A 52-year-old man with atrial fibrillation on warfarin (INR 2.5) is started on amiodarone for rate control. Which of the following is the most appropriate management to prevent warfarin toxicity?

Explanation

## Drug Interaction: Warfarin and Amiodarone **Key Point:** Amiodarone is a potent inhibitor of CYP3A4, CYP2C9, and CYP2C8, which metabolize warfarin. This interaction significantly increases warfarin levels and INR, raising the risk of bleeding. ### Mechanism of Interaction 1. Warfarin is metabolized primarily by **CYP2C9** (S-enantiomer, more potent) 2. Amiodarone inhibits CYP2C9 and reduces warfarin clearance 3. Result: **↑ INR** (typically by 50–100% within 3–5 days) 4. Risk: **Major bleeding** (intracranial, GI, retroperitoneal) ### Management Strategy | Step | Action | Rationale | |------|--------|----------| | **Baseline** | Document current INR (2.5) | Establish reference | | **Dose adjustment** | Reduce warfarin by 30–50% | Prevent supratherapeutic INR | | **Monitoring** | INR every 3–5 days × 2 weeks, then weekly | Detect overcorrection early | | **Target INR** | Maintain 2.0–3.0 | Therapeutic for AF | **High-Yield:** The interaction becomes clinically significant within **3–5 days** of amiodarone initiation. Proactive dose reduction is safer than reactive management of elevated INR. **Clinical Pearl:** Amiodarone has a very long half-life (~40 days). Even after discontinuation, warfarin dose reduction may be needed for weeks. ### Why Other Options Fail - **Increasing warfarin** (Option 0): Compounds the problem; would cause severe supratherapeutic INR and bleeding. - **Assuming no interaction** (Option 2): Dangerous; amiodarone is one of the most significant warfarin interactors. - **Switching to apixaban** (Option 3): Not necessary if warfarin is dose-adjusted. Also, apixaban has its own interactions (e.g., with strong CYP3A4 inhibitors like amiodarone, though less critical than with warfarin).