Antibiotics can affect INR values.

• A. False
• B. True
• C. Never
• D. Only when taking supplements

Answer: (B)

Antibiotics can influence INR values because warfarin’s anticoagulant effect depends on vitamin K–dependent clotting factors and on hepatic metabolism. Many antibiotics disrupt gut bacteria that synthesize vitamin K. With less vitamin K available, warfarin’s effect is potentiated, raising the INR and increasing bleeding risk. That’s why starting an antibiotic in a patient on warfarin often requires closer INR monitoring and possible dose adjustments. Some antibiotics can also alter warfarin metabolism in the liver, which can either raise or lower the INR depending on the drug, but the important point is that antibiotics can change INR values. Monitor for signs of abnormal bleeding and adjust therapy as needed.