Spironolactone

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Spironolactone is an aldosterone antagonist belonging to the class of potassium-sparing diuretics.

It has been used in clinical practice since the 1960s and remains a cornerstone of treatment for heart failure, resistant hypertension, primary hyperaldosteronism, and fluid retention associated with hepatic cirrhosis.

In the United Kingdom, spironolactone is available as tablets (25 mg, 50 mg, and 100 mg) and as an oral suspension for patients who cannot swallow solid dosage forms.

It is a prescription-only medicine (POM).

The importance of spironolactone in cardiovascular medicine was underscored by the landmark RALES trial, which demonstrated a 30% reduction in mortality among patients with severe heart failure when low-dose spironolactone was added to standard therapy.

More recently, NICE has recommended spironolactone as a fourth-line antihypertensive for resistant hypertension, expanding its role in everyday primary care prescribing.

This page provides a comprehensive clinical overview of spironolactone, including its mechanism of action, evidence base, dosing across indications, side effects, critical safety warnings regarding hyperkalaemia, and how to obtain a prescription in the UK.

Important safety information about spironolactone

Before reading further, note the following key safety points about spironolactone.

• Hyperkalaemia (dangerously raised potassium) is the most serious risk. Regular blood tests to monitor serum potassium and kidney function are essential.
• Do not take potassium supplements or use salt substitutes containing potassium chloride while taking spironolactone unless specifically advised by your prescriber.
• Spironolactone is contraindicated in pregnancy due to anti-androgenic effects. Women of childbearing potential must use effective contraception.
• Inform all healthcare professionals treating you that you are taking spironolactone, particularly before surgery or if you are unwell.

What is aldosterone and why does blocking it matter

Aldosterone is a steroid hormone produced by the adrenal cortex as part of the renin-angiotensin-aldosterone system (RAAS).

Its primary physiological role is to regulate sodium and potassium balance and extracellular fluid volume by acting on the mineralocorticoid receptor in the distal nephron.

Aldosterone promotes sodium reabsorption and potassium excretion, leading to water retention and expansion of blood volume.

In heart failure, the RAAS is chronically activated as a compensatory mechanism to maintain cardiac output and blood pressure.

However, sustained aldosterone excess has harmful effects beyond fluid retention.

Aldosterone promotes cardiac fibrosis (deposition of collagen in the myocardium), vascular stiffness, endothelial dysfunction, baroreceptor dysfunction, and sympathetic nervous system activation.

These processes accelerate adverse ventricular remodelling, worsen cardiac function, and increase the risk of arrhythmias and sudden cardiac death.

In primary hyperaldosteronism (Conn syndrome), autonomous aldosterone production by an adrenal adenoma or bilateral adrenal hyperplasia causes hypertension, hypokalaemia, and metabolic alkalosis.

Spironolactone is used both diagnostically (as a therapeutic trial) and as definitive long-term treatment when surgery is not possible.

How spironolactone works: mechanism of action

Spironolactone is a non-selective steroidal antagonist of the mineralocorticoid receptor (MR).

After oral absorption, it is extensively metabolised in the liver to several active metabolites, the most important of which are canrenone and 7-alpha-thiomethylspirolactone.

These metabolites have long half-lives (12 to 24 hours for canrenone), contributing to the sustained pharmacological effect that takes several days to reach full clinical effect.

At the mineralocorticoid receptor in the collecting duct, spironolactone and its metabolites compete with aldosterone for receptor binding.

This prevents translocation of the MR to the nucleus, blocking the transcription of aldosterone-responsive genes including epithelial sodium channels (ENaC) and the Na+/K+-ATPase pump.

The net effect is reduced sodium reabsorption, reduced water retention, and decreased potassium excretion, producing a mild natriuresis and diuresis while conserving potassium.

In the cardiovascular system, MR blockade inhibits aldosterone-mediated fibroblast proliferation and collagen deposition in the myocardium, reduces oxidative stress and inflammation in the vascular endothelium, and improves baroreceptor sensitivity.

These cardioprotective effects are distinct from the diuretic action and are the primary basis for the mortality benefit observed in heart failure trials.

Due to its steroidal structure, spironolactone also binds to androgen receptors (as an antagonist) and progesterone receptors.

This anti-androgenic activity accounts for side effects such as gynaecomastia in men and menstrual irregularity in women, but also underlies its therapeutic use in conditions driven by androgen excess, such as acne and hirsutism in women (though these uses are off-licence in the UK).

Clinical evidence for spironolactone

Heart failure: the RALES trial

The Randomised Aldactone Evaluation Study (RALES), published in 1999, was a double-blind, placebo-controlled trial involving 1,663 patients with severe heart failure (NYHA class III or IV, left ventricular ejection fraction at or below 35%) already receiving an ACE inhibitor, a loop diuretic, and, in most cases, digoxin.

Patients were randomised to spironolactone 25 mg daily or placebo.

The trial was stopped early because of a clear mortality benefit: spironolactone reduced all-cause mortality by 30% (relative risk 0.70), reduced hospitalisation for worsening heart failure by 35%, and improved NYHA functional class.

The benefits were attributed to the cardioprotective effects of MR blockade rather than the diuretic effect alone.

Current NICE guidelines and the European Society of Cardiology recommend an MRA (spironolactone or eplerenone) as a component of optimal medical therapy for heart failure with reduced ejection fraction (HFrEF), alongside an ACE inhibitor (or ARB), a beta-blocker, and, in appropriate patients, an SGLT2 inhibitor.

