digoxin × spironolactone

Spironolactone moderately inhibits P-glycoprotein – digoxin concentration rises by 20–30 %. In addition, spironolactone interferes with immunoassay measurements of digoxin (falsely elevated values).

Check digoxin concentration 2–3 weeks after starting spironolactone, guided by clinical status rather than the level alone. The target in HF is 0.5–0.9 ng/mL. Reduce the digoxin dose by 25 % if needed.