A potassium-sparing (aldosterone-blocking) water tablet
Spironolactone
A water tablet that removes excess fluid while holding on to potassium — used in heart failure, hard-to-control blood pressure, and fluid build-up from liver disease.
What is Spironolactone?
Spironolactone is a potassium-sparing diuretic ("water tablet") that blocks the hormone aldosterone. It helps the body clear excess fluid while keeping potassium, and is used in heart failure, resistant high blood pressure, fluid build-up from liver disease (ascites), and primary hyperaldosteronism. Its main risk is a high blood potassium, so potassium and kidney function are monitored.
Education and reference only. This is a plain-language guide to Spironolactone — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Spironolactone is a "water tablet" (diuretic) with a difference: instead of causing the body to lose potassium like most diuretics, it holds on to it. It works by blocking the hormone aldosterone, and it has several important uses — improving survival and symptoms in heart failure, helping to control blood pressure that has not responded to the usual medicines, and reducing fluid build-up in the abdomen (ascites) caused by liver disease. It is also used for a hormone condition called primary hyperaldosteronism and, at times, for hormonal acne. It is taken as a long-term tablet.
How it works
Spironolactone blocks the receptors that respond to aldosterone, a hormone that normally tells the kidneys to hold on to salt and water and to get rid of potassium. By blocking this signal, spironolactone helps the body remove excess salt and water (reducing fluid overload and blood pressure) while keeping potassium in the body rather than losing it in the urine. In heart failure, blocking aldosterone also reduces harmful effects on the heart itself, which is part of why it improves outcomes.
What it treats
Conditions Spironolactone is used for
Practical use
How to take Spironolactone
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it as prescribed, usually in the morning so any increase in urination is during the day rather than at night.
- Avoid potassium supplements and potassium-based "low-salt" substitutes unless specifically advised, as they add to the risk of a high potassium.
- Attend blood tests to check potassium and kidney function, particularly when starting or increasing the dose or if you become unwell.
- Tell your doctor if you develop vomiting or diarrhoea (dehydration affects potassium and the kidneys), and do not add anti-inflammatory painkillers without checking.
- If you miss a dose, take the next one as usual — do not double up.
Weighing it up
Advantages & disadvantages of Spironolactone
Advantages
- Improves symptoms and survival in heart failure, on top of standard treatment.
- Keeps potassium rather than depleting it, unlike most other diuretics.
- Effective for blood pressure that has resisted other medicines, and for fluid from liver disease.
Disadvantages
- Can raise blood potassium to dangerous levels, especially combined with ACE inhibitors, ARBs or potassium supplements — needs monitoring.
- Can cause breast tenderness or enlargement in men (gynaecomastia).
- Needs caution and monitoring in kidney impairment.
Practical use
Good to know
The main thing to be aware of with spironolactone is potassium: because it keeps potassium in the body, the level can rise too high, especially if you also take an ACE inhibitor or ARB (common heart and blood-pressure medicines), potassium supplements, or salt substitutes (which are often potassium-based). Blood tests check potassium and kidney function after starting and when doses change. A high potassium can be dangerous and may cause muscle weakness or heart-rhythm problems, so it is taken seriously. Because it affects hormones, it can cause breast tenderness or enlargement in men (gynaecomastia), which usually improves if the medicine is changed.
Who should not take it / use with caution
- People with a high blood potassium level, or significant kidney impairment, unless carefully supervised.
- People with Addison's disease (a condition of low adrenal hormones).
- Pregnancy and breastfeeding, except where a specialist judges it necessary.
Monitoring
- Blood potassium level (after starting and with dose changes)
- Kidney function
- Blood pressure and fluid status; signs of gynaecomastia
Side effects
- A high blood potassium (hyperkalaemia) is the key risk — usually silent, which is why blood tests matter; at high levels it can cause muscle weakness or heart-rhythm problems.
- Breast tenderness or enlargement in men, and menstrual changes or breast tenderness in women (hormonal effects).
- Dizziness or low blood pressure, and, less commonly, digestive upset.
Key interactions
- ACE inhibitors, ARBs and other potassium-sparing medicines add to the risk of a high potassium — combinations are monitored closely.
- Potassium supplements and potassium-containing salt substitutes should generally be avoided.
- Anti-inflammatory painkillers (NSAIDs) can reduce its effect and stress the kidneys; care is needed with digoxin and lithium.
Available as: Tablets; a liquid is available for people who cannot swallow tablets.
Answers
Spironolactone: frequently asked questions
Why is potassium so important with spironolactone?
Unlike most water tablets, spironolactone makes the body keep potassium, so the blood level can rise too high — which can be dangerous for the heart. This is more likely if you also take an ACE inhibitor or ARB, potassium supplements, or potassium-based salt substitutes. Blood tests keep an eye on it, especially after starting or changing the dose.
Can spironolactone cause breast enlargement in men?
Yes — because it affects hormone signalling, it can cause breast tenderness or enlargement (gynaecomastia) in some men. It is not dangerous, and usually improves if the medicine is stopped or changed; a related medicine, eplerenone, is less likely to cause it and is sometimes used instead.
When is the best time to take it?
It is usually taken in the morning, so that any increase in passing urine happens during the day rather than disturbing your sleep. Take it consistently as prescribed.
Is Aldactone the same as spironolactone?
Yes — spironolactone is the generic (active-ingredient) name and Aldactone is a brand name; both contain the same active ingredient.
The wider class
About Aldosterone antagonists (potassium-sparing diuretics)
Spironolactone belongs to the aldosterone antagonists (potassium-sparing diuretics) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Spironolactone.
- electronic Medicines Compendium (SmPC): Spironolactone (Aldactone).
- NICE NG106: Chronic heart failure; NG136: Hypertension in adults.
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