amiodarone × metoprolol

Dual mechanism: 1) additive AV conduction slowing and heart rate reduction (amiodarone non-competitively blocks β-receptors, metoprolol selectively blocks β1); 2) amiodarone inhibits CYP2D6, doubling metoprolol concentration. Risk of symptomatic bradycardia, AV block, and hypotension.

The combination is often justified in AF. Start metoprolol at a low dose (12.5–25 mg twice daily) and titrate by heart rate and BP. Monitor ECG (QT, AV conduction) every 3–6 months. In older adults, reduce the dose when resting heart rate drops below 55/min.