Aldactone (Spironolactone)

How it works

Spironolactone works by blocking aldosterone receptors in the kidneys. Aldosterone normally causes the body to retain sodium and water while excreting potassium. By blocking it, spironolactone promotes mild fluid removal without the potassium loss that furosemide causes. Beyond its diuretic effects, spironolactone has important anti-fibrotic properties — it helps slow the scarring and stiffening of heart muscle that worsens DCM over time. This makes it valuable even in dogs whose fluid levels are well controlled.

Common dosage

Typically 1–2 mg/kg once or twice daily. Often started at the lower end and adjusted based on response. Spironolactone is a mild diuretic on its own, so it is almost always used in combination with furosemide rather than as a standalone treatment for heart failure.

Dosages vary — your vet will determine the right dose for your dog.

Side effects to watch for

• ●GI upset (vomiting, diarrhea, reduced appetite)
• ●Facial dermatitis — a rare but recognized side effect, particularly reported in cats but occasionally in dogs
• ●Elevated potassium levels (hyperkalemia) — especially when combined with ACE inhibitors
• ●Lethargy
• ●Rarely: hormonal effects due to its steroid-like structure

Drug interactions

• ●ACE inhibitors (benazepril, enalapril) — both raise potassium levels, so the combination requires regular electrolyte monitoring
• ●Potassium supplements — dangerous combination that can cause life-threatening hyperkalemia
• ●Furosemide — beneficial combination; spironolactone counteracts furosemide's potassium-wasting effect
• ●NSAIDs — can reduce diuretic effectiveness and increase kidney risk
• ●Digoxin — spironolactone can increase digoxin blood levels

Frequently asked questions