Resistant hypertension: PATHWAY-2

The PATHWAY-2 trial, published in 2015, was a randomised, double-blind, placebo-controlled crossover trial comparing spironolactone 25 to 50 mg, bisoprolol, doxazosin, and placebo as add-on therapy in patients with resistant hypertension.

Spironolactone was significantly more effective than bisoprolol, doxazosin, or placebo in reducing both home and clinic blood pressure.

On the basis of this trial, NICE guideline NG136 recommends spironolactone as the preferred fourth-line antihypertensive, provided serum potassium is at or below 4.5 mmol/L.

Dosage and administration

Spironolactone dosing varies by indication.

In heart failure, the starting dose is typically 25 mg once daily, titrated to a maximum of 50 mg once daily, with close monitoring of potassium and renal function.

In resistant hypertension, 25 mg once daily is the usual starting dose. In primary hyperaldosteronism, higher doses of 100 to 400 mg daily are used.

In hepatic cirrhosis with ascites, doses of 100 to 400 mg daily are common, often in combination with furosemide. Spironolactone should be taken with food to improve absorption.

The onset of action is gradual (2 to 3 days for the diuretic effect, up to 2 weeks for the full antihypertensive effect), reflecting the time needed for metabolite accumulation and receptor-level changes.

Patients should be counselled that they may not notice an immediate effect.

Side effects of spironolactone

Hyperkalaemia

The most clinically important adverse effect is hyperkalaemia, which can cause muscle weakness, paraesthesia, fatigue, and potentially life-threatening cardiac arrhythmias (including ventricular fibrillation and cardiac arrest).

The risk is highest in patients with renal impairment, those taking concurrent ACE inhibitors, ARBs, or potassium supplements, and in elderly patients.

Regular monitoring is essential and is not optional.

Anti-androgenic effects

Gynaecomastia and breast tenderness occur in up to 10% of men and are dose-dependent. Menstrual irregularity, breast tenderness, and reduced libido may occur in women.

These effects are related to the non-selective binding of spironolactone to androgen and progesterone receptors. They are reversible on discontinuation, though resolution may take weeks to months.

Eplerenone is more selective for the MR and causes significantly fewer hormonal side effects, but is more expensive and has a narrower range of licensed indications.

Other side effects

Gastrointestinal effects (nausea, vomiting, diarrhoea, abdominal pain) are common but usually manageable by taking the medication with food.

Headache, drowsiness, fatigue, dizziness, leg cramps, and skin rashes have been reported. Hyponatraemia may occur, particularly when spironolactone is used in combination with loop diuretics.

Rare effects include hepatotoxicity, agranulocytosis, and severe cutaneous reactions.

When to seek medical advice

Contact your GP or NHS 111 if you experience muscle weakness, unusual tiredness, tingling or numbness in the hands or feet, nausea, irregular heartbeat, breast swelling or tenderness, severe dizziness, or any new symptoms you believe may be drug-related.

Seek emergency help by calling 999 or attending A&E if you have severe chest pain, a very slow or irregular pulse, collapse, or difficulty breathing.

Report any suspected adverse reactions via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk .

Warnings and precautions

Potassium monitoring

Serum potassium and renal function must be monitored before starting treatment, within one week of initiation or dose adjustment, monthly for the first 3 months, and every 3 to 6 months thereafter.

If potassium rises above 5.5 mmol/L, reduce the dose or stop spironolactone and seek medical review.

Patients should avoid potassium supplements, salt substitutes containing potassium chloride, and excessive intake of potassium-rich foods unless under dietetic supervision.

Drug interactions

The combination of spironolactone with ACE inhibitors or ARBs increases the risk of hyperkalaemia and requires careful monitoring.

NSAIDs (including ibuprofen and naproxen) reduce the diuretic and antihypertensive efficacy of spironolactone and increase potassium levels. Trimethoprim, ciclosporin, and tacrolimus also raise potassium.

Spironolactone may increase lithium levels, requiring dose adjustment and monitoring. Concurrent use with other potassium-sparing diuretics is generally avoided.

Pregnancy and breastfeeding

Spironolactone is contraindicated in pregnancy (Category X equivalent). Its anti-androgenic effects may cause feminisation of a male foetus. Women of childbearing potential must use reliable contraception.

If pregnancy is suspected, stop the medication immediately and consult your prescriber. Spironolactone and canrenone are excreted in breast milk, and breastfeeding is not recommended during treatment.

How to get a spironolactone prescription in the UK

Spironolactone is a prescription-only medicine available on NHS prescription from your GP or specialist.

For heart failure, it is usually initiated and titrated in secondary care or by a GP with a special interest in cardiology, then continued in primary care with ongoing monitoring.

For resistant hypertension, it is often initiated in primary care following NICE NG136 guidance.

The standard NHS prescription charge in England is 9.90 pounds per item; prescriptions are free in Scotland, Wales, and Northern Ireland.

Sources

• Spironolactone 25 mg Tablets, Summary of Product Characteristics (EMC)
• Spironolactone, British National Formulary (BNF)
• NICE NG106: Chronic heart failure in adults: diagnosis and management
• NICE NG136: Hypertension in adults: diagnosis and management
• Spironolactone, NHS
• MHRA Yellow Card Scheme