What Is Quercetin?

Quercetin, a plant pigment, is said to have antioxidant and anti-inflammatory benefits. This chemical compound (flavonoid) is naturally found in foods including apples, onions, teas, berries, red wine, and herbs like ginkgo biloba and St. John's wort. It is also available in supplement form.

Quercetin is sometimes used in various health conditions such as heart disease, cancer, arthritis, and COVID-19. However, there is no good scientific evidence to support taking it for these conditions.

This article discusses the purported uses of quercetin and its available research. It also covers potential side effects and drug-supplement interactions.

Purported Uses of Quercetin

Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian nutritionist, pharmacist, or healthcare provider No supplement is intended to treat, cure, or prevent a disease.

Research on the potential health benefits of quercetin is limited. Due to the lack of human research, there is not enough evidence to support quercetin's use for the following:

• Heart disease
• Cancer
• Arthritis
• Bladder infection
• Allergies
• COVID-19

Moreover, there is also little evidence that taking quercetin by mouth before exercise will decrease fatigue or improve exercise ability. More research is needed as few well-designed, placebo-controlled, peer-reviewed human studies have been published.

Blood Pressure

A meta-analysis of human studies reviewed quercetin's effects on blood pressure control.

The analysis included seven trials consisting of 587 people overall. The results showed that quercetin supplementation significantly reduced systolic and diastolic blood pressure. These reductions were mostly seen in people taking higher doses of 500 milligrams (mg) daily or greater.

Most studies in the analysis involved people with hypertension, except for one that included people with prehypertension and stage 1 disease.

Still, it is unclear if quercetin has much clinical benefit or would be useful as an add-on supplement for people with hypertension. Further research is needed to confirm the findings, look at other dosages, and study potential interactions with antihypertensive medications.

No supplement should replace standard medical care. Always check with your healthcare provider before starting any new supplement.

Inflammation

Some studies suggest quercetin may help lower inflammation in the body.

One small study examined quercetin's possible effects on inflammation, disease severity, and symptoms in women with rheumatoid arthritis. The trial included 50 individuals who were sorted into a quercetin supplementation group or a placebo group for eight weeks. Quercetin users took 500 milligrams (mg) daily.

After eight weeks, those who received quercetin reported significantly reduced early morning stiffness, morning pain, and after-activity pain, according to the results. Quercetin users also had reduced markers of inflammation compared to those in the placebo group.

Still, more evidence is needed to recommend quercetin for this purpose. If you have pain, consult a healthcare provider for medical guidance.

What Are the Side Effects of Quercetin?

Consuming a supplement like quercetin may have potential side effects.

Quercetin is generally well-tolerated when used in appropriate amounts. Some possible side effects of quercetin include:

• Tingling in the arms and legs
• Upset stomach
• Headaches

Check with your healthcare provider before taking quercetin. Be sure to mention any medications you are taking and any pre-existing medical conditions.

Precautions

Pregnant or lactating individuals, children, and people with kidney problems should not take quercetin in any form, as not enough research has been done in these populations.

Dosage: How Much Quercetin Should I Take?

With medical supervision, quercetin has been safely used in amounts of up to 1 gram daily for 12 weeks. There is not enough evidence to know if it is safe for long-term use (longer than 12 weeks).

What Happens if I Take Too Much Quercetin?

As a general guideline, never take more than the manufacturer's recommended dosage. If you experience side effects of any kind, stop taking quercetin and call your healthcare provider.

Interactions

It is essential to carefully read the ingredient list and nutrition facts panel of a supplement to know which ingredients and how much of each ingredient is included. Please review this supplement label with your healthcare provider to discuss any potential interactions with foods, other supplements, and medications.

Medications

Quercetin may interact with the following medications:

• Quinolone antibiotics
• Neoral, Sandimmune (cyclosporine)
• Voltaren (diclofenac)
• Cozaar (losartan)
• Versed (midazolam)
• Mitoxantrone
• Pravastatin
• Minipress (prazosin)
• Seroquel (quetiapine)
• Azulfidine (sulfasalazine)
• Jantoven (warfarin)
• Diabetes medications
• Blood pressure-lowering drugs

Quercetin can also interact with cytochrome (CYP) enzymes and affect how the liver breaks down certain medications, including those known as:

• CYP2C8 substrates
• CYP2C9 substrates
• CYP2D6 substrates
• CYP3A4 substrates

It may also interact with certain drug transporters:

• Organic anion transporter 1 (OAT1) substrates
• OAT3 substrates
• OAT polypeptide substrates

Ask your healthcare provider or pharmacist if you are taking a medication and are unsure if it belongs to any of the above-listed categories.

Herbs and Supplements

Quercetin may interact with herbs and supplements with blood-pressure-lowering effects (e.g., andrographis, casein peptides, niacin) or lower blood sugar (e.g., aloe, bitter melon, chromium).

This is not a complete list of drug and supplement interactions that may occur with quercetin. Tell your healthcare provider or pharmacist about all the substances you take, including prescription and over-the-counter (OTC) drugs, vitamins, and herbal supplements.

How to Store Quercetin

Store quercetin according to manufacturer's directions on the package. Discard as indicated on the packaging.

Sources of Quercetin & What to Look For

Food Sources of Quercetin

Food sources of quercetin include:

• Onions
• Apples
• Black tea
• Green tea
• Buckwheat tea
• Citrus fruits
• Red grapes
• Cherries
• Raspberries
  

Quercetin Supplements

Quercetin can often be purchased online or at health food stores.

When taking quercetin in supplement form, it may be beneficial to choose a product that also contains papain and/or bromelain. These are plant-derived enzymes (fruit extracts) shown to increase the intestine's absorption of quercetin.

Supplements are classified as food products, not drugs. Therefore, it is important to note that they are not strictly regulated by the FDA in the United States. Sometimes, a product may be contaminated or contain products other than those listed on the label. Remember that it is illegal for any company to market a dietary supplement product as a treatment or cure for a specific disease.

Summary

Quercetin is a plant chemical naturally found in certain foods and drinks, like apples, tea, and onions. It is generally recognized as safe by the FDA when used as an ingredient in foods in up to 500 milligrams per serving.

Quercetin is thought to have antioxidant and anti-inflammatory effects. While it's been studied for various health conditions, more research is needed before taking quercetin for any health purpose. Although generally safe, quercetin can interact with many medications and supplements, so it's important to talk to your healthcare provider before taking it.