Uses

Quercetin belongs to a class of water-soluble plant pigments called flavonoids.

What Are Star Ratings?

This supplement has been used in connection with the following health conditions:

Used for Why
3 Stars
Prostatitis
1,000 mg daily
Quercetin has anti-inflammatory and antioxidant effects and may reduce symptoms of chronic prostatitis.

Quercetin, a flavonoid with anti-inflammatory and antioxidant effects, has recently been reported to improve symptoms of NBP and PD. An uncontrolled study reported that 500 mg of quercetin twice daily for at least two weeks significantly improved symptoms in 59% of men with chronic prostatitis. These results were confirmed in a double-blind study, in which similar treatment with quercetin for one month improved symptoms in 67% of men with NBP or PD. Another uncontrolled study combined 1,000 mg per day of quercetin with the enzymes bromelain and papain, resulting in significant improvement of symptoms. Bromelain and papain promote absorption of quercetin and have anti-inflammatory effects as well.

2 Stars
Athletic Performance and Post-Exercise Infection
500 mg twice a day
In one study, quercetin lowered the incidence of upper respiratory tract infections in athletes following intensive exercise.
In a double-blind study of trained athletes, the incidence of upper respiratory tract infections following a three-day period of intensive exercise was significantly lower in people who took quercetin than in those who received a placebo (5% versus 45%). The amount of quercetin used was 500 mg twice a day, beginning three weeks before, and continuing for two weeks after, the intensive exercise.
1 Star
Allergies and Sensitivities
Refer to label instructions
Test tube and animal studies have found some effects from natural antihistamines such as the flavonoid quercetin, though no clinical research has shown whether these substances can specifically reduce allergic reactions.

Many of the effects of allergic reactions are caused by the release of histamine, which is the reason antihistamine medication is often used by allergy sufferers. Some natural substances, such as vitamin C and flavonoids, including quercetin, have demonstrated antihistamine effects in test tube, animal, and other preliminary studies. However, no research has investigated whether these substances can specifically reduce allergic reactions in humans.

1 Star
Asthma
Refer to label instructions
Quercetin, a flavonoid found in many plants, has an inhibiting action on lipoxygenase, an enzyme that contributes to problems with asthma.

Quercetin, a flavonoid found in most plants, has an inhibiting action on lipoxygenase, an enzyme that contributes to problems with asthma. No clinical trials in humans have confirmed whether quercetin decreases asthma symptoms. Some doctors are currently experimenting with 400 to 1,000 mg of quercetin three times per day.

1 Star
Atherosclerosis
Refer to label instructions
Quercetin, a flavonoid, protects LDL cholesterol from damage.

Quercetin, a flavonoid, protects LDL cholesterol from damage. While several preliminary studies have found that eating foods high in quercetin lowers the risk of heart disease, the research on this subject is not always consistent, and some research finds no protective link. Quercetin is found in apples, onions, black tea, and as a supplement. In some studies, dietary amounts linked to protection from heart disease are as low as 35 mg per day.

1 Star
Cataracts
Refer to label instructions
The flavonoid quercetin may help protect against cataracts by blocking sorbitol accumulation in the eye.

The flavonoidquercetin may also help by blocking sorbitol accumulation in the eye. This may be especially helpful for people with diabetes, though no clinical trials have yet explored whether quercetin actually prevents diabetic cataracts.

1 Star
Childhood Diseases
Refer to label instructions
Quercetin is a flavonoid that has shown particularly strong antiviral properties in the test tube.

Flavonoids are a group of compounds found in some plant foods and medicinal herbs. An antiviral action of some flavonoids has been observed in a number of test tube experiments.Quercetin, one of the flavonoids, has shown particularly strong antiviral properties in the test tube; however, one study did not find quercetin to be of benefit to mice with a viral infection. It is not known whether flavonoids can be absorbed in amounts sufficient to exert an antiviral effect in humans, and therefore their possible role in the treatment of childhood exanthems remains unknown.

1 Star
Edema
Refer to label instructions
In one study, the flavonoid quercetin corrected abnormal capillary permeability (leakiness), an effect that might improve edema.
Because coumarin, hydroxyethylrutosides, and diosmin are not widely available in the United States, other flavonoids, such as quercetin, rutin, or anthocyanosides (from bilberry), have been substituted by doctors in an attempt to obtain similar benefits. The effect of these other flavonoids against edema has not been well studied. Also, optimal amounts are not known. However, in one study, quercetin in amounts of 30-50 mg per day corrected abnormal capillary permeability (leakiness), an effect that might improve edema. A similar effect has been reported with rutin at 20 mg three times per day. Doctors often recommend 80-160 mg of a standardized extract of bilberry, three times per day.
1 Star
Gout
Refer to label instructions
In test tube studies, quercetin, a flavonoid, has inhibited an enzyme involved in the development of gout.

In test tube studies, quercetin, a flavonoid, has inhibited an enzyme involved in the production of uric acid in the body. In a double-blind trial, supplementation with 500 mg of quercetin once a day for 4 weeks significantly decreased blood levels of uric acid by an average of 8% in men with uric acid levels in the high-normal range. Decreasing uric acid levels may help prevent gout attacks.

1 Star
Hay Fever
Consult a qualified healthcare practitioner
Quercetin is an increasingly popular treatment for hay fever.

Quercetin is an increasingly popular treatment for hay fever even though only limited preliminary clinical research has suggested that it is beneficial to hay fever sufferers.

