Which antibiotics have a high enough risk of interacting with warfarin to warrant pre-emptive adjustments in warfarin dosage?
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Antibiotics that are less likely to affect the INR are Penicillin, Clindamycin and first- and fourth-generation cephalosporins.
Empiric adjustment should be considered with the use of Bactrim, Rifampin, Fluconazole and Metronidazole. See the chart below for adjustment recommendations for each:
Other antibiotics such as fluoroquinolones (Ciprofloxacin, Levofloxacin) and macrolides (Azithromycin, Clarithromycin and Erythromycin) may affect the INR, but the effect is variable and patient specific.
To put it into clinical practice:
Choose antibiotics with less likelihood to affect the INR such as PCN G, Cindamycin or cephalosporins
If Bactrim or Flagyl can’t be avoided, adjust warfarin dose pre-emptively, and recheck INR 3- 5 days after starting the antibiotics and 3-5 days after discontinuing the antibiotics
When prescribing fluoroquinolones, macrolides, and tetracyclines, do not adjust warfarin dose preemptively with a plan to check INR 5 days after starting the course.
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