“Most drug interactions with warfarin (70%) potentiate its anticoagulant effects and therefore increase the risk of bleeding. Numerous common anti-infectives, many (but not all) NSAIDs, and several SSRIs consistently potentiate warfarin, and using them together requires careful monitoring to reduce the risk of bleeding. With ciprofloxacin, diltiazem, and sertraline, inhibition of cytochromes, selective clearance of isomers, and alterations in the vitamin-K pathway are all possible mechanisms resulting in the potentiation of the anticoagulant effects of warfarin and increasing the risk of bleeding (SOR B).
Other medications, including trazodone, inhibit warfarin’s effect, placing the patient at risk for complications related to inadequate anticoagulation. While clopidogrel independently increases bleeding risk, it neither potentiates nor inhibits warfarin.”
http://jamanetwork.com/journals/jamainternalmedicine/fullarticle/486574