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Warfarin and Amoxicillin/Clavulanate Drug Interaction

at time of writing, Clinical Pharmacy Manager, Department of Pharmacy, St. John's Episcopal Hospital, Far Rockaway, NY; Clinical Assistant Professor, Arnold & Marie Schwartz College of Pharmacy and Health Sciences, Long Island University, Brooklyn, NY; now, Pharmacy Manager, Fidelis Care New York, Quality Health Care Management, Rego Park, NY

Marina Yermolnik

at time of writing, PharmD Student, Arnold & Marie Schwartz College of Pharmacy and Health Sciences, Long Island University

Lewis J Cuni, MD

Co-Chief Hematology, St. John's Episcopal Hospital; Associate Clinical Professor, Medicine, State University of New York in Brooklyn, Brooklyn, NY

Reprints: Liya Davydov PharmD, Fidelis Care New York, QHCM, 95-25 Queens Blvd., Rego Park, NY 11374, FAX 718/896-6819, E-mail ldavydov{at}fideliscare.org, Liyad5{at}aol.com

OBJECTIVE: To report a case of warfarin–amoxicillin/clavulanate potassium (AM/CL) interaction resulting in an elevated international normalized ratio (INR) and hematuria.

CASE SUMMARY: A 58-year-old Hawaiian/Asian/European woman developed an elevated INR and microscopic hematuria as a result of a drug–drug interaction between warfarin and AM/CL.

DISCUSSION:Our report of an increased INR with bleeding complications as a result of an interaction between warfarin and AM/CL is consistent with those in the literature. Although the mechanism for this interaction is not fully known, it is suspected that a decrease in vitamin K–producing gut flora with resulting vitamin K deficiency would be the most likely contributing factor. An objective causality assessment revealed that this adverse drug event as a result of the warfarin and AM/CL interaction was possible.

CONCLUSIONS: An increased INR secondary to warfarin interactions with various antibacterial agents is a known phenomenon. An increased awareness of warfarin–AM/CL interaction and appropriate monitoring are essential to control the INR levels and prevent bleeding complications.

Key Words: amoxicillin/clavulanate potassium, international normalized ratio, warfarin

Published Online, January 27, 2003. www.theannals.com, DOI 10.1345/aph.1C243

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