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What are the Side Effects of Quercetin?

side effects

Quercetin is a flavonoid responsible for pigmenting fruits, flowers, and vegetables. Its antioxidant properties and anti-inflammatory properties protect against free radicals and oxidative stress, making it an effective anti-disease agent. Its potential health benefits include minimizing the risk of heart disease, lowers blood pressure and hypertension, supports a healthy immune system, reduces symptoms of prostatitis, interstitial cystitis, and allergies, and it may even help fight cell damage.

Although a quercetin dietary supplement is highly beneficial and considered safe, it can produce side effects in some people who are pregnant, breastfeeding, or have pre-existing health conditions. That’s why it’s important to talk with your healthcare professional before using it for additional medical advice.

Sources of quercetin with high absorption rates (bioavailability) include citrus fruits, vegetables, red wine, and green tea. You may also get quercetin in supplements.

Although quercetin is highly beneficial and considered safe, it can produce side effects in some people. This article will answer the question, what are the side effects of quercetin, so you can decide if it’s right for you.

What are the Side Effects of Quercetin?

Quercetin may produce the following side effects:

  • Headache
  • Upset stomach
  • Quercetin byproducts may lead to reduced protein function
  • Kidney damage with doses that exceed 1 gram per day
  • May not be safe for people who are pregnant or breastfeeding

You can reduce the risk of side effects by taking breaks between quercetin dosing. The supplement is proven to be safe at 500-milligram doses twice a week for twelve weeks. Risks are unknown when quercetin is taken for extended periods.

Possible Interactions

Quercetin may also interact with medications. You should talk to a provider before taking quercetin if you are taking any of the following:


Quercetin may reduce the effectiveness of certain antibiotics. Talk to your doctor to determine if it will affect the antibiotic you are prescribed.

Anticoagulants (Blood thinners)

Quercetin may enhance the effects of the following blood thinners and increase your risk of bleeding.

  • Aspirin
  • Warfarin (Coumadin)
  • Clopidogrel (Plavix)


Research reveals that quercetin may enhance the effects of the chemotherapy medications doxorubicin and cisplatin.

Additionally, some medical experts feel combining antioxidants with chemotherapy can be harmful. Others believe it can be helpful.

One study showed combining quercetin with doxorubicin helped fight breast cancer cells. Another study showed quercetin reduces cisplatin’s beneficial effects on ovarian cancer cells.

Talk to your oncologist about possible interactions before combining quercetin with chemotherapy medications.


Corticosteroids are anti-inflammatory drugs used to treat arthritis and other conditions. Quercetin may cause corticosteroids to stay in the body longer.

Excessive corticosteroid exposure can cause an adrenal deficiency. Your adrenal glands may become sluggish and reduce your ability to respond to physical stress.


Cyclosporine is an immunosuppressant that is used to treat organ rejection post-transplant. Quercetin may interfere with the absorption of this drug.


Digoxin is used to improve the heart’s strength and efficiency. It can also control heartbeat rate and rhythm. It boosts blood circulation and reduces swelling in the heart patient’s extremities.

Quercetin may increase digoxin risks, including dizziness, nausea, vomiting, changes in vision, and skin rashes.


Fluoroquinolones treat bacterial infections like bronchitis, pneumonia, urinary tract infections, and joint and bone infections. Quercetin may reduce its effectiveness.

Medications Changed By the Liver

Quercetin’s interaction with the liver may alter the effects of medications changed by the liver. It may affect how your body metabolizes these medications.

Now you know the answer to the question, what are the side effects of quercetin? It may produce minor symptoms in certain people. It may cause interactions with certain drugs. It should not be taken by people who are breastfeeding or pregnant. People with kidney disease should avoid excessive doses.

For best results, talk to your doctor and shop around before taking the best quercetin supplement. Start with low doses to see how it affects you. Increase doses in small increments as needed.

Here’s hoping you have a positive quercetin experience.