What Is Quercetin & How Can It Help Support The Immune System? 

Many factors in modern life can weaken the immune system (lack of sleep, high sugar consumption, chronic stress—you get the idea), making it more challenging to maintain good health.

Identifying what undermines your immune system and making lifestyle changes is a great beginning, but nobody is perfect—so supplements like vitamin C and vitamin D are often used to boost immune function.

However, there's another lesser-known nutrient that's gaining attention for its immune-supporting properties: quercetin.

You've likely consumed this bioactive plant compound in foods like apples, onions, or tea, but growing evidence suggests it might be beneficial to supplement it.

Quercetin is part of a group of plant pigments called flavonoids, and it's found in a range of foods such as fruits, vegetables, nuts, and grains. In supplement form, quercetin is often derived from the dried flowers and buds of the Sophora japonica (or Japanese pagoda) tree. 

Like all flavonoids, quercetin is an antioxidant, meaning it helps scavenge and neutralize free radicals that might otherwise accelerate aging, contribute to inflammation, and fuel a range of chronic diseases, from cancer to heart disease. 

Quercetin may be most well known for its ability to ease seasonal allergy symptoms, but it's proving to be quite the versatile plant compound.

Like any nutrient, quercetin isn't a magic bullet for health or immunity in isolation. But the body of existing research on quercetin is promising. 

"The main reason I've prescribed quercetin for over 30 years is because it works, it is safe, and its use is based on solid published evidence," says Robert Rountree, M.D., renowned integrative physician. "I also prescribe it a lot because it is so versatile as a preventive or treatment for so many kinds of health conditions." 

Quercetin may be useful in the prevention and treatment of a wide range of viral infections, including upper respiratory tract infections (URTIs), such as the common cold and the flu.

In one study published in the Journal of Clinical & Experimental Cardiology, supplementation with 1,000 mg of quercetin per day reduced the incidence of URTIs1 after a period of intense exercise—three hours of cycling, three days in a row, during winter. 

This flavonoid has even helped reduce the frequency and severity of herpes outbreaks among Rountree's patients.

According to Rountree, quercetin's mechanism of combating viruses isn't totally understood, but it appears to be a combination of immune modulation, prevention of viral entry into cells, and inhibition of viral replication.

Quercetin also acts synergistically with other immune-boosting nutrients. For example, "Quercetin acts as a zinc shuttle, getting natural antiviral zinc into cells where it can help mitigate viral replication," Vincent Pedre, M.D., integrative physician, recently told mbg. 

"Quercetin's most obvious and immediately noticeable benefit is prevention of histamine-mediated allergic symptoms," says Rountree. "Over the years, I have treated numerous individuals with chronic allergies who were either able to stop or greatly reduce their use of pharmaceutical agents."

More research is needed in humans, but lab studies—including one published in the journal Molecules—show that quercetin can help combat conditional allergic rhinitis3.

Free radicals in the body promote inflammation, but antioxidants help scavenge and neutralize free radicals. So it makes sense that quercetin (a flavonoid antioxidant) would have anti-inflammatory properties. 

In fact, several studies like this one published in the European Journal of Clinical Nutrition have found that supplementing with about 500 mg of quercetin4 per day helps lower levels of the inflammatory biomarker c-reactive protein (CRP), which is elevated in conditions like heart disease, rheumatoid arthritis, metabolic syndrome, and diabetes.

Dementia, Alzheimer's, and other neurodegenerative diseases can be exacerbated by oxidative stress caused by an overabundance of free radicals in the body.

Not only does quercetin have antioxidant properties that counter these effects, but it supports mitochondrial biogenesis (the production of new mitochondria, the energy-revving "powerhouse" of cells) in the brain for enhanced cognition, says Roundtree. 

Numerous flavonoid antioxidants, not just quercetin, have demonstrated anti-cancer properties, which is one reason eating loads of colorful fruits and veggies is so dang good for you. But quercetin, specifically, has been shown to inhibit cancer cell growth and proliferation, or multiplication. 

Quercetin is present in many healthy foods, and eating a variety of the quercetin-rich foods listed above poses no risk (unless you're allergic or have a sensitivity, of course!). Supplements are quite safe too.

"I have found that quercetin is very well tolerated with minimal side effects, the most common of which is mild upset stomach," says Rountree.

Some research suggests15 that quercetin may cause headaches or tingling nerve sensations, but Rountree says he has not seen this among his patients and that quercetin is on his "A-list of supplements" that most people could benefit from taking every day.

Who should avoid quercetin? If you're currently taking another medication, it's always a good idea to talk to your health care provider about potential interactions.

For example, because quercetin can thin the blood and help reduce blood clots, it may interfere with or amplify the effects of blood thinners; there's also some evidence it could interfere with medications that have a narrow therapeutic window, such as transplant anti-rejection drugs.

Most people will realistically only hit up to 80 mg of quercetin per day from dietary sources, according to Rountree.

So supplements are necessary if you want to fight allergies, support immunity, or experience any of the potential quercetin health benefits listed above. In fact, "the dose for prevention of allergic symptoms is from 500 to 3,000 mg daily," he says.

A typical dose of quercetin is 500 to 1,000 mg per day (this is the dosage often used in clinical studies16). However, quercetin is somewhat poorly absorbed through the intestinal wall, so Rountree recommends quercetin phytosome—a form where quercetin is attached to a phospholipid complex17, which has been shown to dramatically enhance absorption18. 

Another way to get more nutritional bang for your quercetin buck is to combine it with vitamin C.

Quercetin is one of those nutrients that's pretty great for overall wellness—as it supports immunity and has anti-inflammatory perks—and it may be particularly beneficial for people struggling with seasonal allergies.

Food sources are abundant (think fruits, veggies, nuts, teas) and contain a variety of other nutrients, but to help manage or prevent a specific ailment, supplementation at doses of 500 to 1,000 mg is typically necessary.

Just remember, as with all supplements, it's important to check in with your doctor first if you have questions or concerns. 

Keep in mind: When it comes to supporting overall immunity, quercetin should be just one component of your routine (nothing works in isolation, after all).