Warfarin and Amoxicillin/Clavulanate Drug Interaction

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.