## Warfarin–Antibiotic Interaction Mechanisms ### Recognized Mechanisms of INR Elevation **Key Point:** Antibiotics increase warfarin effect through three main pathways — NOT through enzyme induction. | Mechanism | Example Antibiotics | Explanation | |-----------|-------------------|-------------| | **Inhibition of vitamin K synthesis** | Amoxicillin, cephalosporins, fluoroquinolones | Eradicate gut flora that produce vitamin K, reducing recycling of vitamin K cofactor | | **Protein binding displacement** | Sulfonamides, NSAIDs (not strictly antibiotic) | Displace warfarin from albumin, increasing free (active) drug concentration | | **CYP450 inhibition** | Metronidazole, fluconazole, macrolides | Inhibit CYP2C9, reducing warfarin metabolism and increasing plasma levels | ### Why Enzyme Induction Is Wrong **High-Yield:** Antibiotics do NOT induce hepatic cytochrome P450 enzymes. Induction (which would DECREASE warfarin effect) is seen with rifampicin, but this is an exception and works in the opposite direction — it lowers INR, not raises it. **Clinical Pearl:** When a patient on warfarin is started on an antibiotic, INR typically rises within 2–3 days due to vitamin K depletion and/or protein displacement. Enzyme induction would cause INR to fall, which is the opposite of what happens with most antibiotics. ### Clinical Management 1. Monitor INR closely (baseline, then 2–3 days after antibiotic start) 2. Reduce warfarin dose if INR rises above therapeutic range 3. Counsel patient on vitamin K-rich foods (consistency is key) 4. Prefer antibiotics with minimal interaction (e.g., cephalosporins over fluoroquinolones if possible) [cite:KD Tripathi 8e Ch 12]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.