Amiodarone × Furosemide

Furosemide (loop diuretic) increases potassium and magnesium loss. Low potassium and magnesium amplify amiodarone's QT-prolonging effect. Furosemide itself does not change amiodarone levels – the danger is the electrolyte shift.

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

The combination is common (atrial fibrillation plus heart failure). Check potassium and magnesium every 1–3 months; if potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic or potassium supplement; if magnesium falls below 0.7 mmol/L, supplement magnesium.