Intracerebral hemorrhage secondary to a warfarin-metronidazole interaction.
BACKGROUND
It has been >25 years since the interaction between warfarin and metronidazole was last reported in the literature. The current case report represents the first documentation of this interaction associated with intracerebral hemorrhage.


METHODS
We present a case of a 78-year-old white woman started on metronidazole (250 mg every 8 hours for 5 days) and levofloxacin (500 mg QD for 6 days) for an upper respiratory tract infection after visiting a walk-in clinic. The patient did not notify any of the health care professionals involved that she was on concomitant warfarin therapy, which had been stable over the last 3 months. Her warfarin dose was 7 mg daily, and her most recent international normalized ratio (INR) reading was 2.5. Nine days after her clinic visit, the patient was admitted to the hospital for a profuse nosebleed with an INR of 8.0 and was found to have an intraparenchymal hemorrhage of the left occipital lobe. The Naranjo adverse drug reaction probability scale indicated that the association with metronidazole was probable and the association with levofloxacin was possible (scores of 7 and 4, respectively). After a 1-week hospital stay, she was discharged.


CONCLUSIONS
This adverse event is highly suggestive of a drug interaction caused primarily by metronidazole, which produces an increase in S-warfarin concentrations. Treatment provided by health care providers who were not familiar with the patient and the use of a different pharmacy (where the pharmacist was unaware of her current medications) likely contributed to the event.
Howard-Thompson
Amanda
A
Department of Pharmacy, Methodist University Hospital, Memphis, Tennessee 38104, USA.
Hurdle
April C
AC
Arnold
Lori B
LB
Finch
Christopher K
CK
Sands
Christopher
C
Self
Timothy H
TH
D000368:Aged; D000890:Anti-Infective Agents / Q000009:adverse effects*; D000925:Anticoagulants / Q000009:adverse effects*; D002543:Cerebral Hemorrhage / Q000139:chemically induced*; D004347:Drug Interactions; D005260:Female; D006801:Humans; D019934:International Normalized Ratio; D008795:Metronidazole / Q000009:adverse effects*; D014859:Warfarin / Q000009:adverse effects*
D000890:Anti-Infective Agents; D000925:Anticoagulants; D008795:Metronidazole; D014859:Warfarin