Quercetin is a flavonoid found in many common foods. If you’ve eaten an apple, sipped red wine, or had onions in your salad, you’ve consumed quercetin. It’s one of the most abundant flavonoids in the human diet.

The name comes from the Latin “quercetum” meaning oak forest, though you’ll find quercetin in far more than oak trees. It gives many fruits and vegetables their colour - the red of apples, the deep purple of certain grapes.

What makes quercetin interesting from a medical perspective is its supposed antioxidant and anti-inflammatory effects. Supplement manufacturers have latched onto these properties, and you’ll now find quercetin capsules in most health food shops. But does the research support the marketing claims? That’s what we’ll examine here.

What is quercetin?

Quercetin belongs to the flavonol subclass of flavonoids. These compounds are plant pigments that have evolved over hundreds of millions of years, primarily to protect plants from UV radiation, pathogens, and herbivores.

In laboratory studies, quercetin shows antioxidant activity, meaning it can neutralise free radicals. It also appears to have anti-inflammatory properties in cell cultures. The question, as always, is whether these test-tube findings translate to meaningful benefits when you swallow a quercetin supplement.

The human body doesn’t absorb quercetin particularly well. Estimates suggest bioavailability is somewhere between 2% and 20%, depending on the food source and what else you’ve eaten 1. Supplement manufacturers sometimes add vitamin C or bromelain to improve absorption, though evidence for this is limited.

Food sources of quercetin

You don’t need supplements to get quercetin. It’s present in a wide variety of foods:

Vegetables: Onions (particularly red onions), broccoli, asparagus, green peppers, tomatoes, leafy greens

Fruits: Apples (the skin contains most of it), cranberries, grapes, blueberries, strawberries, blackcurrants

Beverages: Green tea, red wine

Other sources: Capers (one of the richest sources), ginkgo biloba, ginseng

The typical Western diet provides roughly 10-30 mg of quercetin daily, while supplement doses usually range from 500-1000 mg. That’s a substantial difference, and it’s worth keeping in mind that high-dose supplements may behave differently than dietary quercetin.

What does the research actually show?

1. Adjunctive treatment for COVID-19

During the pandemic, researchers investigated whether quercetin might help with COVID-19 outcomes. One randomised controlled trial enrolled 152 outpatients with COVID-19 and gave half of them 1000 mg of quercetin daily alongside standard care 2.

The quercetin group had fewer hospitalisations, less need for oxygen therapy, and shorter illness duration. Sounds promising, but I have reservations. The trial was open-label (everyone knew who was getting quercetin), the sample size was small, and the study hasn’t been replicated in larger trials.

My take: Interesting preliminary data, but I wouldn’t stock up on quercetin for this purpose. We’d need much larger, blinded studies to draw firm conclusions.

2. Aerobic exercise performance

Given quercetin’s antioxidant properties, some researchers hypothesised it might improve athletic performance by reducing exercise-induced oxidative stress.

A meta-analysis pooled data from 7 randomised controlled trials with 288 participants. The result? Quercetin didn’t significantly improve endurance performance or VO2max 3.

Reality check: This one doesn’t seem to work. Athletes looking for performance gains should probably spend their money elsewhere.

3. Blood pressure regulation

Hypertension affects roughly a third of adults in developed countries. A systematic review and meta-analysis of 7 studies (587 participants) found that quercetin supplementation reduced blood pressure by an average of 3.04/2.63 mmHg (systolic/diastolic) 4.

The effect was most pronounced at doses above 500 mg daily and with more than 8 weeks of use.

Worth noting: A 3 mmHg drop in blood pressure isn’t nothing. At a population level, reductions of this magnitude could meaningfully reduce cardiovascular events. But for an individual, it’s a modest effect. If your blood pressure is 150/95, quercetin alone won’t get you to target.

4. Weight loss

Obesity researchers have explored quercetin because animal studies suggested it might influence fat metabolism.

A meta-analysis of 9 randomised controlled trials (525 participants) found quercetin had no significant effect on body weight, BMI, waist circumference, or waist-to-hip ratio 5.

In practice: Don’t buy quercetin expecting to lose weight. The evidence simply isn’t there.

5. Blood sugar control

For people with metabolic syndrome or type 2 diabetes, blood sugar management is obviously important. A systematic review of 9 trials (781 patients) examined whether quercetin helped 6.

Overall, quercetin didn’t significantly affect fasting glucose, insulin resistance, or HbA1c. However, subgroup analyses suggested some benefit with doses of 500 mg or more taken for at least 8 weeks, and in participants younger than 45.

My assessment: The overall results are negative, but the subgroup findings hint that certain people might benefit. I’d want to see targeted trials in younger people before making recommendations.

6. Inflammation markers

Chronic low-grade inflammation is implicated in cardiovascular disease, diabetes, and possibly cancer. C-reactive protein (CRP) is a commonly measured marker of systemic inflammation.

A meta-analysis of 7 studies (549 participants) found quercetin reduced CRP levels, particularly at doses above 500 mg daily and in people whose baseline CRP was below 3 mg/L 7.

Another meta-analysis of 15 trials confirmed anti-inflammatory effects, showing reductions in both CRP and interleukin-6 in participants with existing health conditions 8.

The catch: Reducing inflammation markers sounds good, but we don’t know if this translates to fewer heart attacks or longer life. CRP is a surrogate endpoint, not a clinical one.

