What Are Quercetin Side Effects?

scientists are studying quercetin

Whether or not quercetin side effects exist is currently the topic of a heated debate among scientists who find conflicting results from their studies.

Quercetin Side Effects

Quercetin is part of a class of water-soluble plant pigments called flavonoids. Flavonoids are chemical compounds found in all vascular plant life and in many of the foods most people eat on a daily basis. It does not seem likely that an element that is so common could cause any adverse reactions - and for the most part this proves true. Questions start to arise when people consume large doses of the flavonoid in vitamin form. Some reputable scientists say this is perfectly safe while other equally reputable scientists say this can cause cancer.

Does Quercetin Cause Cancer

It's first important to note that several studies find that quercetin side effects are non-existent even in high doses and that the flavonoid has many benefits and can be used to heal a variety of ailments. Some of these studies even propose that high doses of the flavonoid may prevent cancer.

On the other hand, a handful of studies from reputable sources claim high doses of quercetin cause cancer. These studies found that some products contained very large amounts of the flavonoid whose potency was severely underestimated.

Researchers found that the high concentrations of the element would actually bind with and damage chromosomes and DNA structures resulting in a cancerous mutation. High concentrations of the flavonoid also disrupted the activity of enzymes and particularly interfered with estrogen and thyroid hormones. These studies claim that there is a documented risk to young children consuming high doses of it and getting sick with a rare form of leukemia.

It's important to remember that the research that indicates quercetin causes cancer always refers to its consumption in high doses. These scientists also found that the flavonoid had protective effects on the body in low levels that only became carcinogenic once it reached an elevated concentration.

Birth Defects

One study found that quercetin caused genetic mutations on a very rudimentary level by conducting experiments with bacteria. The significance of these findings is not fully understood, but they concern some doctors over the flavonoid and its use during pregnancy. Scientists theorize that birth defects could occur in the unborn children of women who consumed high doses of quercetin at the time of conception and throughout pregnancy.

Doses and Uses of Quercetin

Scientists do not know what the optimal intake of quercetin is and do not even know for sure what amount constitutes a true deficiency. However, some doctors recommend taking 200 to 500 mg of the supplement two to three times per day to help treat the following conditions:

  • Prostatitis
  • Allergies
  • Asthma
  • Atherosclerosis
  • Capillary fragility
  • Cataracts
  • Edema
  • Gout
  • Hay fever
  • Peptic ulcer
  • Retinopathy
  • Diabetes

Conversely, the scientists leading the studies that found the flavonoid to cause cancer don't recommend taking any amount of quercetin. These scientists say that while the flavonoid may aid in treating the listed conditions, it's only helpful in smaller amounts than the prescribed 200 to 300 mg two to three times a day. They recommend eating quercetin rich foods to get the right amount of the flavonoid and to make the most of its beneficial properties. Quercetin is found in significant quantities in the following food items:

  • Red wine
  • Grapefruit
  • Onions
  • Apples
  • Black and green teas
  • Leafy green vegetables
  • Nuts
  • Seeds and roots


The flavonoid is known to interact with the following medications:

  • Cyclosporine: Cyclosporine is a suppressive drug that prevents the immune system from rejecting transplanted organs. When the flavonoid was taken in conjunction with this drug, some patients experienced significantly increased blood levels of cyclosporine. Others did not, but because of this discrepancy, it's not recommended to take quercetin supplements with cyclosporine.
  • Estradiol: Estradiol is a semi-synthetic human estrogenic hormone. Injections of this hormone are used to treat menopause, osteoporosis and in replacement therapy for certain other conditions. The flavonoid taken with estradiol is suspected to substantially increase estrogen production in the body. The exact effects of this combination are not known, but it's likely it could cause a hormone imbalance and is therefore not recommended.
  • Felodipine: Felodipine is used to treat high blood pressure, Raynaud's syndrome, and congestive heart failure. Flavonoids are known to break this drug down into inactive pieces. It's definitely not recommended to mix flavonoids with this drug.

Until further research is done, it will not be known for sure if large doses of quercetin combat or cause cancer. However, it is known that small amounts of the flavonoid, like the amounts that naturally occur in healthy foods, are safe and beneficial.

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What Are Quercetin Side Effects?