Sildenafil

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Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction (ED) in adult men.

It is the active ingredient in Viagra and has been available as a generic medicine in the United Kingdom since Pfizer's patent expired in June 2013.

Sildenafil works by increasing blood flow to the penis during sexual stimulation, helping men achieve and maintain an erection sufficient for satisfactory sexual intercourse.

It is available in 25 mg, 50 mg, and 100 mg film-coated tablets.

At 50 mg, sildenafil can be obtained without a prescription from a pharmacy under the brand name Viagra Connect, making it one of the most accessible treatments for ED in the UK.

This page provides a detailed clinical guide to sildenafil, covering how it works, dosage instructions, side effects, safety warnings, and how to obtain the medicine in the United Kingdom.

Erectile dysfunction is a common condition affecting an estimated 4.3 million men in the UK.

It is defined as the persistent inability to achieve or maintain an erection sufficient for satisfactory sexual performance.

ED becomes more prevalent with age, affecting approximately 40% of men at age 40 and nearly 70% by age 70.

Risk factors include cardiovascular disease, diabetes mellitus, hypertension, hyperlipidaemia, obesity, smoking, excessive alcohol consumption, depression, anxiety, and certain medications.

ED may be the first presenting symptom of underlying cardiovascular disease, and men diagnosed with ED should be assessed for cardiovascular risk factors.

The introduction of sildenafil in 1998 transformed the management of ED, providing a safe, effective, and non-invasive oral treatment.

Important safety information about sildenafil

Before reading further, please note these essential safety points.

• Sildenafil must never be taken with nitrate medicines (such as glyceryl trinitrate or isosorbide mononitrate) or recreational nitrites ("poppers"), as the combination can cause a dangerous and potentially fatal drop in blood pressure.
• Do not take sildenafil if you have had a recent heart attack, stroke, or have severe heart failure.
• If you experience an erection lasting longer than 4 hours (priapism), seek immediate medical attention at your nearest A&E department or call 999.
• Sildenafil does not protect against sexually transmitted infections or pregnancy. Use appropriate contraception and barrier protection.
• Do not exceed one dose (maximum 100 mg) in any 24-hour period.

Understanding erectile dysfunction

An erection is a vascular event that depends on the coordinated interaction of psychological stimuli, nerve signalling, and blood flow.

During sexual arousal, parasympathetic nerve fibres release nitric oxide (NO) from non-adrenergic, non-cholinergic (NANC) nerve terminals and endothelial cells within the corpus cavernosum.

Nitric oxide activates soluble guanylate cyclase in smooth muscle cells, catalysing the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP).

Rising cGMP levels reduce intracellular calcium, leading to relaxation of the trabecular smooth muscle and dilation of the helicine arteries.

This allows blood to flow into the lacunar spaces of the corpora cavernosa under arterial pressure.

As the corpora expand, the subtunical venules are compressed against the tunica albuginea, restricting venous outflow and trapping blood within the penis.

This veno-occlusive mechanism produces and maintains rigidity.

Erectile dysfunction occurs when this process is disrupted at any point. Vascular causes (atherosclerosis, endothelial dysfunction) are the most common organic aetiology.

Neurogenic causes include diabetic neuropathy, spinal cord injury, multiple sclerosis, and pelvic surgery. Hormonal causes, principally hypogonadism, account for a smaller proportion.

Psychogenic ED, driven by performance anxiety, depression, or relationship difficulties, is common in younger men and may coexist with organic factors.

Many men have a combination of physical and psychological contributors.

How sildenafil works

Sildenafil citrate is a potent and selective inhibitor of PDE5, the enzyme responsible for degrading cGMP in the corpus cavernosum.

By blocking PDE5, sildenafil prevents the breakdown of cGMP, thereby prolonging and amplifying the smooth muscle relaxation initiated by nitric oxide release during sexual arousal.

This enhances blood flow into the corpora cavernosa and improves the quality and duration of erections. Crucially, sildenafil does not initiate an erection on its own.

Sexual stimulation is required to activate the nitric oxide/cGMP pathway; sildenafil simply enhances the natural physiological response.

Sildenafil has some selectivity for PDE5 over other phosphodiesterase isoenzymes. It has approximately 10-fold selectivity over PDE6, which is found in the retina and is responsible for phototransduction.

This relatively weak inhibition of PDE6 accounts for the occasional visual side effects (blue-tinged vision, increased light sensitivity) reported by some users.

Sildenafil has over 80-fold selectivity for PDE5 over PDE1 (found in the brain, heart, and vascular smooth muscle) and over 700-fold selectivity over PDE2, PDE3, PDE4, PDE7, PDE8, PDE9, PDE10, and PDE11.

