Proven Benefits of Quercetin, Side Effects and Drug Interactions

Have you ever wondered what what top superfoods like red wine, green tea, kale, and blueberries all have in common? The answer is quercetin, a natural compound tied to longevity, heart health, endurance, immunity, and more.

What Is Quercetin?

Quercetin is an antioxidant that belongs to a class of water-soluble plant substances called flavonoids, which are present in certain fruits and vegetables. 

While the best way to get quercetin is through your diet, it also is available in supplement form, as pills or capsules. Sometimes, quercetin in supplement form is packaged with bromelain, an enzyme found in pineapples, because both have anti-inflammatory properties (Quercetin & Bromelain for inflammation). Most dietary supplements contain a chemical compound, aglycone, which is the free form of quercetin.

There are foods high in quercetin that you can add to your diet. Arguably, red onions are some of the most potent sources of quercetin; one study found that absorption of onion-derived quercetin was 52% compared to only 24% in supplement form. In fact, onions can give you three times the amount of quercetin than you’d get from apples. Other food sources that are high in quercetin include:

• Organically grown apples, grapes and tomatoes
• Dark cherries and dark berries like blueberries, blackberries and bilberries (eaten in moderation)
• Cocoa, unsweetened
• Hot green chili peppers
• Lingonberries

In fact, quercetin is considered to be the most widely distributed and extensively studied flavonoid, according to a 2018 review. 

We have compiled and summarised essential information below in layman's terms so that you can understand and make a better informed decision.

This article contains information and links to list of quercetin related scientific references in various categories. This list is a work-in-progress list as new evidence might be added from time to time.

Here is the list of proven benefits of quercetin (by category):

Quercetin and Allergies

Quercetin has powerful anti-inflammatory and antihistamine properties that can help relieve the symptoms of nasal allergies. In a clinical study involving people suffering from nasal allergies, those who used a nasal spray containing quercetin experienced significantly reduced symptom that were comparable to oral antihistamine drugs. (Reference)

Experts suggest that quercetin can be taken at levels of 400-500 mg three times a day.

Quercetin and COVID-19

Quercetin is a pigment that is found in plants, vegetables, and fruits, and serves as an immune nutrient offering many health benefits. Elderberry, red onions, white onions and cranberries are the richest sources of quercetin. It is a flavonoid and antioxidant that may help to reduce inflammation, infections, and allergies. Research has found that quercetin may be particularly beneficial for viral respiratory infections.

As of March 2021, 6 studies have been launched to investigate the benefits of quercetin against COVID-19. Ultimately, the results of the above trials will offer more definitive evidence.

Quercetin was initially found to provide broad-spectrum protection against SARS coronavirus in the aftermath of the SARS epidemic that broke out across 26 countries in 2003. Now, some doctors are advocating its use against SARS-CoV-2, in combination with vitamin C, noting that the two have synergistic effects.

Incidentally, ascorbic acid (vitamin C) and the bioflavonoid quercetin (originally labeled vitamin P) were both discovered by the same scientist — Nobel prize winner Albert Szent-Györgyi. Quercetin and vitamin C also act as an antiviral drug, effectively inactivating viruses. 

June 19, 2020, Marik published the paper, “Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19)” in the journal Frontiers in Immunology, which notes:

“Ascorbic acid is a crucial vitamin necessary for the correct functioning of the immune system. It plays a role in stress response and has shown promising results when administered to the critically ill. Quercetin is a well-known flavonoid whose antiviral properties have been investigated in numerous studies.

There is evidence that vitamin C and quercetin co-administration exerts a synergistic antiviral action due to overlapping antiviral and immunomodulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy.

Safe, cheap interventions which have a sound biological rationale should be prioritized for experimental use in the current context of a global health pandemic.”

There is evidence that vitamin C and quercetin co-administration exerts a synergistic antiviral action due to overlapping antiviral and immunomodulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy.

Quercetin and Vitamin C are also part of the FLCCC protocol for prevention and early out-patient treatment of COVID-19.

