Quercetin Side Effects with Meds: What You Need to Know

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 power, but it doesn’t play nice with every medication. Many people assume natural equals safe, but quercetin can change how your body handles drugs—sometimes in dangerous ways. It’s not just about stomach upset; it can interfere with how your liver processes medications, making them too strong or too weak.

For example, if you’re on blood thinners, like warfarin or aspirin, used to prevent clots, quercetin might boost their effect and raise your risk of bleeding. Same goes for antibiotics, such as cefadroxil or other fluoroquinolones, which rely on precise absorption to work. Quercetin can block their uptake in the gut, leaving you unprotected against infection. And if you’re managing high blood pressure, with drugs like hydrochlorothiazide or other diuretics, quercetin might lower it too far, causing dizziness or fainting.

It’s not just about the big names. Even over-the-counter painkillers like ibuprofen can become riskier when mixed with quercetin. The same goes for supplements like melatonin or St. John’s wort—quercetin doesn’t just sit there quietly. It changes how your body absorbs, breaks down, or eliminates other substances. That’s why pharmacists flag it in allergy alerts, even if it’s not a true allergen. The real danger? You won’t always feel it coming. Side effects can sneak up days later, or show up as a sudden drop in energy, unexplained bruising, or a headache that won’t quit.

You don’t need to quit quercetin entirely—but you do need to know who you’re taking it with. If you’re on more than one medication, especially for heart, immune, or mental health conditions, talk to your pharmacist before adding anything new. They check for interactions every day. Don’t wait for a side effect to happen. A quick check can save you from a hospital visit.

Below, you’ll find real cases and clear breakdowns of how quercetin plays with common drugs—from blood pressure meds to antibiotics and beyond. No fluff. Just what you need to know to stay safe.