Clinical evidence for sildenafil

Sildenafil has been studied in over 150 clinical trials involving more than 8,000 men.

The pivotal trials demonstrated that sildenafil significantly improved erectile function compared with placebo across a range of aetiologies, including psychogenic ED, organic ED, and mixed ED.

In a landmark trial published in the New England Journal of Medicine in 1998, 69% of all sexual attempts were successful with sildenafil compared with 22% with placebo.

Global assessments of improved erections were reported by 82% of patients on 100 mg, 74% on 50 mg, and 72% on 25 mg, compared with 24% on placebo.

Sildenafil has also demonstrated efficacy in difficult-to-treat subgroups, including men with diabetes mellitus, spinal cord injury, post-radical prostatectomy (nerve-sparing), and depression.

In diabetic men, a meta-analysis showed a 63% improvement in erections with sildenafil compared with 19% with placebo.

Long-term studies have confirmed sustained efficacy and safety over periods exceeding 4 years.

Sildenafil compared with other PDE5 inhibitors

Four PDE5 inhibitors are currently available in the UK: sildenafil, tadalafil (Cialis), vardenafil (Levitra), and avanafil (Spedra). All four work through the same mechanism but differ in their pharmacokinetic profiles.

Sildenafil has an onset of action of 30 to 60 minutes and a duration of approximately 4 to 6 hours.

Tadalafil has a much longer half-life (17.5 hours), giving a duration of action of up to 36 hours, and is also available as a daily low-dose (2.5 mg or 5 mg) option.

Vardenafil has a similar onset and duration to sildenafil. Avanafil (Spedra) has the fastest onset of the four, with some men responding within 15 minutes.

There is no strong evidence that any single PDE5 inhibitor is more effective than the others.

Choice is guided by patient preference, lifestyle considerations (spontaneity versus planned intercourse), side-effect profile, and cost.

Generic sildenafil is typically the least expensive option, which is why it is recommended as first-line treatment in most UK guidelines.

Dosage and administration

The recommended starting dose of sildenafil is 50 mg, taken approximately one hour before anticipated sexual activity.

Depending on efficacy and tolerability, the dose may be adjusted to 25 mg or 100 mg. The maximum recommended dose is 100 mg per day.

Do not take more than one dose in any 24-hour period.

Sildenafil can be taken with or without food.

However, a high-fat meal (such as a fry-up or takeaway) may delay absorption and reduce peak plasma concentration by approximately 29%, which can delay the onset of action.

For optimal results, take sildenafil on an empty stomach or after a light meal.

Onset of action is typically 30 to 60 minutes, with peak plasma concentrations reached at approximately 60 minutes.

The duration of effect is approximately 4 to 6 hours, during which an erection can be achieved with appropriate sexual stimulation.

This does not mean a continuous erection for 4 to 6 hours; rather, the ability to respond to sexual stimulation is enhanced during this window.

A lower starting dose of 25 mg is recommended for men aged 65 and over, those with significant hepatic impairment, severe renal impairment (creatinine clearance below 30 mL/min), or those taking potent CYP3A4 inhibitors.

Side effects of sildenafil

Very common and common side effects

Headache is the most frequently reported side effect, affecting approximately 10% to 15% of users. It is caused by vasodilation and typically resolves within a few hours.

Paracetamol can be taken to relieve it. Facial flushing (warmth and redness of the face) occurs in roughly 10% of users and is a direct result of vasodilation.

Nasal congestion (blocked or stuffy nose) is another common vasodilatory effect. Dyspepsia and nausea occur in a small percentage of users.

Dizziness may occur, particularly when standing up quickly, due to the mild blood pressure-lowering effect of sildenafil.

Uncommon and rare side effects

Visual disturbances, including a blue or green tinge to vision (cyanopsia), increased sensitivity to light (photophobia), and blurred vision, are uncommon and related to weak inhibition of PDE6 in the retina.

These effects are transient and resolve as the drug is cleared. Back pain and myalgia have been reported. Palpitations and tachycardia are uncommon.

Serious rare side effects

Priapism (a painful erection lasting more than 4 hours) is a medical emergency. If this occurs, attend your nearest A&E department immediately or call 999.

Untreated priapism can cause irreversible damage to the erectile tissue.

Non-arteritic anterior ischaemic optic neuropathy (NAION) has been reported very rarely; seek urgent medical attention if you experience sudden painless loss of vision in one eye.

Sudden sensorineural hearing loss has also been reported very rarely; if you notice sudden hearing loss or ringing in the ears, stop taking sildenafil and see a doctor immediately.

