When taking any form of antibiotic while on coumadin, it is important to closely monitor PT/INR. While antibiotics eradicates bacteria in the guts, Vitamin K2 is produced by the synthesis of vitamin K by the bacteria present in the colon. Washing off the bacteria in the colon depletes vitamin K source. Coumadin is then well absorbed and INR level will readily increase or double. Taking antibiotics due to infection can not be avoided. I would suggest more frequent lab draw for PT/INR.
Reference:
1. Vitamin K – Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Vitamin_K
2. Microbial Fermentation
biology.about.com/library/organs/bldigestlargeint4.htm
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