Warfarin Institute of America
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LEVOFLOXACIN INTERACTIONS WITH WARFARIN
Ravnan and Locke report two cases where the INRs became elevated when patients took levofloxacin. Both simply had their warfarin stopped until the INR returned to the desired range. Neither patient suffered harm and both returned to their previous warfarin doses after the levofloxacin was stopped. The first patient, a 70-year-old man had his INR in the desired range for eight months before the incident occurred. He took the levofloxacin for ten days and five days after it was stopped, his INR was 5.7. The second patient was a 75-year-old man who had just had his warfarin dose reduced. He took the levofloxacin for five days and one day later, his INR was 7.9.
The authors point out that the prescribing information for levofloxacin states that there was no significant prothrombin time change when healthy volunteers took this combination of medications. This illustrates the difficulty in extrapolating data from healthy volunteers to ill patients. The authors also correctly point out that the elevated INR could be due to the patients infections having a effect on the warfarin. Nonetheless, sick patients taking levofloxacin can have elevated INRs.
The authors also give three guidelines for managing or avoiding the interaction:
- The patient should self-monitor for signs and symptoms of bleeding
- Monitor the INR frequently
- Use an alternate antibiotic
Jones and Fugate reported on four cases of elevated INRs when levofloxacin was added to warfarin therapy. The patients ranged in age from 34 to 81 years old. The INRs were 3.5, 8.1 and 11.5 after 11, 5 and 4 days of levofloxacin. These patients experienced no serious problems. A fourth patient had minor bleeding and a slightly elevated INR on day two of therapy. The warfarin dose was reduced by 19% during the time that levofloxacin was given.
I'm not sure if this was the effect of levofloxacin or that the patients were sick and simply changed their eating patterns. For the millions of people who have taken this combination to have generated only six cases in the medical literature, it seems to me that the combination is probably safe.
Editor's Note: In my clinic, I often see patients who live up to 100 miles away. It does not seem practical to ask an ill person to make several 200 mile trips to have their INR monitored. In these cases, I have instructed them to decrease their warfarin dose on two or three days while they take an antibiotic. I have no data on what their INRs were during this time, but there have been no episodes requiring any medical treatment.
References:
Ravnan SL et al. Levofloxacin and warfarin interaction. Pharmacotherapy 2001; 21:884-885.
Jones CB et al. Levofloxacin and warfarin interaction. Ann Pharmacother 2002:36;1554-1557.
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Last updated November 13, 2004