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Quercetin Side Effects

What are the possible side effects of quercetin?


Quercetin is a natural flavonoid found abundantly in fruits and vegetables, especially – raspberries, apples, red grapes, red onions, citrus fruits, cherries, and green leafy vegetables.

The daily intake of quercetin with a typical American diet is estimated to range between 1 and 30 milligrams, with a median of 10 milligrams per day.


This flavonoid has been reported to exhibit anti-carcinogenic, antioxidative, anti-microbial, anti-viral, anti-inflammatory, anti-thrombotic, anti-aggregatory, anti-aging, and vasodilating effects.

Reduces The Effects of Stress

Stress is a natural mental and physical reaction to life experiences. All people experience stress from time to time.

However, chronic stress can cause numerous symptoms and negatively affects your overall well-being. Signs and symptoms of chronic stress include:

  • insomnia;
  • irritability;
  • headaches;
  • anxiety;
  • depression;
  • stomach upset;
  • change in sex drive;
  • fatigue;
  • chest pain;
  • muscle tension.

Quercetin has shown that it can reduce the effects of chronic stress in nerves while preventing the depletion of nerve antioxidants due to stress.

Antioxidant Properties

Antioxidants help stop or prevent cell damage caused by free radicals – compounds that are produced naturally in your body (due to metabolism) or due to the environment (like – pollution or tobacco smoke).

This antioxidant activates the essential step leading to the production of cellular glutathione (a cell’s primary antioxidant), according to a 2009 study.

In addition, when it is combined with other foods such as papayas or pineapples, quercetin has a synergistic response to reducing inflammation.


Cancer is a diverse group of hundreds of diseases in which some of the body’s cells begin to multiply out of control.

According to the data, about 1.7 million new cancer cases occurred in the United States. Breast cancer is the most frequent cancer in women worldwide, with over 1.8 million new cases diagnosed every year.

It has been demonstrated that this flavonoid inhibits the growth of cancer cell lines and that the anti-proliferative activity of this antioxidant is mediated by a Type II Estrogen-Binding Site.


Prostatitis is the inflammation and swelling of the prostate gland – a walnut-sized gland that is part of the male reproductive system and is located below the bladder.

Prostatitis accounts for over 2.1 million visits a year to outpatient urology practices in the US. This flavonoid is recognized in many different studies as being beneficial for chronic prostatitis sufferers, typically at a dose of 500 mg per day.


Quercetin is available in supplement form, as pills or capsules. Most dosages range from 200 to 500 milligrams, taken 15 minutes before meals.

Its bioavailability is really poor. Numerous factors like solubility, glucose moieties, vitamin C status, and human factor can affect its bioavailability.

Papain (a proteolytic enzyme extracted from the leaf and fruit of the papaya plant) and bromelain (an enzyme extract derived from the stems of pineapples) may assist the absorption of quercetin in the intestine.

Side Effects of Quercetin

Quercetin is a safe supplement that you can use in conjunction with conventional methods or as the main therapy.

But, some people can experience some side effects, including:

  • upset stomach;
  • headaches;
  • tingling in the arms and legs;
  • flushing of the skin;
  • excessive sweating;
  • diarrhea;
  • vomiting;
  • nausea;
  • persistent abdominal cramps;
  • heartburn;
  • acid reflux;
  • indigestion.

Moreover, this supplement may inhibit an enzyme called CYP3A4, which is involved in the metabolism of a variety of drugs.


The thyroid gland is located below Adam’s apple and wrapped around the trachea. Through the hormones it produces, the gland influences most metabolic processes in the human body.

According to a 2014 study conducted at the Department of Medicine and Sciences of Aging, Pescara, flavonoids can interfere with thyroid function.

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Homocysteine is a naturally occurring amino acid that is produced as part of the body’s methylation process.

Elevated homocysteine levels may act on blood platelets and increase the risks of clot formation as well as it may contribute to plaque formation by damaging arterial walls.

This supplement may increase homocysteine levels due to increased methylation. Hence, it is recommended to be cautious when using this supplement.


Some people can experience an allergic reaction after taking this supplement. This commonly occurs when the body mistakenly recognizes quercetin as a potentially harmful chemical and produces an immune response against it.

The majority of allergic reactions occur within minutes after exposure to the allergen. Possible symptoms include:

  • hives;
  • low blood pressure;
  • swelling (angioedema);
  • fast heart rate (tachycardia);
  • flushing;
  • lightheadedness;
  • itching;
  • dizziness;
  • wheezing;
  • chest tightness;
  • shortness of breath;
  • cough;
  • a runny nose;
  • diarrhea;
  • nasal congestion;
  • vomiting;
  • nausea;
  • abdominal cramping;
  • abdominal pain.

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Drug Interactions

Quercetin supplements can enhance the effects of anticoagulants like Clopidogrel (Plavix) or Warfarin (Coumadin). This can increase your risk of bleeding.

In addition, quercetin:

  • may decrease the effectiveness of fluoroquinolones (antibiotics which are used to treat respiratory and urinary tract infections);
  • could increase the risk of side-effects associated with digoxin (a medication used to treat various heart conditions);
  • may interact with the absorption of cyclosporine (an immunosuppressant medication that reduces the body’s ability to fight illness);
  • could cause corticosteroids (anti-inflammatory medicines prescribed for a wide range of conditions) to stay in the body for longer;
  • may interact with chemotherapeutic medications (certain drugs that are used to kill cancer cells or to stop them from growing).

Pregnancy & Breastfeeding

There are no well-done clinical studies regarding the safe use of this flavonoid by pregnant or breastfeeding women.

Therefore, if you are pregnant or breastfeeding, do not take quercetin as a supplement before you contact your doctor.

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Food Sources

It is best to take quercetin naturally, from your nutrition. Best food sources of quercetin include:

  • capers (180 mg/100 grams);
  • kale (7.7 mg/100 grams);
  • apples (4.7 mg/100 grams);
  • hot peppers (15 mg/100 grams);
  • red onions (33 mg/100 grams);
  • elderberries (42 mg/100 grams).

Image source – Shutterstock

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