Basic Information

Spironolactone is an aldosterone antagonist that used as a potassium-sparing diuretic. It is most commonly used in animals for congestive heart failure or for those with ascites. Spironolactone is an older drug but recent research still supports its use.

Spironolactone competitively inhibits aldosterone at the mineralocorticoid receptor of target cells in the distal renal tubules. It increases the excretion of sodium, chloride, and water, while sparing potassium, ammonium, and phosphate. It appears to have its greatest effect in animals with elevated aldosterone and to have less effect in animals with normal aldosterone levels. Spironolactone has been shown to have some additional cardioprotective effects in humans with elevated aldosterone. It may also act as a vasodilator through a mechanism similar to calcium channel blockers.

Spironolactone can be thought of as a prodrug as it is rapidly metabolized in the liver to canrenone and other active metabolites. It is primarily eliminated via the kidney. In humans, oral absorption is improved by administration with food. Spironolactone is not thought of as an emergency drug as peak diuresis may not occur until two to three days of therapy.

Dogs and Cats

Spironolactone is used to help manage refractory edema or fluid retention due to congestive heart failure, cirrhosis of the liver, ascites, hypertension, feline primary hyperaldosteronism, and nephrotic syndrome. It is used with furosemide, digoxin, and ACE 1 inhibitors in dogs with chronic congestive heart failure. The addition of spironolactone may allow a lower dose of furosemide. Research in both humans and in dogs would indicate improved survival when spironolactone is added to the conventional congestive heart failure treatment.

Side Effects

• The most-common side effects are due to dehydration and/or electrolyte imbalances (hyperkalemia and hyponatremia). These are generally reversible by discontinuing the drug.
• Higher doses of spironolactone may cause anorexia, vomiting, and diarrhea

Precautions

• Spironolactone should not be used in animals with hyperkalemia, significant kidney dysfunction, or Addison’s disease.
• Spironolactone crosses the placenta and has some anti-androgen properties in humans. It should only be used in pregnant or nursing animals when the benefits outweigh the risks due to the potential for feminization of fetuses. Spironolactone is found in maternal milk.

Drug Interactions

• The half life of digoxin may be increased. Additional monitoring may be warranted.
• Spironolactone should not be used with other potassium-sparing diuretics or potassium supplements because of the possibility of hyperkalemia.
• Spironolactone may decrease the effects of Mitotane.
• The diuretic effects of spironolactone may be decreased by salicylates.
• Spironolactone may increase the effects of neuromuscular blocking agents.

Overdose

There is little information available on overdose in animals. If an oral overdose is recognized promptly, gut-emptying protocol may be of benefit. Monitoring and support of hydration status and electrolyte status should be performed.