Lipitor (atorvastatin) does not significantly affect INR levels in patients taking warfarin. Studies show that lipitor, at typical doses, does not interact strongly with warfarin or alter its anticoagulant effect, as measured by INR. Patients should monitor INR regularly when starting or stopping lipitor to ensure stable anticoagulation, but major interactions are unlikely based on available data [1][2].
Sources:
1. DrugPatentWatch.com
2. Additional pharmacological references on statin and warfarin interactions.