Antibiotic-Warfarin Interaction — EECC MCQ

Correct answer: D — Amoxicillin reduces gut flora that synthesise vitamin K, decreasing vitamin K availability and potentiating warfarin's anticoagulant effect — this is a common and clinically significant interaction that requires more frequent INR monitoring during antibiotic courses

Multiple antibiotic classes interact with warfarin through various mechanisms: (1) Gut flora disruption (amoxicillin, other broad-spectrum antibiotics): intestinal bacteria synthesise menaquinone (vitamin K2), contributing to overall vitamin K supply. Antibiotics that deplete these bacteria reduce vitamin K availability, potentiating warfarin's effect; (2) CYP450 inhibition (metronidazole, fluconazole, trimethoprim — inhibit CYP2C9, the primary S-warfarin metabolising enzyme; erythromycin/clarithromycin — inhibit CYP3A4, affecting R-warfarin); (3) Reduced absorption (rifampicin — also a potent CYP inducer, reducing warfarin levels). The 2024 ESC AF Guidelines and BNF recommend: increased INR monitoring frequency during antibiotic courses in warfarin patients (check INR within 3-5 days of starting antibiotics). This interaction is one of the most common causes of supratherapeutic INR in clinical practice.

Reference: BNF; ESC (2024): AF Guidelines