Quercetin Side Effects: What You Need to Know

Quercetin is one of the most-studied and prevalent antioxidant flavonoids found in foods such as deeply colored fruits and vegetables.REF#595,REF#596

For example, the superfood status given to berries, kale, and green tea is partly related to their Quercetin antioxident content.

Quercetin has also demonstrated a variety of health benefits, including supporting:

Quercetin has a long history of safe use when consumed in foods and as a supplement.

However, Quercetin supplements can have rare side effects if taken in large amounts.

In this article, we’ll discuss the rare side effects of Quercetin and its potential contraindications.

How Quercetin Works

As an antioxidant, Quercetin works by scavenging free radicals—unstable molecules that can cause cell damage and oxidation if left unchecked.

Free radical damage can lead to various ailments related to inflammatory response, aging, and immunity.

This is partly why eating plenty of fresh fruits and vegetables is healthful; in addition to containing vitamins, minerals, and fiber, they provide extra antioxidant support in the form of Quercetin and other antioxidants.

Is Quercetin Safe to Take as a Supplement?

Quercetin is ubiquitous in our food supply, especially for those who eat a plant-based diet rich in deeply colored foods like berries and leafy green vegetables.

There are no reports of adverse events from excess consumption of Quercetin from foods.

On the contrary, experts from nearly all diet and nutrition backgrounds recommend eating more Quercetin- and antioxidant-rich foods to support health.

But are Quercetin supplements safe?

Per MedlinePlus, a Government Health site by the National Institutes of Health, Quercetin is likely safe for most people when taken short-term.REF#597

Quercetin has been safely used in doses of up to 1 gram daily for 12 weeks.

It's unknown whether long-term use or higher doses are safe, and there isn’t enough data on taking Quercetin during pregnancy or breastfeeding to determine safety.

However, Quercetin supplements have demonstrated excellent safety when taken short-term with very few rare side effects.

There are some concerns and theories on possible interactions regarding contraindications or interactions with medications, but no studies have demonstrated conclusive contraindications.

What are those side effects and possible contraindications? Let’s explore them now.

6 Rare Quercetin Side Effects

#1: Headache

A study published in the journal Urology tracked the effects of 1000 mg of Quercetin per day given to chronic prostatitis patients for one month.

They had one report of headaches during the treatment, which stopped when treatment ceased.REF#598

#2: Tingling of Extremities

In the same Urology study, another participant reported tingling in the extremities while taking 1000 mg of Quercetin daily for one month.REF#598

Symptoms resolved when the participants stopped taking Quercetin.

#3: High-dose, Intravenous Quercetin may Cause Nausea, Vomiting, Sweating, Fushing, Dyspnea, and Kidney Toxicity

In a phase I clinical trial, cancer patients who were unresponsive to standard treatments, received intravenous infusions of Quercetin at doses greater than or equal to 10.5 mg/kg body weight (~756 mg of quercetin for a 70 kg individual).REF#599

Side effects reported nausea, vomiting, sweating, flushing, and dyspnea (difficulty breathing).REF#599

Higher doses of Quercetin intravenous infusion of up to 51.3 mg/kg body weight (~3,591 mg of quercetin) were associated with renal (kidney) toxicity without evidence of nephritis (kidney inflammation), infection, or obstructive uropathy.REF#600

Remember, these side effects are related to high amounts of Quercetin given intravenously to cancer patients and are not the result of taking a Quercetin supplement by mouth.

#3: Potential to Enhance Nephrotoxic Effects in Mice with Kidney Damage

A 2018 Review published in Molecular Nutrition and Food Research noted the potential for kidney inflammation in mice with pre-existing kidney damage given supplemental Quercetin.REF#601

The authors also noted: “Among the numerous published human intervention studies, adverse effects following supplemental quercetin intake have been rarely reported, and any such effects were mild.”

#4: Potential to Promote Tumor Growth in Animal Models

The same Review from Molecular Nutrition and Food Research also noted the potential for supplemental Quercetin to promote tumor growth in mice, especially those with estrogen-dependent cancer.REF#602

There is no evidence to suggest Quercetin supplements encourage tumor growth in humans.

#5: Quercetin + Bromelain May Act as a Blood Thinner

There is evidence that the enzyme, Bromelain, found in pineapple, may have a blood-thinning effect.REF#603

Bromelain is often combined with Quercetin in supplements designed to support heart health, inflammatory response, and pain.

If you’re concerned about this possible side effect, look for Quercetin supplements without Bromelain or talk to your doctor.

#6: Quercetin, and other Antioxidants, May Inhibit Iron Absorption

You may have heard that drinking tea with a meal can reduce the amount of iron you absorb from that meal.

That’s because flavanol antioxidants, like Quercetin, have been shown to bind to non-heme iron (the type of iron found in plants, not meats) when taken with meals.REF#604

This generally only affects those with iron deficiency (particularly vegans or vegetarians who rely primarily or solely upon plants for iron) who take Quercetin with meals. And it may not affect everyone.

If you have iron deficiency, talk to your doctor about possibly taking Quercetin a couple of hours after eating to avoid potential disruption of iron absorption.

Can Quercetin Interact With Medications?

As previously mentioned, no studies confirm Quercetin can affect medications.

However, some doctors, researchers, and pharmacists believe it may negatively interact with certain drugs, includingREF#605

Antibiotics
Blood thinners
Chemotherapy

Always discuss any supplements you’re taking with your doctor or pharmacist.

Is Quercetin Safe?

As you’ve learned, Quercetin is very safe to consume in foods and offers a range of health benefits.

It’s also demonstrated excellent safety when taken as a supplement short-term, with few rare side effects.

How much is safe, and what’s considered short-term?

Generally, up to 500 milligrams taken twice daily for 12 weeks appears to be very safe.

If you’d like to take Quercetin longer, speak with your doctor first.

Again, there is no recommendation for limiting Quercetin intake from foods; quite the opposite!

How to Find a Quality Quercetin Supplement

If you’re interested in taking a Quercetin supplement, we recommend following this criterion in your selection:

Look for a brand that does third-party testing for identity, purity, and contaminants.
Avoid supplements that contain artificial flavors, colors, preservatives, fillers, or excipients (such as magnesium stearate or talc).
If you’re considering an herbal formula with Quercetin, look for certified organic herbs to avoid pesticide residues.

Gaia Herbs offers several supplements containing Quercetin and other beneficial herbs, including

To learn more about Quercetin, check out: