Which combination is most likely to cause a clinically significant drug interaction?

• A. Digoxin, Amiodarone
• B. Warfarin, Ramipril
• C. Lithium, Bendroflumethiazide
• D. Amitriptyline, Loperamide

Answer: (A)

When a drug with a narrow therapeutic index is affected by another medication, small changes in its level can have big, dangerous effects. Digoxin has a very small margin between effective and toxic levels. Amiodarone can raise digoxin levels by inhibiting pathways that clear it (notably P-glycoprotein and aspects of renal clearance). The result is higher digoxin exposure and a real risk of toxicity, which can present as nausea, confusion, visual changes, and most concerning, serious heart rhythm problems like bradycardia or AV block. Because both drugs can affect heart rhythm, the combination markedly increases the danger of life-threatening cardiac events. That combination therefore stands out as the most likely to produce a clinically significant interaction, requiring close monitoring of digoxin levels, kidney function, electrolytes, and ECG, with dose adjustment or avoidance if possible.

There are interactions in the other pairs, but they are generally less likely to cause such a profound or immediate risk. For example, warfarin with an ACE inhibitor like ramipril can raise bleeding or potassium issues in some settings, but it’s not as predictable or dramatic a pharmacokinetic interaction as digoxin with amiodarone. Lithium with a thiazide diuretic is a well-known and important interaction because diuretics can raise lithium levels, so this pair also needs monitoring, but the specific combination tends to be managed with careful dosing and monitoring rather than producing an abrupt, high-risk toxicity on a routine basis.