Quercetin and Inflammation

Flavonoids (aka bioflavonoids) including quercetin, are key anti-inflammatories that act as antioxidants, which means they fight the natural process of oxidation that takes place over time as we age. Quercetin can help stop damaging particles in the body known as free radicals, which negatively impact how cells work. It can also reduce expression of inflammatory genes such as interleukin.

Research now shows that inflammation is the root of most diseases, including heart disease, cancer, cognitive decline, some mental disorders, and autoimmune disorders. At this time, practitioners and patients report using quercetin to effectively fight a variety of conditions related to inflammation, including:

• “Hardening of the arteries” (atherosclerosis)
• High cholesterol
• Heart disease and circulation problems
• Insulin resistance and diabetes
• Eye-related disorders, including cataracts
• Allergies, asthma, and hay fever
• Stomach ulcers
• Cognitive impairment
• Gout
• Cancer
• Viral infections
• Chronic fatigue syndrome
• Inflammation of the prostate, bladder, and ovaries
• Chronic infections of the prostate
• Skin disorders, including dermatitis and hives

Quercetin and Heart Health

While it may not directly improve your love life, quercetin might be just what the doctor ordered in terms of a healthy heart and vascular system. One of the most prevalent flavonoids in the food we eat, this polyphenol is also one of the most studied, including on our cardiovascular system.

www.OneDayMD.com

Eating lots of deeply colored fruits and veggies that contain flavonoids is linked to a lower risk of cardiovascular disease, and even death, in older adults. Quercetin has also been connected to reduced risk for type 2 diabetes and obesity, which have many of the same risk factors as heart disease.

In those already experiencing some level of cardiovascular disease with high blood pressure and cholesterol levels, studies show quercetin can significantly reduce these two markers, demonstrating its therapeutic uses. (Reference)

Another clinical trial conducted in 2005 at the University of Utah with 41 pre-hypertensive or stage 1 hypertensive subjects suggest that quercetin supplementation may help control high blood pressure. Pre-hypertension was defined as having a blood pressure of 120–139/80–89, while those with 140–159/90–99 were categorized with stage 1 hypertension (as per clinically accepted guidelines at the time). Potential study candidates were carefully screened for chronic disease or other possible interfering factors (i.e., blood pressure medication or other supplementation), and only non-smokers who were not pregnant or nursing were allowed to complete the study. Participants were randomly divided into two groups, and for four weeks, took 365 mg of quercetin twice a day (based on in vivo animal studies) or a placebo. Results demonstrated that quercetin supplements reduced both systolic and diastolic pressures by five to seven points for those suffering from hypertension, regardless of age, gender, or weight, with no changes in those with borderline normal blood pressures. Given the statistical data, these reductions are considered significant and are associated with decreased risks of mortality from stroke and heart disease. (Reference)

Quercetin and Atherosclerosis

Results of a clinical study involving 12 healthy men of normal weights and blood pressures suggest that oral ingestion of 200 mg of quercetin could improve the functioning of arteries by significantly increasing production of vasodilating nitric oxide molecules while reducing levels of endothelin-1, a vasoconstricting peptide. (Reference, Loke et al)

Another clinical study demonstrated that quercetin’s antioxidant properties may also reduce the cardiovascular risks of atherosclerosis that patients in end-stage kidney failure typically suffer from. Thirty-eight patients undergoing regular hemodialysis for renal failure were divided into two groups: 26 given 100 ml of concentrated red grape juice (RGJ) a day over two weeks, and the remaining 12 as a control group not taking the RGJ supplement. A group of 15 healthy volunteers were also included in the study. (Reference)

Quercetin is the main polyphenol constituent of RGJ (red grape juice), and results of the study were substantial increases in HDL (the “good” cholesterol) as well as clinically significant decreases in LDL (the “bad” cholesterol), both of which lower the risks of atherosclerosis. Interestingly, even though not show in most previous studies, this study demonstrated the same effects on cholesterol levels in the healthy test subjects as in the unhealthy patients. One explanation may be the substantially higher amount of RGJ supplement given in this study (644 mg/day vs. 432 mg/day). (Reference)

Quercetin and Pain

Taking quercetin supplements may help lower pain associated with arthritis, as well as infections, including those of the prostate and respiratory tract. That’s because studies suggest quercetin reduces inflammatory pain. For example, there’s some evidence from several small studies that people experiencing bladder pains from infections (causing an urgent need to urinate, swelling, and burning) have fewer symptoms when taking quercetin supplements.

