- Hollman PC, et al. (1997). Relative bioavailability of the antioxidant flavonoid quercetin from various foods in man. FEBS Letters, 418(1–2), 152–56.
- Nijveldt RJ, et al. (2001). Flavonoids: a review of probable mechanisms of action and potential applications. American Journal of Clinical Nutrition, 74(4), 418–425.
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any supplement, especially if you are pregnant, have a medical condition, or take prescription medication.
