Is sildenafil a diuretic? This is a common question, particularly amongst patients taking cardiovascular medications. The short answer is no—sildenafil is not a diuretic. Sildenafil belongs to a class of medicines called phosphodiesterase type 5 (PDE5) inhibitors, primarily used for erectile dysfunction and pulmonary arterial hypertension. Diuretics, by contrast, increase urine production by acting on the kidneys and are prescribed for conditions such as hypertension, heart failure, and oedema. Whilst both medication types can affect blood pressure, they work through entirely different mechanisms. Understanding this distinction is essential for safe medication use, particularly when combining sildenafil with other cardiovascular treatments.

Is Sildenafil a Diuretic?

No, sildenafil is not a diuretic. This is a common misconception that arises because both medications can affect blood pressure and cardiovascular function, but they work through entirely different mechanisms and serve distinct therapeutic purposes.

Sildenafil belongs to a class of medicines called phosphodiesterase type 5 (PDE5) inhibitors. It is primarily licensed in the UK for treating erectile dysfunction (marketed as Viagra) and pulmonary arterial hypertension (marketed as Revatio). The Medicines and Healthcare products Regulatory Agency (MHRA) has approved sildenafil for these specific indications, and there is no official link between sildenafil and diuretic activity.

Diuretics, by contrast, are medications that increase urine production by acting on the kidneys. They are commonly prescribed for conditions such as hypertension, heart failure, and oedema (fluid retention). Examples include furosemide, bendroflumethiazide, and spironolactone. These medicines work by altering sodium and water reabsorption in different parts of the kidney tubules.

Whilst sildenafil may cause mild vasodilation (widening of blood vessels) that can lead to a modest reduction in blood pressure, this effect does not involve increased urine output or changes to fluid balance typical of diuretics. Understanding this distinction is important for patient safety, as combining sildenafil with certain medications—particularly nitrates (including nicorandil and amyl nitrite or 'poppers') or riociguat—can cause dangerous drops in blood pressure. If you are taking diuretics or other cardiovascular medications and are considering sildenafil, it is essential to discuss this with your GP or specialist, as the combination may increase the risk of postural hypotension.

What Is Sildenafil and How Does It Work?

Sildenafil is a medicine that works by inhibiting the enzyme phosphodiesterase type 5 (PDE5). This enzyme is found in high concentrations in the smooth muscle of blood vessels, particularly in the corpus cavernosum of the penis and in the pulmonary vasculature. By blocking PDE5, sildenafil prevents the breakdown of cyclic guanosine monophosphate (cGMP), a chemical messenger that promotes smooth muscle relaxation and vasodilation.

In the context of erectile dysfunction, sexual stimulation triggers the release of nitric oxide in penile tissue, which increases cGMP levels. Sildenafil enhances this natural process by maintaining higher cGMP concentrations, thereby improving blood flow to the penis and facilitating an erection. It is important to note that sildenafil does not cause spontaneous erections—sexual stimulation is still required for the medication to be effective.

For pulmonary arterial hypertension (PAH), sildenafil works by relaxing the blood vessels in the lungs, reducing pulmonary vascular resistance and improving exercise capacity. This use is typically initiated and monitored by specialists in secondary care.

Sildenafil is usually taken orally, with effects beginning approximately 30–60 minutes after ingestion and lasting for up to four hours. A high-fat meal may delay the onset of action. The standard dose for erectile dysfunction is 50 mg, which may be adjusted to 25 mg or 100 mg depending on efficacy and tolerability. For PAH, the dosing regimen differs and is determined by specialist physicians.

While most sildenafil products require a prescription, Viagra Connect (sildenafil 50 mg) is available from pharmacies without a prescription following an assessment by a pharmacist. Common side effects include headache, facial flushing, indigestion, nasal congestion and visual disturbances—all related to its vasodilatory properties rather than any diuretic action.

Sildenafil interacts with several medications, particularly CYP3A4 inhibitors (such as erythromycin, ketoconazole and ritonavir) which may increase sildenafil levels. Grapefruit juice should be avoided, and alcohol intake limited. Sildenafil must never be used with nitrates or riociguat due to the risk of severe hypotension.

Differences Between Sildenafil and Diuretics

Understanding the fundamental differences between sildenafil and diuretics is crucial for safe medication use and appropriate treatment selection.

