Which substance's dosing must be decreased when starting a patient on Amiodarone?

When a patient is started on Amiodarone, the dosing of Warfarin must be decreased due to the significant interaction between these two medications. Amiodarone is known to enhance the anticoagulant effects of Warfarin by inhibiting CYP2C9, which is responsible for metabolizing Warfarin. This interaction can lead to increased levels of Warfarin in the body, raising the risk of bleeding complications. Therefore, it is essential to monitor the patient's INR (International Normalized Ratio) closely and adjust the Warfarin dose accordingly.

In contrast, the other substances listed do not require a dose adjustment when initiated alongside Amiodarone. Furosemide, often used as a diuretic, does not interact in a way that necessitates a change in dosing with Amiodarone. Atorvastatin, a cholesterol-lowering medication, does not require a reduced dose due to its metabolic pathway being less affected by Amiodarone. Lastly, Aspirin, an antiplatelet agent, does not have a documented interaction that mandates a dose adjustment when used with Amiodarone.