Which antibiotic class should be avoided in patients taking warfarin due to vitamin K interactions?

• A. Macrolides
• B. Cephalosporins
• C. Fluoroquinolones
• D. Aminoglycosides

Answer: (B)

The concept being tested is how antibiotics can affect vitamin K–dependent coagulation in someone taking warfarin. Warfarin works by inhibiting the vitamin K–dependent gamma-carboxylation of clotting factors II, VII, IX, and X, which lowers the body's ability to form clots. Vitamin K comes not only from the diet but also from gut bacteria that synthesize it. When broad-spectrum antibiotics suppress these gut bacteria, vitamin K production drops, tipping the balance toward increased anticoagulation and a higher risk of bleeding. That’s why this antibiotic class is avoided or used with careful INR monitoring in patients on warfarin.

Some other antibiotics can interact with warfarin, but not primarily through vitamin K depletion. Macrolides can raise warfarin levels via metabolic interactions, fluoroquinolones can influence coagulation through various mechanisms, and aminoglycosides have minimal effect on vitamin K–dependent coagulation.