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Warfarin and antibiotics

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

The anticoagulant effect of warfarin and other coumarins is reduced by rifamycins.

Antibiotics that enhance the anticoagulant effect include:

  • cephamandole
  • chloramphenicol
  • ciprofloxacin
  • co-trimoxazole
  • erythromycin
  • metronidazole
  • ofloxacin
  • sulphonamides

There is possible enhancement of the anticoagulant effect by:

  • azithromycin
  • aztreonam
  • clarithromycin
  • nalidixic acid
  • neomycin
  • tetracyclines
  • trimethoprim

There is anecdotal evidence that the INR can be altered following a course of oral broad spectrum antibiotic e.g. ampicillin.


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