Which of the following medications requires serum levels to be monitored regularly?

Digoxin is a cardiac glycoside used primarily to treat heart failure and certain arrhythmias. Regular monitoring of serum digoxin levels is crucial due to its narrow therapeutic index. This narrow therapeutic index means that the difference between a therapeutic dose and a toxic dose is very small, making patients susceptible to toxicity even with slight deviations in dosage or when interacting medications are introduced.

Furthermore, factors such as renal function can significantly affect digoxin levels, as the drug is primarily excreted through the kidneys. Monitoring these serum levels helps ensure that patients receive an effective dose while minimizing the risk of toxicity, which can lead to serious complications such as cardiac arrhythmias, gastrointestinal symptoms, and neurological effects.

In comparison, while other medications listed may require monitoring for different reasons—such as monitoring kidney function in patients on metformin, or thyroid function tests for levothyroxine—these do not typically necessitate the regular serum level checks associated with digoxin. Amoxicillin, an antibiotic, generally does not require such monitoring in standard practice.