When you take quercetin, a natural flavonoid found in apples, onions, and berries that’s often used as a supplement for inflammation and allergies. It’s known for its antioxidant properties, but what most people don’t realize is that quercetin can change how your body handles prescription drugs. Just like calcium or iron can block thyroid meds, quercetin can interfere with antibiotics, blood thinners, and even thyroid treatments. It doesn’t always cause harm—but it doesn’t always stay harmless either.
Quercetin affects enzymes in your liver, especially CYP3A4 and CYP2C9, which are responsible for breaking down over 50% of common medications. If you’re on warfarin, a blood thinner that requires precise dosing to prevent clots or bleeding, quercetin might make your INR levels swing unpredictably. That’s the same risk you get from alcohol or NSAIDs, and it’s just as dangerous. If you’re taking azithromycin, an antibiotic linked to heart rhythm issues when mixed with other QT-prolonging substances, quercetin could add to that risk by slowing how fast your body clears the drug. Even if you’re on thyroid medication like levothyroxine, quercetin may reduce absorption, just like calcium supplements do. It’s not about avoiding quercetin entirely—it’s about knowing when it’s safe and when it’s not.
People often assume natural means safe, but that’s not true when drugs are involved. A study in the Journal of Clinical Pharmacology found that quercetin increased blood levels of cyclosporine by nearly 30% in transplant patients, raising the chance of kidney damage. That’s not a small bump—it’s a red flag. If you’re on any medication that has a narrow therapeutic window, like NTI drugs or antiepileptics, you’re playing with fire by adding quercetin without checking. And if you’re older, taking multiple pills, or have liver issues, your body can’t clear it as easily, making interactions more likely.
You’ll find real stories in the posts below—from people who thought quercetin was just a harmless berry extract, only to end up with dizziness, bruising, or weird heart palpitations. Others figured out how to time their doses to avoid clashes with their thyroid meds or antibiotics. This isn’t theory. It’s lived experience. What you’re about to read isn’t a warning to stop taking it. It’s a guide to taking it smartly.
Quercetin supplements can dangerously raise levels of common medications by inhibiting liver enzymes. Learn which drugs are at risk, who's most vulnerable, and what to do to stay safe.