Spironolactone

Aldosterone Antagonist Potassium-sparing Diuretic

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 1-2 mg/kg q12h Duration: Chronic (long-term)
Notes: For heart failure, use in combination with loop diuretics and ACE inhibitors. For hyperaldosteronism, higher doses (up to 4-6 mg/kg/day) may be needed.
Cat PO 1-2 mg/kg q12h Duration: Chronic (long-term)
Notes: Start at lower end of dose range. Monitor renal function and electrolytes. For hyperaldosteronism, doses up to 3-4 mg/kg/day may be used.
Horse PO 0.5-1 mg/kg q12h Duration: Variable
Notes: Limited data; use with caution.
Rabbit PO 1-2 mg/kg q12h Duration: Variable
Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications
  • Adjunctive therapy for congestive heart failure
  • Treatment of primary hyperaldosteronism
  • Management of edema and ascites
Dog (Canine)
  • Adjunctive therapy for congestive heart failure (CHF) due to degenerative mitral valve disease or dilated cardiomyopathy
  • Treatment of primary hyperaldosteronism (Conn's syndrome)
  • Management of ascites and edema associated with hepatic cirrhosis
  • Treatment of hypertension (especially when associated with hyperaldosteronism)
Cat (Feline)
  • Adjunctive therapy for hypertrophic cardiomyopathy (HCM) with congestive heart failure
  • Treatment of primary hyperaldosteronism
  • Management of refractory edema or ascites

Pharmacology & Mechanism of Action

Drug Class: Aldosterone Antagonist | Pharmacological Group: Potassium-sparing Diuretic

Mechanism of Action: Spironolactone is a competitive aldosterone antagonist. It binds to the mineralocorticoid receptor in the distal convoluted tubule and collecting duct of the nephron, blocking aldosterone-mediated sodium reabsorption and potassium excretion. This results in increased excretion of sodium and water, while conserving potassium. Additionally, spironolactone has anti-fibrotic and anti-remodeling effects on the myocardium, which are beneficial in heart failure.

Pharmacodynamics: Spironolactone produces diuresis, but its diuretic effect is relatively mild compared to loop diuretics. Its primary benefits in heart failure are due to aldosterone blockade, which reduces cardiac fibrosis, improves endothelial function, and decreases sympathetic activation. It also lowers blood pressure in hypertensive animals. The onset of diuretic effect is slow (2-3 days) and peak effect may take up to 2 weeks.

⚡ Pharmacokinetics Summary

Absorption: Spironolactone is well absorbed after oral administration, with a bioavailability of approximately 90% in dogs. Food increases absorption.
Distribution: Spironolactone is extensively bound to plasma proteins (greater than 90%). It distributes widely into tissues, with high concentrations in the liver and kidneys. It crosses the placenta and is excreted in milk.
Metabolism: Spironolactone undergoes extensive first-pass metabolism in the liver to active metabolites, primarily canrenone and 7α-thiomethylspironolactone. These metabolites contribute to the drug's therapeutic effect.
Excretion: Metabolites are excreted primarily in the urine, with some biliary excretion. The elimination half-life of spironolactone is short (1-2 hours), but its active metabolites have longer half-lives (canrenone: 10-35 hours in dogs).
Half-Life: Dog: 1-2 hours (parent drug), 10-35 hours (active metabolites); Cat: approximately 2-3 hours (parent drug), but active metabolites may be longer.
Bioavailability: Approximately 90% in dogs; may be lower in cats.
Protein Binding: >90%

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg, 100 mg (PO)
Oral Suspension (compounded) 5 mg/mL, 10 mg/mL (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to spironolactone
  • Hyperkalemia
  • Addison's disease (hypoadrenocorticism)
  • Acute renal failure or severe renal impairment
  • Anuria
  • Concurrent use of other potassium-sparing diuretics (e.g., amiloride, triamterene)
  • Concurrent use of potassium supplements
Warnings & Clinical Precautions:
  • Use with caution in patients with renal insufficiency, hepatic disease, or diabetes mellitus.
  • Monitor serum potassium and renal function periodically, especially in patients on ACE inhibitors or NSAIDs.
  • May cause electrolyte imbalances (hyperkalemia, hyponatremia).
  • In cats, may cause vomiting, anorexia, and weight loss; use with caution.
  • May cause gynecomastia in male animals.
  • Use with caution in animals with pre-existing hyperkalemia or those at risk for hyperkalemia.
  • In food animals, withdrawal times must be observed; extra-label use is prohibited in some countries.

Adverse Effects & Reactions

Common:

  • Hyperkalemia
  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Polyuria/polydipsia
  • Dehydration

Serious / Severe:

  • Severe hyperkalemia (can cause cardiac arrhythmias)
  • Renal dysfunction
  • Hypotension
  • Metabolic acidosis

Rare:

  • Gynecomastia
  • Mammary tumors in dogs (prolonged use)
  • Blood dyscrasias
  • Skin reactions

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
ACE inhibitors (e.g., enalapril, benazepril) Additive hyperkalemia; risk of hyperkalemia increases. High
Potassium supplements Severe hyperkalemia. High
NSAIDs (e.g., carprofen, meloxicam) Reduced diuretic effect and increased risk of renal dysfunction and hyperkalemia. Moderate
Digoxin Spironolactone may increase digoxin half-life and serum levels; monitor for digoxin toxicity. Moderate
Other diuretics (e.g., furosemide) Additive diuretic effect; may cause excessive fluid and electrolyte loss. Moderate
Corticosteroids May enhance potassium loss, counteracting spironolactone's potassium-sparing effect. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Hyperkalemia (weakness, bradycardia, cardiac arrhythmias)
  • Hypotension
  • Dehydration
  • Lethargy
  • Gastrointestinal signs (vomiting, diarrhea)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Discontinue the drug. Monitor serum potassium and ECG. For severe hyperkalemia, administer intravenous fluids, calcium gluconate (to stabilize cardiac membrane), insulin with dextrose, sodium bicarbonate, and/or potassium-binding resins (e.g., sodium polystyrene sulfonate). In severe cases, dialysis may be necessary. Provide supportive care for dehydration and electrolyte imbalances.

Food Animal Withdrawal Times

Spironolactone is not approved for use in food animals. Extra-label use in food animals is prohibited in many countries due to lack of withdrawal data. If used, a conservative withdrawal time should be established in consultation with a veterinary pharmacologist.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats in some countries (e.g., FDA-approved for dogs in the US for heart failure). Not approved for food animals.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited under AMDUCA if an approved animal drug exists that is labeled for the condition and species. Spironolactone is not approved for food animals, so extra-label use is not permitted in food-producing animals. In companion animals, extra-label use is allowed under veterinary discretion.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Spironolactone is a valuable adjunctive therapy for congestive heart failure in dogs and cats, particularly when used with an ACE inhibitor and a loop diuretic. It has been shown to reduce morbidity and mortality in dogs with heart failure. In cats, it may be used for heart failure and hyperaldosteronism, but careful monitoring is required due to potential adverse effects. Always monitor serum potassium and renal function within 1-2 weeks after starting therapy and periodically thereafter. Adjust doses based on clinical response and electrolyte status. In cases of hyperaldosteronism, higher doses may be necessary. Spironolactone has a slow onset of action; do not use as a sole diuretic in acute pulmonary edema. Consider compounding if tablet sizes are not appropriate for small patients.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)