Flavonoids are also linked to reduced symptoms of prostatitis (inflammation of the prostate) and rheumatoid arthritis(RA). There’s evidence that when patients with RA switch from eating a “typical Western diet” to one higher in antioxidant-rich foods (like uncooked berries, fruits, vegetables, nuts, roots, seeds, and sprouts), they experience less pain and fewer recurring symptoms.

Quercetin May Improve Energy and Endurance

Quercetin is added to some athletic supplements because it’s believed to help increase athletic performance and endurance, likely because of its positive effects on blood flow. Researchers from the School of Applied Physiology at the Georgia Institute of Technology found that, on average, “quercetin provides a statistically significant benefit in human endurance exercise capacity (VO2 max) and endurance exercise performance.”

Other studies show that quercetin helps increase immune function and prevents susceptibility to illnesses that can occur when someone trains intensely and experiences exhaustion. One study found evidence that taking a dosage of 500 mg of quercetin twice daily helped protect cyclers from exercise-induced respiratory infections.

Because it can boost energy, quercetin could affect sleep patterns. One study found evidence that it may alter the sleep-wake cycle partly through activation of GABA receptors. However, insomnia is generally not believed to be a common side effect of taking quercetin.

Quercetin and Cancer

A Boston University School of Medicine study published in the Journal of Biological Regulators and Homeostatic Agents shows a link between a nutrient-dense diet rich in quercetin and a lowered risk of cancer. Quercetin seems to have potential chemo-preventive activity and might have a unique antiproliferative effect on cancer cells, making it an effective addition to any natural treatment approach. Research shows that this may result from the modulation of either EGFR or estrogen-receptor pathways. Recent studies have also found that quercetin can help stop the processes involved in cell proliferation and mutation, the growth of tumors, and symptoms related to typical cancer treatments, such as radiation or chemotherapy.

Animal studies suggest that quercetin effectively suppresses breast tumor growth, and further studies demonstrated that quercetin enhanced the therapeutic effects of Adriamycin® while suppressing its toxic effects on normal cells.

An animal study with rats conducted in 2009 demonstrated that quercetin’s antioxidant effects may act to prevent hepatocellular carcinoma (HCC). 

Another animal study showed that a combination of pterostilbene (derivative of resveratrol) and quercetin significantly inhibited the growth of liver metastases from malignant melanoma and also increased the lifespan of the subject mice.

Quercetin and Skin Health

Capable of blocking “mast cells,” which are immune cells critical in triggering allergic reactions, inflammatory disease, and autoimmune disease, quercetin can help protect skin from the effects of disorders like dermatitis and photosensitivity. Flavonoids like quercetin block the release of many proinflammatory cytokines, such as IL-8 and TNF, which helps stop symptoms related to skin inflammation, even in people who don’t find relief from other conventional treatments or prescriptions.

Studies have found that quercetin has antioxidant and anti-inflammatory effects that help fight allergic and inflammatory diseases as well as some prescriptions when taken in oral supplement form. For example, some people take quercetin for eczema since it can inhibit the secretion of histamine and pro-inflammatory markers.

Quercetin and Liver Health

Recent research has shown that quercetin has protective effects when administered to rats with ethanol-induced acute liver injury. Researchers concluded that “quercetin, by multiple mechanisms interplay, demonstrates hepatoprotective effect on liver-injury induced by alcohol by increasing ethanol metabolizing enzyme activities, increasing antioxidant system activities against oxidative stress, and lowering the expressions of proinflammatory cytokines.”

