Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Methimazole may indeed increase the effect of Warfarin. How much it s going to affect it cannot be predicted on an individual basis. In such cases we have to balance the risk of any particular patient for bleeding complications - for example a patient who s over 65, drinks a few drinks every day, has uncontrolled hypertension and a history of major bleeding is at high risk for a new major bleeding event. For a patient at low bleeding risk monitoring might be enough (measuring the INR during the first few days of concurrent use). For high risk patients perhaps reducing the dosage of Warfarin a bit might be more appropriate. Monitoring for a few days should be enough in most cases. Hope I have answered your query. Let me know if I can assist you further.