If you are being treated with any of the following medications, you should not use quercetin supplements without first talking to your health care provider.

Anticoagulants (blood thinners) -- Quercetin may enhance the effect of these drugs, increasing your risk for bleeding:

• Warfarin (Coumadin)
• Clopidogrel (Plavix)
• Aspirin

Chemotherapy -- Test tube and animal studies suggest that quercetin may enhance the effects of doxorubicin and cisplatin, two chemotherapy medications used to treat cancer. In addition, some doctors believe taking antioxidants at the same time as chemotherapy can be harmful, while others believe it can be helpful. Talk to your oncologist before taking any supplements if you are undergoing chemotherapy.

Corticosteroids -- Quercetin may cause these drugs to stay in the body longer.

Cyclosporine -- Quercetin may interfere with the body's absorption of this drug, which is used to suppress the immune system.