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 Adverse Cases

Back Reported Adverse Drug Reaction Cases
Fluoroquinolone antibiotics ...Interactions with warfarin

The potential interaction between warfarin and fluoroquinolones (ciprofloxacin, norfloxacin, moxifloxacin, gatifloxacin)* was reported in a 1993 review.1 ADRAC has received 20 reports of this interaction, implicating ciprofloxacin (9 reports), norfloxacin (11) and moxifloxacin (1). One of the reports involves both ciprofloxacin and norfloxacin. As yet, no reports have been received with gatifloxacin, which has had little use.

Of the 20 ADRAC reports, one patient had vaginal bleeding; for the other patients the coagulation disorder was detected on laboratory testing. In eight cases an INR of greater than 7 was recorded. In one case, the patient's INR rose from a stable baseline of around 2.0 to more than 10, four days after commencing moxifloxacin. Two days after moxifloxacin and warfarin were withdrawn and treatment with vitamin K was given, the INR had fallen to 1.2. No symptomatic event occurred.

Health Canada has reported 57 cases of this interaction up to Jan 2004.2 Gatifloxacin was also implicated in the Canadian series. In 16 of the 57 cases the patient was hospitalised and four patients aged 70-90 years with complex medical conditions died.

Although significant pharmacokinetic interactions have not been demonstrated in interaction studies, the Product Information for each of the fluoroquinolones and for warfarin warn that an increased effect of warfarin is possible, and that the INR should be closely monitored when a fluoroquinolone and warfarin are administered concomitantly.

Possible mechanisms of this interaction include decreased warfarin cytochrome P450-mediated metabolism, and reduction in gut flora that produce vitamin K.

ADRAC advises health professionals to consider the possibility of this interaction and monitor the INR when fluoroquinolones and warfarin are used concomitantly.

  1. Marchbanks CR. Drug-drug interactions with fluoroquinolones. Pharmacotherapy 1993;13:23S-28S.
  2. Morawiecka I. Fluoroquinolones and warfarin: suspected interactions. Canadian Adv Reaction Newsletter 2004;14:1-2.
Australian Adverse Drug Reactions Bulletin
Volume 25, Number 1, February 2006


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