Major
Spironolactone × tacrolimus
Potassium-sparing diuretics (mineralocorticoid receptor antagonists)×Calcineurin inhibitor (immunosuppressant)
Mechanism
Spironolactone retains potassium via aldosterone receptor blockade. Tacrolimus reduces renal potassium secretion via a distal tubular inhibitory effect. Additive hyperkalaemia.
Symptoms
Muscle weakness, paraesthesia, slowed pulse, arrhythmias. ECG: peaked T waves, widened QRS. Severe cases progress to cardiac arrest.
Management
Avoid the combination in transplant patients. Alternative diuretics: furosemide (loop, removes potassium) or hydrochlorothiazide. If spironolactone is needed, check potassium and creatinine every 1–2 weeks.