Azathioprine appears to increase
warfarin requirements, and cases of bleeding have occurred when
azathioprine is stopped. Dose increases of up to 4-fold have been needed. Mercaptopurine appears to interact similarly with acenocoumarol.
Monitor the anticoagulant effect if
azathioprine or mercaptopurine is added or
withdrawn and adjust the
warfarin dose as necessary. Similar precautions would
seem necessary with any coumarin.