Asking on behalf of my dad, he is 60, underwent bypass surgery a year ago, has been taking warfarin 10 years back. Recently his INR keep dropping even after taking more Warfarin, we went back to the doctor , after running all the test and scan, the doctor can t tell what is the problem, but advice him to take more warfarin, this time with 6mg for 2 days. Could there be underlying disease that has been missed? For example virus infection or cancer? What could be the most common cause? INR Reading Oct30 2.77 (Warfarin=3MG) Jan27 1.03 (Warfarin=3.5MG) Feb16 1.48 (Warfarin=4MG) Mar02 1.16 (Warfarin=4.5MG)