Quercetin benefits: what's known for hay fever and immunity, how well it's absorbed, and whether you need it with zinc or bromelain

What quercetin is and what it’s used for

Quercetin is a flavonoid, a plant pigment from the polyphenol family. It’s the most common flavonoid in our food, found in red onions, apples, leafy greens and tea. It isn’t a vitamin, and there’s no recommended intake.

In lab studies, quercetin acts as an antioxidant and reduces the release of histamine, the substance behind sneezing, itching and swelling in allergies. That’s where the label claims come from:

• easing hay fever;
• supporting immunity and warding off colds;
• “helping zinc work”.

Memorial Sloan Kettering (MSKCC), a New York cancer hospital, notes that most of this comes from lab studies, and it’s still unclear whether the effects happen in the human body.

Quercetin in food

Even the richest foods contain only tens of milligrams per 100 g. According to Germany’s Federal Institute for Risk Assessment (BfR), supplements often provide up to 1,000 mg a day, far more than people usually get from food.

Figures from the Linus Pauling Institute. Content varies with variety and cooking. A 100 g portion of red onion holds several times less than a single capsule:

Quercetin supplement absorption

Flavonoids in general are poorly absorbed: little gets in, and what does is quickly processed and excreted. The form matters:

• Onions beat apples. In a 2012 study, peak blood levels after onion powder were about 4 times higher than after apple peel powder.
• Quercetin dihydrate is the standard form in capsules and the best studied.
• Phytosome is quercetin bound to lecithin. It was designed to improve absorption and was used in some recent trials.

Rutin is another form: quercetin bound to a sugar. It’s sold on its own and in “citrus bioflavonoid” supplements.

Quercetin and blood pressure

This is a less famous but relatively well-studied use. In a crossover trial of 96 people with metabolic disorders, 150 mg of quercetin a day lowered systolic blood pressure by 2.6 mmHg. Diastolic pressure and other markers didn’t change. Trials at 500–730 mg a day found similar results.

A drop of 2–3 mmHg is small. Quercetin doesn’t replace blood pressure medicines, and if you take them, check with your GP first.

Quercetin for hay fever: what’s known

This is the most popular use, but the data are modest.

• A Japanese trial (2022). 66 adults with pollen allergy took 200 mg of quercetin phytosome or a placebo for 4 weeks. The quercetin group reported less eye itching, sneezing and runny nose on a questionnaire.
• A 2025 meta-analysis pooled 13 trials of polyphenols in allergic rhinitis (823 people). Nasal symptoms improved, but certainty was low to very low, and quality of life didn’t change significantly. It also included polyphenols other than quercetin.

For hay fever, the NHS suggests starting with a pharmacist, who can recommend antihistamine tablets, drops or sprays and steroid nasal sprays. Quercetin is, at most, something to discuss as an add-on.

Quercetin for immunity

The largest trial ran in the US in 2010. 1,002 adults took 500 mg or 1,000 mg of quercetin a day, or a placebo, for 12 weeks. Overall, it didn’t change how often people caught colds or how bad they were.

A difference appeared only in a subgroup: physically fit adults over 40 (325 people) taking 1,000 mg had 31% fewer sick days. Subgroup findings like this need confirming.

Small COVID-19 trials gave quercetin alongside treatment. A meta-analysis of 6 of them found fewer hospital admissions but not fewer deaths, and its authors called for large trials. Options with stronger evidence are in our immunity section.

Quercetin and zinc, bromelain and vitamin C

iHerb has many formulas that combine quercetin with bromelain, vitamin C and zinc. The evidence behind each pairing is thin:

• Zinc. In a 2014 experiment, quercetin carried zinc ions into mouse liver cancer cells and artificial membrane bubbles. This hasn’t been tested in people. Count the zinc alongside your other supplements; our guide to zinc explains the limits.
• Bromelain. A pineapple enzyme added “for absorption”. We haven’t found reliable human studies that confirm it. Bromelain itself is covered in our guide to digestive enzymes.
• Vitamin C. Safe at these doses, but a normal diet usually covers your needs. Our vitamin C guide explains when a supplement makes sense.

How to take quercetin

There’s no official dose. These figures come from research:

• 150–1,000 mg a day is the dose range used in clinical trials.
• Up to 1,000 mg a day for up to 3 months produced no significant side effects, according to the Linus Pauling Institute.
• Side effects at 1,000 mg are rarely reported: headache and tingling in the hands and feet.
• Long-term high doses are poorly studied, so taking it in courses is sensible.

Start with 250–500 mg a day and review after 4–8 weeks.

Quercetin and your kidneys

In a phase I trial, people with cancer received very high doses of quercetin by intravenous infusion. At 1,700 mg per square metre of body surface, kidney damage was severe enough to stop the dose rising, and some kidney toxicity occurred at lower doses too.

That was an infusion, not a tablet. But a 2018 review by Germany’s BfR notes that in animal studies quercetin worsened damage in kidneys that were already injured. So if you have kidney disease, avoid quercetin supplements.

Quercetin interactions with medicines

• Ciclosporin. In rats, quercetin cut blood levels of ciclosporin by 43%. This hasn’t been tested in people, but the authors advise transplant patients to avoid quercetin, because too little of the drug risks rejection.
• Warfarin. In one reported case, a 79-year-old man on a stable warfarin dose had a rise in INR after starting quercetin; it returned to normal when he stopped.
• Medicines broken down by CYP3A4 and CYP2C19. In lab studies, quercetin inhibited these enzymes. The clinical importance is unknown.
• Chemotherapy. As an antioxidant, quercetin may interfere with some cancer drugs.

If you take regular medicines, check combinations in our vitamin compatibility chart and talk to your GP or pharmacist.

Who should avoid quercetin

• People with kidney disease.
• Transplant recipients and anyone taking ciclosporin.
• Pregnancy and breastfeeding. Safety hasn’t been established.
• Oestrogen-sensitive tumours – in an animal study, quercetin made oestrogen-driven breast tumours worse.
• Children – only if a doctor recommends it.

How to choose quercetin on iHerb

1. Decide why you want it. If you’re not sure you need a supplement at all, try our vitamin check.
2. Pick the form: quercetin dihydrate as the default; phytosome if you’ll pay for better absorption.
3. Check the dose: 250–500 mg per capsule is easy to work with.
4. Look at the add-ons. Zinc, vitamin C and bromelain raise the price, and zinc needs adding to your other supplements.
5. Compare the price per 500 mg of quercetin and prefer products with independent quality testing (USP, NSF and similar).

Our guide to reading a supplement label explains the Supplement Facts panel. A related polyphenol from grapes is covered in our guide to resveratrol.