A 2017 study found evidence indicating that quercetin attenuates liver inflammation and fibrosis in mice through inhibiting macrophages infiltration. Researchers believe it “holds promise as a potential therapeutic agent for human fibrotic liver disease,” a condition triggered by liver injury and inflammation.

Quercetin May Protect the Brain

There’s mounting evidence that quercetin offers neuroprotective benefits due to its ability to defend the brain against oxidation and inflammation, leading to potentially less risk for cognitive conditions such as Alzheimer’s disease and dementia.

A 2018 study concluded that “findings suggest a possible new protective role for dietary flavonoids on Alzheimer’s disease (AD).” The study found that quercetin helps ameliorate cognitive dysfunction and may help reduce destruction of neurons.

Quercetin and Herpes Infections

In the lab, studies have demonstrated quercetin’s antiviral effect against both herpes simplex virus type 1 (HSV-1) and 2 (HSV-2). Quercetin extracted from apples inhibited both HSV-1 and HSV-2 replication by greater than 50%.

The in vitro antiviral effects of quercetin on herpesviruses (HSV-1, 2) and adenoviruses (ADV-3,−8,−11) suggest inhibition of early stage viral replication in a dose dependent manner (for HSV-1 100% inhibition at 60 mg/L) (PubMed, PubMed) as well as inhibition of viral DNA and RNA polymerase (PubMed, PubMed, PubMed).

Quercetin and Canker Sores (Aphtous Ulcers)

Canker sores, also called aphthous ulcers, are small, shallow lesions that develop on the soft tissues in your mouth or at the base of your gums. Unlike cold sores, canker sores don't occur on the surface of your lips and they aren't contagious. They can be painful, however, and can make eating and talking difficult.

A clinical trial was conducted to assess quercetin’s effects on canker sores, and the results were promising.

In the forty male patients randomly assigned to apply quercetin topical cream 2-3 times a day to canker sores, 90% had completely healed ulcers in 4-7 days. By 10 days, all 20 test subjects had resolved sores. Compared to the control group treated with a medicated mouthwash, the quercetin cream group healed faster and found the cream easy to use.

Quercetin Side Effects

When taken by mouth: Quercetin is POSSIBLY SAFE for most people when taken by mouth short-term. Quercetin has been safely used in amounts up to 500 mg twice daily for 12 weeks. It is not known if long-term use or higher doses are safe.

When taken by mouth, quercetin can cause headache and tingling of the arms and legs. Very high doses might cause kidney damage.

When given by IV: Quercetin is POSSIBLY SAFE when given intravenously (by IV) in appropriate amounts (less than 722 mg). Side effects may include flushing, sweating, nausea, vomiting, difficulty breathing, or pain at the injection site. But larger amounts given by IV are POSSIBLY UNSAFE. There have been reports of kidney damage at higher doses.

Special Precautions & Warnings:

Pregnancy and breast-feeding : There isn't enough reliable information to know if quercetin is safe to use when pregnant or breast-feeding. Stay on the safe side and avoid use.

Kidney problems : Quercetin might make kidney problems worse. Don't use quercetin if you have kidney problems.

Quercetin Interactions

Be cautious with the following combination below! An adverse outcome could occur. Talk with your health provider before taking this combination.

Antibiotics (Quinolone antibiotics) interacts with QUERCETIN

Taking quercetin along with some antibiotics might decrease the effectiveness of some antibiotics. Some scientists think that quercetin might prevent some antibiotics from killing bacteria. But it's too soon to know if this is a big concern.

Some of these antibiotics that might interact with quercetin include ciprofloxacin (Cipro), enoxacin (Penetrex), norfloxacin (Chibroxin, Noroxin), sparfloxacin (Zagam), trovafloxacin (Trovan), and grepafloxacin (Raxar).

Cyclosporin (Neoral, Sandimmune) interacts with QUERCETIN

Cyclosporin (Neoral, Sandimmune) is changed and broken down by the liver. Quercetin might decrease how quickly the liver breaks down cyclosporin (Neoral, Sandimmune). Taking quercetin might increase the effects and side effects of this medication. Before taking quercetin talk to your healthcare provider if you take cyclosporin (Neoral, Sandimmune).