1 Star
Type 1 Diabetes
Refer to label instructions
Quercetin may be helpful for its ability to reduce levels of sorbitol-a sugar that accumulates in cells and damages the nerves, kidneys, and eyes of people with diabetes.
Doctors have suggested that quercetin might help people with diabetes because of its ability to reduce levels of sorbitol-a sugar that accumulates in nerve cells, kidney cells, and cells within the eyes of people with diabetes and has been linked to damage to those organs. Clinical trials have yet to explore whether quercetin actually protects people with diabetes from nerve damage (neuropathy), nephropathy, or eye damage (retinopathy).
1 Star
Type 2 Diabetes
Refer to label instructions
Quercetin may be helpful for its ability to reduce levels of sorbitol-a sugar that accumulates in cells and damages the nerves, kidneys, and eyes of people with diabetes.
Doctors have suggested that quercetin might help people with diabetes because of its ability to reduce levels of sorbitol-a sugar that accumulates in nerve cells, kidney cells, and cells within the eyes of people with diabetes-and has been linked to damage to those organs. Clinical trials have yet to explore whether quercetin actually protects people with diabetes from neuropathy, nephropathy, or retinopathy.

How It Works

How to Use It

Some doctors recommend 200-500 mg of quercetin taken two to three times per day. Optimal intake remains unknown.

Where to Find It

Quercetin can be found in onions, apples, green tea, and black tea. Smaller amounts are found in leafy green vegetables and beans.

Possible Deficiencies

No clear deficiency of quercetin has been established.

Interactions

Interactions with Supplements, Foods, & Other Compounds

Since flavonoids help protect and enhance vitamin C, quercetin is often taken with vitamin C.

Interactions with Medicines

Certain medicines interact with this supplement.

Types of interactions: Beneficial Adverse Check

Replenish Depleted Nutrients

  • none

Reduce Side Effects

  • none

Support Medicine

  • none

Reduces Effectiveness

  • none

Potential Negative Interaction

  • Estradiol

    Studies have shown that grapefruit juice significantly increases estradiol levels in the blood. One of the flavonoids found in grapefruit juice is quercetin. In a test tube study, quercetin was found to change estrogen metabolism in human liver cells in a way that increases estradiol levels and reduces other forms of estrogen. This effect is likely to increase estrogen activity in the body. However, the levels of quercetin used to alter estrogen metabolism in the test tube were much higher than levels found in the body after supplementing with quercetin.

    There is evidence from test tube studies that another flavonoid in grapefruit juice, naringenin, also has estrogenic activity. It has yet to be shown that dietary or supplemental levels of quercetin (or naringenin) could create a significant problem.

  • Felodipine

    Quercetin is a flavonoid found in grapefruit juice, tea, onions, and other foods; it is also available as a nutritional supplement. Quercetin has been shown in test tube studies to inhibit enzymes responsible for breaking down felodipine into an inactive form. This interaction may result in increased blood levels of felodipine that could lead to unwanted side effects. Until more is known about this interaction, patients taking felodipine should avoid supplementing with quercetin.

Explanation Required

  • Cyclosporine

    In an animal study, oral administration of quercetin (50 mg per 2.2 pounds of body weight) at the same time as cyclosporine decreased the absorption of cyclosporine by 43%. However, in a study of healthy human volunteers, supplementing with quercetin along with cyclosporine significantly increased blood levels of cyclosporine, when compared with administering cyclosporine alone. Because the effect of quercetin supplementation on cyclosporine absorption or utilization appears to be unpredictable, individuals taking cyclosporine should not take quercetin without the supervision of a doctor.

The Drug-Nutrient Interactions table may not include every possible interaction. Taking medicines with meals, on an empty stomach, or with alcohol may influence their effects. For details, refer to the manufacturers' package information as these are not covered in this table. If you take medications, always discuss the potential risks and benefits of adding a supplement with your doctor or pharmacist.

Side Effects

Side Effects

No clear toxicity has been identified. Early quercetin research suggested that large amounts of quercetin could cause cancer in animals.1 Most,2 , 3 , 4 but not all,5 current research finds quercetin to be safe or actually linked to protection from cancer.

Quercetin has been shown to cause chromosomal mutations in certain bacteria in test tube studies.6 Although the significance of this finding for humans is not clear, some doctors are concerned about the possibility that birth defects could occur in the offspring of people supplementing with quercetin at the time of conception or during pregnancy.

References

1. Ishikawa M, Oikawa T, Hosokawa M, et al. Enhancing effect of quercetin on 3-methylcholanthrene carcinogenesis in C57B1/6 mice. Neoplasma 1985;43:435-41.

2. Hertog M, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and cancer risk in the Zutphen Elderly Study. Nutr Cancer 1994;22:175-84.

3. Castillo MH, Perkins E, Campbell JH, et al. The effects of the bioflavonoid quercetin on squamous cell carcinoma of head and neck origin. Am J Surg 1989;351-5.

4. Stavric B. Quercetin in our diet: from potent mutagen to probable anticarcinogen. Clin Biochem 1994;27:245-8.

5. Barotto NN, López CB, Eyard AR, et al. Quercetin enhances pretumourous lesions in the NMU model of rat pancreatic carcinogenesis. Cancer Lett 1998;129:1-6.

6. Stoewsand GS, Anderson JL, Boyd JN, Hrazdina G. Quercetin: a mutagen, not a carcinogen in Fischer rats. J Toxicol Environ Health 1984;14:105-14.