A 59-year-old man on warfarin is prescribed antibiotics for a suspected infection; which antibiotic is most appropriate with his warfarin?

• A. Erythromycin
• B. Amoxicillin
• C. Cefalexin
• D. Ciprofloxacin

Answer: (C)

Warfarin’s effect depends on liver metabolism and vitamin K–dependent clotting factors. Antibiotics can interact with warfarin in two main ways: by reducing gut bacteria that synthesize vitamin K, which can tilt the balance toward more anticoagulation, and by inhibiting the enzymes that metabolize warfarin, increasing its levels. Because of these interactions, some antibiotics markedly raise bleeding risk, while others have little effect.

Erythromycin is a macrolide that strongly inhibits the enzymes responsible for breaking down warfarin, often causing a significant increase in INR and bleeding risk. Ciprofloxacin can also raise INR, but the effect is variable. Amoxicillin can modestly affect INR as well, due to changes in gut flora, but the risk is generally smaller. Cefalexin, a first-generation cephalosporin, typically has minimal impact on warfarin metabolism and on gut vitamin K production, making it the safer option for a patient on warfarin.

So, cefalexin is chosen to minimize interaction with warfarin. Nonetheless, starting any antibiotic in a patient on warfarin requires monitoring the INR (usually within 48–72 hours) and adjusting the warfarin dose if needed, with attention to any signs of bleeding.