Which drug requires plasma concentration monitoring due to narrow therapeutic window?

• A. Acetaminophen
• B. Amoxicillin
• C. Ibuprofen
• D. Digoxin

Answer: (D)

Drugs with a narrow therapeutic window require plasma concentration monitoring to keep them effective but not toxic. Digoxin fits this pattern because the range between the dose that benefits the heart and the dose that can cause toxicity is small and varies a lot between individuals. Its clearance depends mainly on kidney function, so changes in renal function, age, or interactions with other drugs can lead to significant shifts in blood levels. Because of this, clinicians monitor digoxin trough levels and adjust dosing accordingly, aiming for a specific therapeutic range that depends on the indication (often lower targets for atrial fibrillation and slightly higher for heart failure). Therapeutic drug monitoring helps catch accumulation early, reducing risks of toxicity such as nausea, confusion, visual disturbances, and dangerous arrhythmias.

In contrast, the other drugs listed aren’t routinely monitored for plasma levels. Acetaminophen levels aren’t used to guide normal dosing; they’re checked mainly in suspected overdose or to assess hepatotoxicity. Amoxicillin and ibuprofen don’t require regular plasma level monitoring in standard practice.