Digoxin × Furosemide

Furosemide is a loop diuretic that increases urinary loss of potassium and magnesium. Low serum potassium (hypokalaemia) and low magnesium increase digoxin binding to the cardiac sodium-potassium ATPase and intensify its toxicity even when digoxin plasma levels are within range. Furosemide does not change digoxin concentration directly – the danger is the electrolyte shift.

Nausea, anorexia, visual disturbances (yellow-green halos around lights), arrhythmias ranging from premature beats to ventricular tachycardia. Confusion in older adults. Symptoms emerge once potassium falls below 3.5 mmol/L.

Check serum potassium before starting the combination, again at 1–2 weeks, and every 1–3 months thereafter. Monitor magnesium in parallel. If potassium drops below 4 mmol/L in a digoxin patient, add a potassium-sparing diuretic (spironolactone, eplerenone) or oral potassium. Replete magnesium when levels are below 0.7 mmol/L.