furosemide × spironolactone

Opposing effects on potassium: furosemide depletes potassium, spironolactone retains it. A target combination in HF and resistant hypertension. The potassium balance depends on the dose ratio and renal function – in CKD and diabetes, spironolactone dominates and hyperkalemia risk rises.

Check potassium and creatinine 3–7 days after initiation or dose change, then every 1–3 months. Target potassium is 4.0–5.0 mmol/L. If K+ > 5.5 mmol/L, reduce the spironolactone dose.