A patient is taking warfarin and starts a new antibiotic. What key monitoring or action should be considered?

• A. Check INR and monitor for signs of bleeding; alert clinician if interaction suspected
• B. Stop all medications immediately
• C. Increase antibiotic dosage as needed
• D. Ignore potential interactions and continue as prescribed

Answer: (A)

When a patient on warfarin starts a new antibiotic, the key idea is that antibiotics can alter warfarin’s effect, increasing bleeding risk. The appropriate action is to check the INR after starting the antibiotic and to monitor for any signs of bleeding, then alert the clinician if an interaction is suspected. This happens because many antibiotics can raise warfarin’s effect by reducing gut vitamin K–producing bacteria or by inhibiting hepatic enzymes that metabolize warfarin, leading to a higher INR. Obtain an INR check as advised (often within a few days) and continue watching for bleeding symptoms such as unusual bruising, gum or nasal bleeding, blood in urine or stool, or severe headaches with weakness or confusion. Do not stop all medications or automatically increase the antibiotic dose; these decisions require clinician input to adjust the warfarin dose or consider an alternative antibiotic.