What Is Quercetin?

Quercetin is a chemical found naturally in a number of foods including apples, onions, teas, berries, and red wine. This flavonoid is also found in some herbs such as ginkgo biloba and St. John's wort.

Quercetin acts as an antioxidant, neutralizing free radicals—the chemical by-products that harm cell membranes and damage DNA. Available as a dietary supplement, quercetin also possesses antihistamine and anti-inflammatory properties.

What Is Quercetin Used For?

In alternative medicine, quercetin is said to help with the following conditions:

• Allergies
• Cardiovascular disease
• Inflammation
• Colds

So far, results to support the benefits of quercetin are mixed—with some conditions examined only in test tubes or on animals. Here's how the research shakes out:

Allergy Relief

Quercetin is thought to prevent the release of histamine—an inflammatory chemical involved in allergic symptoms such as sneezing and itching—from certain immune cells. Although lab experiments suggest that quercetin may help fight allergic conditions like allergic rhinitis, most have been performed in vitro or in animals. Researchers recommend further studies on humans to prove a correlation.

High Blood Pressure

A 2016 review of randomized controlled trials found quercetin significantly reduced both systolic and diastolic blood pressure, particularly in diabetics who were given at least 500 milligrams per day. It's still unclear the precise dosage and duration to see the most benefits.

Athletic Endurance

Quercetin may be no better than a placebo when it comes to enhancing athletic performance, according to a 2011 review of 11 previous studies. All studies showed a boost in exercise endurance via VO2 max—oxygen consumption during physical activity—when people ingested quercetin but the effect was minimal.

Another study found a more impressive link. A 2013 study analyzing 60 male students who've participated in athletics for at least three years saw improved lean body mass, total body water, basal metabolic rate, and total energy expenditure after taking quercetin.

Cancer

Studies on cell cultures have shown that quercetin may help slow the growth of some types of cancer cells. Some in vitro and animal-based research indicates that quercetin may protect against certain types of cancer, such as leukemia and lung cancer. For example, a 2010 study looked at the relationship between quercetin intake and lung cancer risk in 38 non-tumor lung tissues and found an inverse correlation—the higher the intake of quercetin, the lower the risk.

However, since there is currently a lack of human studies on quercetin's cancer-fighting effects, it's too soon to tell whether quercetin might play a significant role in cancer prevention.

Possible Side Effects

Quercetin is generally well-tolerated when used in appropriate amounts. Some have reported tingling in the arms and legs, as well as upset stomach and headaches when taking quercetin orally. Very high doses—greater than 1 gram per day—might cause kidney damage.

Supplements haven't been tested for safety and due to the fact that dietary supplements are largely unregulated, the content of some products may differ from what is specified on the product label. Also keep in mind that the safety of supplements in pregnant women, nursing mothers, children, and those with medical conditions or who are taking medications—particularly antibiotics—has not been established.

Dosage and Preparation

Under the care of medical supervision, quercetin has been safely used in amounts up to 1,000 mg twice daily for 12 weeks. There is not enough evidence to know if it is safe for long-term use.

The appropriate dose for you may depend on factors including your age, gender, and medical history. Speak to your healthcare provider to get personalized advice if you choose to take this supplement.

What to Look For

Food sources of quercetin include teas, onions, apples, buckwheat, and pau d'arco. 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.

Due to the lack of supporting research, it's too soon to recommend quercetin for any health purpose. If you're considering using it, consult your primary care provider first. Self-treating a condition and avoiding or delaying standard care may have serious consequences.