Quercetin Side Effects: What You Need to Know Before Taking It
When you hear quercetin, a natural flavonoid found in apples, onions, and berries that’s marketed as an anti-inflammatory and antioxidant supplement. Also known as a plant pigment with potential immune-modulating effects, it’s often sold as a standalone pill or bundled with vitamin C and zinc for cold prevention. But just because it’s natural doesn’t mean it’s harmless. People take it hoping to reduce allergies, ease joint pain, or boost endurance—but many don’t realize it can interfere with medications, cause stomach upset, or even trigger reactions in sensitive individuals.
Flavonoid side effects, the unwanted reactions that can occur when consuming high doses of plant-based compounds like quercetin aren’t always obvious. Unlike prescription drugs, supplements like quercetin aren’t required to prove safety before hitting shelves. Studies show that doses above 1,000 mg per day may cause headaches, tingling in the limbs, or nausea. Some users report kidney stress, especially if they already have kidney issues or are taking other drugs cleared by the kidneys. And here’s the catch: quercetin can block enzymes your liver uses to break down common meds like blood thinners, antibiotics, and even chemotherapy drugs. That’s not theoretical—it’s been documented in clinical reports.
Quercetin supplements, over-the-counter products that vary wildly in purity, dosage, and added ingredients are a minefield. One brand might contain 500 mg of pure quercetin; another might have 250 mg plus caffeine and artificial fillers. There’s no standardization. People with autoimmune conditions should be especially careful—quercetin can suppress immune activity, which might sound good until you’re fighting an infection and your body can’t respond properly. Pregnant women, nursing mothers, and kids should avoid it unless a doctor says otherwise. Even healthy adults can have bad reactions if they mix it with alcohol or other supplements like green tea extract or resveratrol.
What you’ll find below isn’t a list of marketing claims. These are real cases, documented interactions, and patient experiences pulled from actual medical reports. You’ll see how quercetin interacts with antibiotics like doxycycline, how it might affect people on warfarin, and why some users saw no benefit but still paid the price in side effects. This isn’t about whether quercetin works—it’s about whether it’s safe for you.
