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

So many aspects of modern life can impair immune system functioning (poor sleep, excessive sugar intake, chronically elevated stress levels—you get the picture), making it harder to stay healthy. Identifying your immune-sapping triggers and adjusting your lifestyle accordingly is a great start, but no one's perfect—so, we often reach for supplements like vitamin C and vitamin D for extra immune strength. But there's another less well-known nutrient gaining attention for its immune-supporting potential: quercetin. 

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. 

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 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."

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. 

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 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. 

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.