Quercetin Side Effects: Stomach, Thyroid and Drug Interactions

Quercetin is the most abundant flavonoid present in various types of vegetables, fruits, teas, wine, and many other food items.

Onions have the highest known quercetin content (200-600 mg/kg), with fruits in a close second with between 2 and 250 mg/kg.

It is actually considered to be a plant pigment and that explains why it is found in abundance in deep colorful fruits and veggies.

It’s essential to note that a few studies have shown that quercetin has no side effects even in high doses and that the substance has many health benefits. It can also be used to heal a variety of ailments, such as:


Among different diseases, cancer is still considered the most detrimental to the survival of humans.

This flavonoid has been said to have health benefits on people suffering from cancer, plus it may also lower the risk of certain cancers.

In addition, this substance also deactivates enzymes that help cancer cells to grow, such as ovarian, bladder, breast, and colorectal cancer.

Bone Healthbone health run exercise

It interacts with bones at a fundamental level, positively influencing the behavior of both bone-building cells: osteoblasts and osteoclasts.

Also, it is a natural compound that assists in optimal bone health.

Antioxidant Properties

Because of its outstanding antioxidant effects, this compound is extremely effective in managing numerous inflammatory conditions that have a negative impact on human health, such as chronic fatigue, heart disease, and arthritis.

Physical Exerciserunning exercise

It is possible that this substance can improve energy levels; as a matter of fact, it is used as an ingredient in certain athletic supplements.

This is because of its capacity to improve blood circulation.

A meta-analysis published in 2012 found that individuals who took this flavonoid increased their performance levels and their levels of endurance.


Gout is a common form of arthritis that is characterized by swelling and tenderness in the joint, and sudden attacks of pain.

Gout typically occurs in only one joint at a time. It occurs in about 4 percent of the adults in the United States – about 2 million women and 6 million men.

Some foods can cause gout when you eat too much of them, especially:

  • organ meat;
  • red meat;
  • shellfish.

Consuming too many alcoholic drinks can also lead to this painful condition.

Quercetin has been shown to inhibit uric acid production in a manner similar to the drug Allopurinol (brand name Zyloprim).

Heart Healthheart disease blood pressure

There are recent studies conducted on some human populations which established that various types of flavonoids are capable of helping reduce the risk of atherosclerosis.

Moreover, its capacity to prevent oxidative stress and its anti-inflammatory properties again come to the fore when it comes to your most vital organ.

Side Effects of Quercetin

Minor side effects may result from quercetin supplement use, such as blurred vision, swollen feet, and ankles, dizziness, fluid accumulation in the knee, dull ache or feeling of pressure or heaviness in legs, headache, itching skin near damaged veins, pounding in the ears, nervousness, or a slow or fast heartbeat.

Furthermore, taking very high doses of this flavonoid (more than 1 gram per day) may interfere with your kidney health.

In addition, some preliminary evidence exists that it may generate a byproduct, which leads to loss of protein function in your body. Other side effects include:

Drug InteractionsPain medications drugs

It may interfere with the absorption and effect of a few types of drugs, including antibiotics because as an antioxidant, it competes for the same cellular receptors as many types of antibiotic medications.

Also, it may interfere with the body’s absorption of cyclosporine, an immunosuppressant drug broadly used in organ transplantation to prevent rejection.

Moreover, the supplement must not be taken with calcium channel blockers for hypertension or with some of the hormone replacement drugs.

Thyroid FunctionThyroid

Your thyroid gland is a small gland, normally weighing less than one ounce. The organ is located in the front part of the neck below the voice box and has the shape of a butterfly.

The thyroid keeps your metabolism under control through the action of the thyroid hormone, which makes it by extracting iodine from the blood and incorporating it into thyroid hormones.

This strong flavonoid inhibits thyroid-restricted gene expression and thyroid function and decreases the expression of the thyroid peroxidase, thyrotropin receptor, and thyroglobulin genes.

READ MORE: High Galactagogue Foods and Herbs

AllergySymptoms of a mushroom allergy

An allergic reaction to the supplement may occur in some individuals as a result of your immune system over-responding and mistake the flavonoid for a potentially toxic or harmful substance.

When you experience an allergic reaction, one major mediator is histamine, a natural compound that the body releases when it perceives an invader has entered and is attacking.

Histamine can cause itching, swelling, flushing, redness, and much discomfort.

READ MORE: Lipoflavonoid Side Effects

Stomachabdominal pain

Individuals with sensitive stomachs may also experience acid reflux or heartburn if larger doses are consumed on an empty stomach.

Symptoms of acid reflux disease include:

  • wheezing;
  • bloating;
  • unexplained weight loss;
  • nausea;
  • hiccups;
  • dysphagia;
  • burping;
  • black stools.

READ MORE: Triclosan in Antibacterial Soap Side Effects

During Pregnancypregnant woman

There is no study regarding its safety for pregnant women but nor is there any negative evidence. If you feel concerned, take the safe option and consult your healthcare specialist.

READ MORE: Feijoa (Acca sellowiana) – Side Effects


It is known that small amounts of the bioflavonoid, like the amounts that naturally occur in fruits, legumes, and vegetables, are 100% safe and beneficial.

For example, the average American adult consuming a balanced diet consumes approximately 30 to 50 milligrams per day.

Featured image credit – Shutterstock

READ THIS NEXT: Famous People With Simian Lines



6 thoughts on “Quercetin Side Effects: Stomach, Thyroid and Drug Interactions”

  1. A question on dosage: I am currently taking 500 mg a day as a supplement capsule which includes 1000 mg of ascorbic acid and 50 mg of bromelain. I take no medications other that a calcium supplement.

    You listed 1 gram a day long term is considered high, and a level that can cause damage, but did not mention if there was a lower boundary in which possible damage might occur. I am getting well above the 30 to 50 from food you referenced.

    Thank you for the article and the information contained.

  2. Hi, had pericarditis,heart attack and taking synthetic100mcg. Lung scarring, on metoprolol and colanor for tacky cardiac heart rate to stay in correct range. Is it ok to take citranox and kidney stage 3?

  3. Why is it not safe to take this with channel blocker’s?
    If I take this supplement in the morning and the channel blocker at night is that OK?


Leave a Comment