The anticoagulant effect of warfarin is assessed by the measurement of prothrombin time (PT). Failure of elevation of PT indicates the decreased effect of warfarin. Rifampicin is an enzyme inducer and can decrease the effect of warfarin. Rifampicin is the most effective drug for tuberculosis and should not be replaced. Acenocoumarol is also an oral anticoagulant. Its metabolism is also subjected to induction by rifampicin. Effect of warfarin starts in 4-5 days. Therefore if we increase the dose of warfarin, it will take 5 days to prevent DVT. For immediate action, we should start heparin or LMW heparin. However, heparin needs to be given by injection, thus not suitable for long-term use. Therefore, the next step will be to shift to heparin but long-term treatment will be by increasing the dose of warfarin.