Medications changed by the liver interacts with QUERCETIN

Some medications are changed and broken down by the liver. Quercetin might decrease how quickly the liver breaks down some medications. Taking quercetin along with these medications that are changed by the liver might increase the effects and side effects of your medication. Before taking quercetin talk to your healthcare provider if you take any medications that are changed by the liver.

Some medications that are changed by the liver include paclitaxel (Taxol), rosiglitazone (Avandia), amiodarone (Cordarone), docetaxel (Taxotere), repaglinide (Prandin), verapamil (Calan, Isoptin, Verelan), celecoxib (Celebrex), diclofenac (Voltaren), fluvastatin (Lescol), glipizide (Glucotrol), ibuprofen (Advil, Motrin), irbesartan (Avapro), losartan (Cozaar), phenytoin (Dilantin), piroxicam (Feldene), tamoxifen (Nolvadex), tolbutamide (Tolinase), torsemide (Demadex), warfarin (Coumadin), amitriptyline (Elavil), codeine, flecainide (Tambocor), haloperidol (Haldol), imipramine (Tofranil), metoprolol (Lopressor, Toprol XL), ondansetron (Zofran), paroxetine (Paxil), risperidone (Risperdal), tramadol (Ultram), venlafaxine (Effexor), lovastatin (Mevacor), clarithromycin (Biaxin), cyclosporine (Neoral, Sandimmune), diltiazem (Cardizem), estrogens, indinavir (Crixivan), triazolam (Halcion), verapamil (Calan, Isoptin, Verelan), alfentanil (Alfenta), fentanyl (Sublimaze), losartan (Cozaar), fluoxetine (Prozac), midazolam (Versed), omeprazole (Prilosec), lansoprazole (Prevacid), propranolol (Inderal), fexofenadine (Allegra), amitriptyline (Elavil), amiodarone (Cordarone), citalopram (Celexa), sertraline (Zoloft), ketoconazole (Nizoral), itraconazole (Sporanox) and others.

Medications moved by pumps in cells (P-glycoprotein Substrates) interacts with QUERCETIN

Some medications are moved by pumps in cells. Quercetin might make these pumps less active and increase how much of some medications get absorbed by the body. This might cause more side effects from some medications.
Some medications that are moved by these pumps include diltiazem (Cardizem), verapamil (Calan, Isoptin, Verelan), digoxin (Lanoxin) cyclosporine (Neoral, Sandimmune), saquinavir (Invirase), amprenavir (Agenerase), nelfinavir (Viracept), loperamide (Imodium), quinidine, paclitaxel (Taxol), vincristine, etoposide (VP16, VePesid), cimetidine (Tagamet), ranitidine (Zantac), fexofenadine (Allegra), ketoconazole (Nizoral), itraconazole (Sporanox), and others.

Quercetin Dosage

Most quercetin studies use a dosage of around 500 mg per day, although some studies use a dosage of 500mg taken twice per day.

Most supplements have a similar dosage, offering 500 mg to 1,200 mg of quercetin per serving.

For prevention of COVID-19, the FLCCC recommends 250 mg daily for prevention and the Zelenko protocol recommends 500 mg daily for prevention. If you wish to stick to the dosage which is within the range of the FLCCC - Zelenko protocols; that would be 250 mg to 500 mg daily. 

In some studies, researchers have given participants up to 5,000mg of quercetin per day with no reported side effects.

Quercetin has poor bioavailability. You might take a 1,200mg quercetin supplement, but your body only absorbs a small percentage of it. That’s why many quercetin supplements contain vitamin C or bromelain, as some evidence suggests they boost absorption.

There’s also some evidence that quercetin has a synergistic effect when combined with other flavonoid supplements. That’s why some quercetin supplements contain resveratrol, catechins, genistein, and other flavonoids.

Related: Best Quercetin Supplement 2020