warfarin and antibiotics

Last reviewed 01/2022

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.