Warfarin Institute of America
DEDICATED TO YOUR HEALTH SINCE 2000
AZATHIOPRINE INTERACTION WITH WARFARIN (Coumadin, Jantoven)
I THINK THAT THIS IS A VERY DANGEROUS INTERACTION. IT IS OFTEN OVERLOOKED BY BOTH THE PRESCRIBING PHYSICIAN AND THE PHARMACIST FILLING THE PRESCRIPTION BECAUSE THEY HAVE NEVER SEEN IT. DO NOT UNDERSTIMATE THE SERIOUS NATURE OF THIS INTERACTION!
One person who had azathioprine added to her warfarin (Coumadin, Jantoven) had her INR dropped dramatically. Before her INR went back to the therapeutic range, we had tripled her warfarin (Coumadin, Jantoven) dose. When the azathioprine dose was reduced by half, her INR increased quickly. Because we were looking for this to happen, she did not have an adverse event.
A reader of this website wrote to say that her warfarin (Coumadin, Jantoven) dose had to be raised by 50% after azathioprine was started.
Havrda et al. reported on the case of a woman with a deep vein thrombosis and steroid dependent ulcerative colitis. She had completed a short-course of warfarin (Coumadin, Jantoven) when azathioprine was added for a steroid-sparing effect. Another DVT developed and she was restarted on warfarin (Coumadin, Jantoven). During the first course of warfarin (Coumadin, Jantoven), 35 mg/wk were sufficient to maintain a therapeutic INR. After the azathioprine was started, up to 120 mg of warfarin (Coumadin, Jantoven) were required to maintain a therapeutic INR. The warfarin (Coumadin, Jantoven) dose had to be adjusted upward and downward to coincide with adjustments in the azathioprine dosage.
I have managed several people with this interaction and it has never been easy.
Besides my experience, the medical journal article references for these articles are:
Havrda DE et al. A case report of warfarin resistance due to azathioprine and review of the literature. Pharmacotherapy 2201;21:355-357
Rotenberg M. et al. Effect of Azathioprine on the Anticoagulant Activity of Warfarin. Ann Pharmacother 2000:34;120-122.
Rivier, G. Warfarin and Azathioprine: A Drug Interaction Does Exist. Am J Med 1993;95:342.
Singleton JD. Warfarin and Azathioprine: An Important Drug Interaction. Am J Med 1992;92:217. If you wish to look these up, print this page and take it to your librarian.
It is most important that you have your INR monitored if azathioprine is added to warfarin (Coumadin, Jantoven) or discontinued from warfarin (Coumadin, Jantoven).
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Last modified March 25, 2006