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

Warnings and precautions

Absolute contraindications

Sildenafil must never be taken alongside organic nitrates in any form, including glyceryl trinitrate (GTN) sprays, patches, or tablets, isosorbide mononitrate, and isosorbide dinitrate.

The combination causes severe, unpredictable hypotension that can be life-threatening.

Recreational nitrites (amyl nitrite, butyl nitrite, commonly known as "poppers") are equally dangerous in combination with sildenafil and must be avoided.

Riociguat, a guanylate cyclase stimulator used for pulmonary hypertension, is also contraindicated with sildenafil.

Cardiovascular considerations

Sexual activity carries a degree of cardiac risk. Men with cardiovascular conditions where sexual activity is inadvisable should not take sildenafil.

These include recent myocardial infarction (within 90 days), unstable angina, severe heart failure (NYHA class IV), uncontrolled arrhythmias, and stroke within 6 months.

Men with stable cardiovascular disease who are physically capable of moderate exercise (equivalent to brisk walking or climbing two flights of stairs without symptoms) can generally use sildenafil safely.

The Princeton III Consensus recommends cardiovascular risk stratification before prescribing PDE5 inhibitors.

Drug interactions

Alpha-blockers (doxazosin, tamsulosin, alfuzosin) used for benign prostatic hyperplasia or hypertension can cause additive hypotension when combined with sildenafil.

Allow at least 4 hours between doses, and start sildenafil at 25 mg.

Potent CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir, saquinavir, erythromycin, clarithromycin) increase sildenafil plasma levels significantly; use a lower starting dose of 25 mg.

CYP3A4 inducers (rifampicin, phenytoin, carbamazepine) may reduce the effectiveness of sildenafil.

Special populations

Patients with anatomical penile deformity (Peyronie disease, angulation, cavernosal fibrosis) or conditions predisposing to priapism (sickle cell disease, multiple myeloma, leukaemia) should use sildenafil with caution.

Men with hereditary degenerative retinal disorders (such as retinitis pigmentosa) should not use sildenafil.

Patients who have lost vision in one eye due to NAION should avoid PDE5 inhibitors entirely.

Erectile dysfunction and cardiovascular risk

Erectile dysfunction and cardiovascular disease share common risk factors, including hypertension, diabetes, hyperlipidaemia, smoking, and obesity. ED may precede symptomatic cardiovascular events by 3 to 5 years.

NICE guidance and the European Association of Urology guidelines recommend that men presenting with ED should undergo cardiovascular risk assessment, including blood pressure measurement, fasting lipid profile, fasting glucose or HbA1c, and assessment of smoking status and body mass index.

Treating modifiable risk factors may improve both cardiovascular health and erectile function.

Lifestyle measures alongside sildenafil

Sildenafil is most effective when combined with healthy lifestyle changes. Regular physical exercise improves endothelial function and cardiovascular fitness, both of which support erectile function.

Weight loss in overweight or obese men has been shown to improve ED independently of medication.

Smoking cessation improves vascular health and can enhance the response to PDE5 inhibitors.

Reducing alcohol consumption (the NHS recommends no more than 14 units per week) helps maintain erectile function.

Addressing psychological factors through cognitive behavioural therapy or psychosexual counselling can be beneficial, particularly in men with performance anxiety or relationship difficulties.

The combination of medication and lifestyle modification produces better outcomes than either approach alone.

How to get sildenafil in the UK

There are several ways to obtain sildenafil in the United Kingdom.

Generic sildenafil (25 mg, 50 mg, or 100 mg) is available on NHS prescription from your GP or an authorised online prescriber registered with the General Pharmaceutical Council (GPhC).

Since 2014, NHS England has allowed GPs to prescribe generic sildenafil for ED without the previous restrictions that limited NHS-funded prescriptions to specific clinical groups.

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

Prepayment certificates are available for those who need multiple prescription items.

Sildenafil 50 mg is also available without a prescription as a pharmacy medicine under the brand name Viagra Connect.

You can purchase it from a pharmacy after a short consultation with the pharmacist to ensure it is safe for you to use.

The pharmacist will ask about your medical history, current medicines, and cardiovascular risk factors.

Sildenafil can also be obtained from authorised online pharmacies and prescribers registered with the GPhC or Care Quality Commission (CQC). Always verify that the service is legitimately registered before purchasing medicines online.

Sources

• Sildenafil film-coated tablets, Summary of Product Characteristics (EMC)
• Sildenafil, British National Formulary (BNF)
• Erectile dysfunction, NICE CKS
• Sildenafil (Viagra), NHS
• MHRA Yellow Card Scheme