63-year-old woman with suspected digoxin toxicity; which medication is most likely responsible for the interaction?

• A. Ramipril
• B. Clarithromycin
• C. Simvastatin
• D. Paracetamol

Answer: (B)

Digoxin has a very small margin between effective and toxic levels, so drugs that raise its concentration can precipitate toxicity. The interaction in focus is with a macrolide antibiotic. Clarithromycin inhibits intestinal P-glycoprotein, the transporter that pumps digoxin back into the gut. When this transporter is inhibited, more digoxin is absorbed and cleared more slowly, raising serum levels and the risk of toxicity. This mechanism makes clarithromycin the most likely culprit in a patient with suspected digoxin toxicity.

The other medications listed don’t have this effect. Ramipril is an ACE inhibitor and doesn’t raise digoxin levels; simvastatin interacts with some antibiotics in other ways but not by increasing digoxin toxicity; paracetamol is generally safe with digoxin.