Quercetin: Benefits, Side Effects, Interactions and Dosage 2021

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. According to an article in Current Research in Nutrition and Food Science, the bioavailability of quercetin absorbed by your body varies, depending on the source where you get it, as well as your individual body’s metabolism of it. Sugar and dietary fat and fiber also affect it.

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.

When looking for and using quercetin supplements, it’s important to maintain a nutritious, balanced diet with fiber and healthy fats in it, as research shows that accumulation of quercetin in the body is dependent on “factors such as glucose moieties, solubility, human factor, vitamin C status and food matrix.”

Specifically, research also says that quercetin supplementation is best achieved when consumed with a fat-enriched diet.

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

Benefits of Quercetin

Quercetin has been linked to the following health benefits:

• Possessing anti-inflammatory, antioxidant properties that can help combat certain diseases. One study found that 500 mg of quercetin a day could improve symptoms of rheumatoid arthritis.
• Helping prevent cancer cell growth from breast, colon, prostate, endometrial and lung.
• Counteracting allergic reactions and helping with immune responses.
• Helping to prevent osteoporosis
• Addressing cognitive decline, including Alzheimer’s and dementia.
• Assisting in control of blood pressure. A study in 2002 found that quercetin can have vasodilator effects.
• Potentially helping to lower blood sugar levels. One study found that dosages of 500 mg per day “significantly reduced” fasting glucose levels.

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: Quercetin and COVID-19: Any evidence to support using if for COVID-19?