Spironolactone is a mild diuretic that can be used to manage different conditions from heart failure to hepatic failure, hyperaldosteronism, hypertension and kidney disease. It’s a very useful drug… but there are loads of different uses! So read on to see why your vet might have prescribed it…

But how can one single drug affect so many body systems? 

Spironolactone is an aldosterone receptor antagonist, which means it blocks the action of the steroid hormone aldosterone. Aldosterone is produced by the adrenal glands in response to low blood pressure and acts on the kidneys to increase water and sodium retention, as well as excreting potassium. 

By blocking the action of aldosterone, spironolactone increases water and sodium elimination while reducing elimination of potassium. For this reason, it is called a potassium sparing diuretic.

This mixture of properties can be very useful for us:

Congestive heart failure:

During congestive heart failure, the heart is unable to pump blood forward and fluid starts accumulating behind the heart. This further increases the amount of blood that needs to be pumped. By increasing the amount of water eliminated by the kidneys, spironolactone reduces the amount of fluid accumulation associated with congestive heart failure; contributing to a reduction in the heart’s workload. Furthermore, it acts on the heart muscle (myocardium) and blood vessels to reduce formation of scar tissue and changes in heart and blood vessels shape associated with cardiac disease (remodelling), slowing its progression.

High blood pressure:

Increased elimination of water results in a reduced circulating blood volume and, consequently, reduction in blood pressure. If the blood pressure is too high for whatever reason, spironolactone helps prevent accumulation of fluid, and so helps bring it back down. Although this drug isn’t usually used for this alone.

Fluid accumulation in the abdomen (ascites):

Similarly to what happens in heart failure, fluid can also accumulate in the abdomen, secondary to hepatic failure. This can happen due to increased pressure in the vessels passing through the liver and/or secondary to decrease production of protein by the failing liver (hypoproteinaemia). An increase in water elimination reduces the pressure in the blood vessels. Consequently, it reduces movement of water from the vessels to the abdominal cavity.

Electrolyte imbalances:

In the kidneys, movement of water is accomplished mainly through electrolyte pumps. Water usually follows the movement of sodium and sodium is eliminated in exchange for potassium. By blocking the effects of aldosterone, spironolactone contributes to the elimination of sodium and water by absorbing potassium. For this reason, it can be used to manage conditions that result in low serum potassium (hypokalaemia).

We sometimes need this if we are treating a dog who is on other diuretics (water tablets) like frusemide, as in higher doses, this can lead to too much potassium loss – the aldosteron can help rebalance things.

Hyperaldosteronism secondary to adrenal tumours:

As explained above, aldosterone is produced by the adrenal glands in response to low blood pressure. Certain tumours of the adrenal glands produce excessive amounts of aldosterone independently of the body’s needs. In these cases, spironolactone can be used to block the effects of excess aldosterone (hyperaldosteronism).

High testosterone levels:

Very rarely, spironolactone may be used to reduce blood testosterone levels. Because of the way it blocks one steroid hormone (aldosterone), it can also interfere with others, especially testosterone. However, this is a very old-fashioned use, and in most cases there are other drugs that would be used in preference – or else we would castrate the patient (chemically or surgically).

Can spironolactone be used in every patient with the conditions above and what are the possible side effects to look for?

Due to its effects in electrolyte levels, spironolactone should not be used in patients with conditions that result in high potassium and low sodium, such as Addison’s disease. Furthermore, it should not be used in combination with other drugs that affect the kidneys (such as non-steroidal anti-inflammatory drugs) in animals with kidney disease. Your veterinary surgeon will likely recommend periodic blood tests to check electrolyte levels and kidney function.

Spironolactone is contraindicated during pregnancy, lactation or animals intended for breeding, because of its effects on sex hormones.

Spironolactone is a bit of a “wonder-drug” but as with all medicines, there are upsides and downsides. So your vet will have carefully considered all of the options before deciding that this is the best. If you’re still confused, give them a call and ask them to explain their reasoning – most vets will be happy to help!

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