Quercetin Side Effects and Interactions 2023

By Dr. Frank Yap, MD -

Quercetin is an antioxidant that belongs to a class of water-soluble plant substances called flavonoids, which are present in certain fruits and vegetables. It can be found in high quantities in tomatoes, red wine, onions, green tea, apples, berries, and Gingko Biloba.

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.

Quercetin side effects are uncommon as this is considered to be one of the safest supplements you can take. However, Quercetin has a number of interactions, especially with certain medications like antibiotics and blood-thinning medicine.

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.

Some side effects of bioflavonoids (the active ingredient contained in Quercetin) may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
  • Blurred vision
  • dizziness
  • dull ache or feeling of pressure or heaviness in legs
  • fluid accumulation in the knee
  • headache
  • itching skin near damaged veins
  • nervousness
  • pounding in the ears
  • red or scaling skin
  • slow or fast heartbeat
  • swollen feet and ankles
  • tingling of the arms and legs
  • stomach upset
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 - WebMD

Quercetin may interact with a number of medications:

  • Antibiotics: May prevent the action of antibiotics
  • Blood thinners: Could increase the effects of blood-thinning medication
  • Chemotherapy: May interact with chemotherapeutic medication
  • Corticosteroids: Could cause these medications to stay in your body for longer
  • Cyclosporine: May interact with the absorption of this medicine
  • Digoxin: Could increase the risk of side-effects associated with this chemical agent
  • Fluoroquinolones: May decrease the effectiveness of these medicines
  • Medications altered/activated by the liver: Quercetin may change how your body metabolizes medicine that is activated in the liver 

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.

Conclusion: Side Effects of Quercetin

In summary, Quercetin is a great supplement for anybody looking to boost their health and well-being. It offers immune support and has powerful anti-inflammatory effects. Quercetin may also help to improve energy levels and to reduce stress. To check out the full list of benefits of quercetin with scientific references, visit proven benefits of quercetin.

Quercetin side effects are minimal and this plant-based dietary supplement is generally considered safe at the recommended dosages. High doses may cause headaches and an upset stomach.

There are some warnings about Quercetin side effects regarding interactions with various medications (listed above), and it should not be used by pregnant or breastfeeding women.


Medical Disclaimer: Not intended to treat, diagnose, or cure any disease or ailment. Please read and fully understand potential adverse effects before using this product. These statements have not been reviewed by the FDA. Please consult your doctor before using any supplements, especially if you have any medical conditions.

Related: Best Quercetin Supplement 

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