7. Cholesterol levels

A systematic review of 16 placebo-controlled trials (1575 participants with metabolic syndrome and related conditions) found quercetin supplementation reduced total cholesterol, LDL cholesterol, and CRP 9.

However, quercetin didn’t improve triglycerides or raise HDL (the “good” cholesterol).

Worth considering: The lipid effects are modest but consistent. For someone already doing everything else right (diet, exercise, possibly statins), I’m not sure quercetin would add much. But it’s not nothing.

8. Upper respiratory tract infections

A randomised controlled trial gave 1002 participants either a quercetin-containing supplement or placebo for 12 weeks during cold season 10.

Overall, quercetin didn’t reduce the incidence or severity of upper respiratory infections. However, a subgroup of healthy participants over 40 who took 1000 mg daily did have milder infections and fewer sick days.

My take: The overall result was negative. The subgroup finding is intriguing but needs confirmation. I wouldn’t rely on quercetin to ward off colds.

9. Allergic conditions

This is where quercetin gets called a “natural antihistamine,” and the laboratory evidence is genuinely interesting. Quercetin appears to stabilise mast cells (which release histamine), reduce pro-inflammatory cytokines, and inhibit IgE antibody formation in test tubes 11.

The problem? Most of this evidence comes from cell cultures and animal studies. Human trials are scarce. If you have seasonal allergies, I wouldn’t count on quercetin to replace your antihistamines.

Reality check: The mechanisms are plausible, but we need human trials. Lab findings don’t always translate to real-world benefits. For more options on managing allergies, see our article on health foods that may help with allergies.

Side effects of quercetin

At typical supplement doses (up to 1000 mg daily for up to 12 weeks), quercetin appears reasonably safe for most healthy adults 12.

Reported side effects include:

Nausea
Headache
Stomach discomfort
Tingling sensations in the arms or legs (paraesthesia)

Long-term safety data beyond 12 weeks is limited. As with many supplements, we simply don’t have the extensive safety monitoring that pharmaceutical drugs receive.

Contraindications and drug interactions

1. Impaired liver or kidney function

The safety of quercetin in people with compromised liver or kidney function hasn’t been established. Since these organs process and excrete supplements, there’s theoretical concern about accumulation or adverse effects.

2. Pregnancy and breastfeeding

No adequate safety data exists for quercetin supplementation during pregnancy or lactation. The quercetin in food is presumably fine, but concentrated supplements are a different matter.

3. Quinolone antibiotics

Quercetin may reduce the absorption of quinolone antibiotics, potentially making them less effective. If you’re taking ciprofloxacin, norfloxacin, levofloxacin, or similar antibiotics, avoid quercetin supplements.

4. Drugs metabolised by cytochrome P450 enzymes

This is where things get complicated. Quercetin can inhibit several cytochrome P450 enzymes, potentially increasing blood levels of medications metabolised through these pathways. The list is extensive and includes:

Heart and blood pressure medications: Verapamil, diltiazem, losartan, propranolol, warfarin

Cholesterol medications: Lovastatin, fluvastatin

Immunosuppressants: Cyclosporin

Psychiatric medications: Amitriptyline, imipramine, paroxetine, fluoxetine, sertraline, haloperidol

Pain medications: Codeine, tramadol, fentanyl

Other common drugs: Omeprazole, lansoprazole, diclofenac, ibuprofen

If you take prescription medications, discuss quercetin supplementation with your doctor or pharmacist before starting. This isn’t a supplement to combine casually with a medicine cabinet full of other drugs.

The bottom line

Quercetin has some genuinely interesting properties. The anti-inflammatory effects on biomarkers seem real, and there are hints of benefits for blood pressure and possibly blood sugar in certain populations.

But let’s be honest about what we don’t know. We don’t have long-term safety data. We don’t know whether improving inflammation markers actually prevents disease. The effects, where they exist, are generally modest.

For most people eating a varied diet with plenty of fruits and vegetables, supplemental quercetin probably isn’t necessary. The quercetin in your onions and apples comes packaged with fibre, other polyphenols, and vitamins that may work better together than isolated compounds.

If you’re interested in quercetin’s anti-inflammatory potential, start with food sources. If you’re considering supplements, stick to doses under 1000 mg daily, take breaks, and be aware of the drug interactions. And if you’re taking multiple medications, please talk to your doctor first.

References

Bioavailability of quercetin in humans: https://pubmed.ncbi.nlm.nih.gov/26091183/
Quercetin and COVID-19 trial: https://pubmed.ncbi.nlm.nih.gov/34135619/
Quercetin and exercise performance meta-analysis: https://pubmed.ncbi.nlm.nih.gov/22805526/
Quercetin and blood pressure meta-analysis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5015358/
Quercetin and weight loss meta-analysis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6497115/
Quercetin and glycaemic control meta-analysis: https://www.ncbi.nlm.nih.gov/pubmed/30848564
Quercetin and CRP meta-analysis: https://www.ncbi.nlm.nih.gov/pubmed/28537580
Quercetin anti-inflammatory effects meta-analysis: https://www.ncbi.nlm.nih.gov/pubmed/31213101
Quercetin and lipid profile meta-analysis: https://www.ncbi.nlm.nih.gov/pubmed/31017459
Quercetin and respiratory infections trial: https://www.ncbi.nlm.nih.gov/pubmed/20478383
Quercetin and allergic mechanisms review: https://www.ncbi.nlm.nih.gov/pubmed/27187333
Quercetin safety review: https://www.ncbi.nlm.nih.gov/pubmed/29127724