When a patient begins warfarin therapy with a new antibiotic, what should a clinician first check?

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Multiple Choice

When a patient begins warfarin therapy with a new antibiotic, what should a clinician first check?

The crucial idea is that warfarin has a narrow therapeutic window and is highly susceptible to drug interactions. When a patient on warfarin starts a new antibiotic, the clinician should first check for any potential interactions, because many antibiotics can increase warfarin’s effect by inhibiting hepatic metabolism or by reducing gut vitamin K production, leading to higher INR and bleeding risk. If an interaction exists, the approach typically involves adjusting the warfarin dose or choosing an alternative antibiotic and increasing INR monitoring (often within 48–72 hours after starting the antibiotic, then as needed). While allergy history, renal function, and age are important for other reasons, they do not address the immediate and common risk posed by combining warfarin with a new antibiotic.

The crucial idea is that warfarin has a narrow therapeutic window and is highly susceptible to drug interactions. When a patient on warfarin starts a new antibiotic, the clinician should first check for any potential interactions, because many antibiotics can increase warfarin’s effect by inhibiting hepatic metabolism or by reducing gut vitamin K production, leading to higher INR and bleeding risk. If an interaction exists, the approach typically involves adjusting the warfarin dose or choosing an alternative antibiotic and increasing INR monitoring (often within 48–72 hours after starting the antibiotic, then as needed). While allergy history, renal function, and age are important for other reasons, they do not address the immediate and common risk posed by combining warfarin with a new antibiotic.

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