Warfarin Institute of America
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THAT IS A GOOD QUESTION
AZITHROMYCIN INTERACTION WITH WARFARIN
Azithromycin has a long half-life. This means that it takes many days for the body to rid itself of azithromycin after you stop taking it. This complicates the management of this interaction.
After about three days on azithromycin, your INR can increase to a level where you are at a danger for having a bleeding episode. At this point, you usually have only two more days of azithromycin. It is important that the prescriber not overlook the long azithromycin half-life. Your warfarin dose will need to be held or decreased for several days. You should have another INR test done about five days after you finish azithromycin.
Another factor confounding the management of this interaction is that when you are sick, you probably are not eating well. When you do not eat well, your intake of vitamin K is lessened because you are probably eating less vegetables. Vitamin K is the direct opposition to warfarin.
McCall et al (Pharmacotherapy. 2004 Feb;24(2):188-94) retroactively reviewed the charts of twenty people who took azithromycin and twenty who did not and found that there was no interaction. However, they studied any INR that was done within 30 days of the azithromycin therapy. This was plenty of time for the INR to have gone back down to normal.
Whether or not this is a "true" interaction or whether the azithromycin is a marker that you are not feeling well, the INR can be greatly elevated when someone is taking this combination. I saw one elderly lady who was bleeding from the pores in her face from this interaction. She had noticed bruising on a Friday night and held her warfarin until I saw her on Monday, so her INR had come back to the desired range. You should not be lulled into a false sense of security because this is not a "true" interaction. I also do not think that official package insert has a strong enough statement about this phenomenon It only states that healthy volunteers had no significant interaction.
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Last modified April 22, 2006