Brand-specific effects depend on formulation and excipients, but common quercetin-supplement side effects include nausea, GI discomfort, headache, and rarely allergic reactions. Reflux/bloating may worsen in sensitive individuals. Interaction-like symptoms can occur in people on chronic meds. If you use anticoagulants, have planned surgery, or kidney/liver disease, seek medical advice. Stop and seek care for serious warning signs (rash, breathing difficulty, severe pain, black stools).