Mechanism of action: Sildenafil works by inhibiting PDE5, leading to smooth muscle relaxation and vasodilation primarily in specific vascular beds. Diuretics, however, act directly on the kidneys to increase sodium and water excretion through urine. Different classes of diuretics work at various sites in the nephron—loop diuretics (e.g., furosemide) act on the loop of Henle, thiazides (e.g., bendroflumethiazide) on the distal convoluted tubule, and potassium-sparing diuretics (e.g., spironolactone) on the collecting duct.

Therapeutic indications: Sildenafil is licensed for erectile dysfunction and pulmonary arterial hypertension (specialist-initiated). Diuretics are prescribed for hypertension, heart failure, chronic kidney disease, liver cirrhosis with ascites, and various oedematous conditions. These are entirely separate clinical scenarios requiring different pharmacological approaches.

Effects on fluid balance: Diuretics directly increase urine output and reduce total body fluid volume, which is their primary therapeutic effect. Sildenafil does not have diuretic action and does not alter fluid balance through kidney effects. Whilst sildenafil may cause mild blood pressure reduction through vasodilation, this is a secondary effect and not the result of fluid loss.

Side effect profiles: Diuretics commonly cause electrolyte imbalances (particularly hypokalaemia with loop and thiazide diuretics), dehydration, and postural hypotension. Sildenafil's adverse effects relate to vasodilation—headaches, flushing, visual disturbances (including colour vision changes), and nasal congestion. Importantly, sildenafil is contraindicated with nitrates (used for angina) and riociguat due to the risk of severe hypotension. While diuretics and nitrates can be prescribed together, the combination may contribute to hypotension and requires careful monitoring.

Monitoring requirements: Patients on diuretics require regular monitoring of renal function and electrolytes. Sildenafil for erectile dysfunction does not typically require routine blood test monitoring, though cardiovascular assessment is important before initiation. For pulmonary arterial hypertension, sildenafil use is specialist-initiated with ongoing monitoring in secondary care.

When to Speak to Your GP About Sildenafil

There are several important situations when you should consult your GP regarding sildenafil use, whether you are considering starting the medication or are already taking it.

Before starting sildenafil, you should discuss your complete medical history with your GP, particularly if you have:

• Cardiovascular disease, including recent heart attack, stroke, or unstable angina

• Severe heart failure or significant arrhythmias

• Low blood pressure (hypotension) or uncontrolled high blood pressure

• Liver or kidney impairment requiring dose adjustment

• Retinitis pigmentosa or other inherited eye conditions

• History of priapism (prolonged erection) or conditions predisposing to it

• Anatomical deformation of the penis

Medication interactions are particularly important. You must inform your GP if you are taking:

• Nitrates (such as glyceryl trinitrate for angina), nicorandil, or recreational 'poppers' (amyl nitrite) – these must never be used with sildenafil

• Riociguat – this combination is contraindicated

• Alpha-blockers (for prostate problems or hypertension) – you should be stable on alpha-blocker therapy before starting sildenafil, usually at a reduced dose of 25 mg, and consider separating dosing times

• CYP3A4 inhibitors (e.g., erythromycin, clarithromycin, ketoconazole, itraconazole, HIV protease inhibitors) – these may increase sildenafil levels; dose adjustments may be needed

• CYP3A4 inducers (e.g., rifampicin) – these may reduce sildenafil effectiveness

Avoid grapefruit juice and limit alcohol when taking sildenafil.

Seek urgent medical attention if you experience:

• An erection lasting more than four hours (priapism)—this is a medical emergency

• Sudden vision loss or hearing loss

• Chest pain during or after sexual activity – call 999 immediately and inform emergency services that you have taken sildenafil. Do NOT take nitrate medications (such as GTN spray or tablets) if you have taken sildenafil within the last 24 hours

• Severe dizziness or fainting

For ongoing treatment, contact your GP if sildenafil is not effective at the prescribed dose, if you experience troublesome side effects, or if your health status changes (such as new cardiovascular symptoms or starting new medications). Your GP can review whether sildenafil remains appropriate, consider dose adjustments, or explore alternative treatments. Remember that sildenafil obtained from unregulated sources may be counterfeit or unsafe—always obtain medication through legitimate NHS or registered pharmacy channels.

If you experience any side effects, talk to your doctor, pharmacist or nurse. This includes any possible side effects not listed in the package leaflet. You can also report side effects directly via the Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Frequently Asked Questions