CoQ10 may interfere with warfarin treatment, although the evidence is mixed. There are some reports suggesting that CoQ10 decreases the effectiveness of warfarin (Coumadin), which could necessitate an increase in warfarin dosage. On the other hand, there's a study which suggests the opposite (that CoQ10 may increase the risk of bleeding when on warfarin). And another study suggests no effect so long as your INR (bleeding time) is stable. To play it safe, consult with your doctor about using CoQ10 with warfarin.
However, it's not clear whether it CoQ10 affects Plavix treatment. For more information, see the Cautions and Concerns section of the CoQ10 and Ubiquinol Supplements Review >>
Also see the Encyclopedia article about Warfarin.