Quercetin is a plant pigment with antioxidant and anti-inflammatory properties, used most commonly to manage allergies, support cardiovascular health, and boost immune function. It’s found naturally in onions, apples, berries, and broccoli, and is widely available as a supplement in doses up to 500 milligrams twice daily.
How Quercetin Works in the Body
Quercetin belongs to a class of plant compounds called flavonoids. At the cellular level, it does two key things: it neutralizes unstable molecules called free radicals that damage cells, and it dials down inflammatory signals before they spiral out of control.
On the antioxidant side, quercetin activates a protective pathway in your cells that ramps up production of your body’s own antioxidant enzymes. Think of it as flipping a switch that tells your cells to reinforce their defenses against oxidative damage. It also binds to metals like iron and copper that would otherwise accelerate that damage.
On the inflammation side, quercetin blocks a central inflammatory signaling molecule called NF-κB. When NF-κB is suppressed, your body produces fewer inflammatory compounds, including several that drive swelling, pain, and tissue damage. This dual action, reducing oxidative stress while calming inflammation, is the foundation for nearly every health benefit quercetin is associated with.
Allergy and Histamine Relief
Quercetin’s most popular use is as a natural option for seasonal allergies. It works differently from antihistamines you’d buy at the pharmacy. Rather than blocking histamine after it’s released, quercetin stabilizes the mast cells that release histamine in the first place. By keeping those cells from degranulating in response to pollen or other allergens, it reduces symptoms at the source.
In one study, people who took a quercetin-containing supplement for four weeks reported less eye itching, less frequent sneezing, reduced nasal discharge, and better sleep. A separate study concluded that quercetin had no significant side effects and could be a good supplement for managing allergic conditions. It won’t replace a prescription antihistamine during a severe reaction, but for people with mild to moderate seasonal symptoms, it can take the edge off.
Blood Pressure and Heart Health
Quercetin has a measurable effect on blood pressure. A meta-analysis published in the Journal of the American Heart Association, pooling data from nine randomized controlled trial arms, found that quercetin supplementation lowered systolic blood pressure by about 3 mm Hg and diastolic by about 2.6 mm Hg on average. At doses of 500 milligrams per day or higher, the reductions were larger: roughly 4.5 mm Hg systolic and 3 mm Hg diastolic.
Those numbers might sound modest, but in population-level terms, even a 2 to 5 mm Hg drop in systolic blood pressure is associated with meaningful reductions in stroke and heart disease risk. The effect likely comes from quercetin’s ability to reduce oxidative stress in blood vessel walls, improving their ability to relax and dilate.
Immune Support and the Zinc Connection
Quercetin acts as a zinc ionophore, meaning it helps shuttle zinc into your cells. Research published by the American Chemical Society showed that quercetin forms a complex with zinc, carries it across cell membranes, and then releases it inside the cell where zinc can do its work. This matters because intracellular zinc interferes with viral replication, and many people don’t have enough zinc inside their cells even when their dietary intake is adequate.
This is why quercetin and zinc supplements are often sold together. Quercetin essentially acts as a delivery vehicle, making the zinc you consume more effective at supporting immune cell function. The combination gained widespread attention during the COVID-19 pandemic, though the evidence for preventing any specific infection remains limited.
Exercise Performance
Athletes sometimes use quercetin to improve endurance. A study published in the International Journal of Sport Nutrition and Exercise Metabolism found that seven days of quercetin supplementation produced a 3.9% increase in VO2 max compared to placebo. VO2 max is the maximum amount of oxygen your body can use during intense exercise, and a nearly 4% improvement is notable for a supplement. The effect likely comes from quercetin’s influence on mitochondrial function, helping muscle cells produce energy more efficiently. That said, the research on exercise performance is thinner than the evidence for quercetin’s cardiovascular or allergy benefits, and not every study has replicated this result.
Top Food Sources
You don’t need a supplement to get quercetin. It’s abundant in everyday foods:
- Onions (especially red onions) are the richest common source
- Apples with the skin on
- Cherry tomatoes
- Broccoli
- Blueberries
Cooking reduces quercetin content somewhat, so raw or lightly cooked preparations deliver more. A diet heavy in colorful fruits and vegetables can easily provide meaningful amounts without supplementation, though the doses used in clinical trials (500 mg or more) are difficult to reach through food alone.
Dosage, Safety, and Side Effects
Most clinical studies use 500 milligrams twice a day, and that dose appears safe for up to 12 weeks. Longer-term safety data is limited. Human studies have shown that doses up to 1,000 milligrams per day don’t cause adverse effects on kidney function, even over several months, despite older concerns about potential kidney stress at very high doses.
The most important safety consideration is drug interactions. Quercetin can interfere with blood thinners like warfarin in two ways: it displaces the drug from proteins in the blood (increasing the amount of active drug circulating freely), and it inhibits the liver enzyme responsible for breaking warfarin down. Both effects amplify warfarin’s blood-thinning action, raising the risk of bleeding. If you take warfarin, other anticoagulants, or anti-inflammatory medications like diclofenac, talk to your prescriber before adding quercetin.
For most people, quercetin supplements are well tolerated. Mild digestive discomfort is the most commonly reported side effect. Taking it with food generally helps, and pairing it with a fat source can improve absorption since quercetin is fat-soluble.