What is the main consequence of starting amiodarone in a patient already taking digoxin?

Starting amiodarone in a patient already taking digoxin leads to increased digoxin concentrations primarily due to the pharmacokinetic interactions between the two medications. Amiodarone is known to inhibit P-glycoprotein, an important transporter that helps in the efflux of digoxin from cells. This inhibition results in reduced clearance of digoxin, consequently causing elevated serum digoxin levels.

Furthermore, amiodarone also has effects on hepatic metabolism, and while digoxin is not heavily metabolized by the liver, the alteration in the normal excretion pathways due to amiodarone's effect can lead to higher levels of digoxin in circulation.

Therefore, when a patient is started on amiodarone while already on digoxin, careful monitoring of digoxin levels is imperative to prevent toxicity, as the risk for adverse effects increases significantly with higher concentrations.