Phosphodiesterase-5 (PDE5) inhibitor Pregnancy: Not indicated for use by women. There are no adequate and well-controlled studies in pregnant or breast-feeding women. No relevant adverse effects were found in reproduction studies in rats and rabbits following oral administration. There was no effect on sperm motility or morphology after single 100 mg oral doses in healthy volunteers.

Sildenafil

Brand names: Viagra, Revatio (PAH)

Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction and, in other formulations, pulmonary arterial hypertension.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: One 50 mg tablet taken with water approximately one hour before sexual activity

Route: Oral

Frequency: As required; the maximum recommended dosing frequency is once per day

Max: 100 mg. For the prescription-only product, based on efficacy and tolerability the dose may be increased to 100 mg or decreased to 25 mg; the maximum recommended dose is 100 mg

SOURCE: eMC UK SPC, 'Aronix 50 mg Film-Coated Tablets', §4.2. INDICATION CAVEAT: this SPC covers erectile dysfunction only — it contains NO regimen for pulmonary arterial hypertension (e.g. the 20 mg three-times-daily product), which must be sourced separately if this page covers PAH. Taking with food may delay the onset of activity compared with the fasted state. Patients should be advised they may need to take it a number of times on different occasions (a maximum of one 50 mg tablet per day) before achieving an erection satisfactory for sexual activity, and to consult a doctor if still unsuccessful after several attempts. ELDERLY: no dosage adjustment required in patients aged 65 years or over. HEPATIC IMPAIRMENT: clearance is reduced (e.g. cirrhosis) — individuals with mild to moderate hepatic impairment must consult their doctor before taking it, since a 25 mg tablet may be more suitable; use in severe hepatic impairment is contraindicated. CO-MEDICATION: with CYP3A4 inhibitors (ketoconazole, itraconazole, erythromycin, cimetidine) a 25 mg tablet may be more suitable — ritonavir is contraindicated; patients on alpha blockers (e.g. alfuzosin, doxazosin, tamsulosin) should be stabilised on alpha blocker therapy before sildenafil is initiated and a 25 mg tablet may be more suitable. PAEDIATRICS: not indicated for individuals below 18 years of age. US labelling (cross-check): the recommended dose is 50 mg as needed approximately 1 hour before sexual activity, but may be taken anywhere from 30 minutes to 4 hours before; start at 25 mg with alpha-blockers, with strong CYP3A4 inhibitors or erythromycin, in patients over 65 years, hepatic impairment and severe renal impairment; with ritonavir do not exceed a maximum single dose of 25 mg in a 48-hour period.

Dose adjustments

Renal

No dosage adjustment is required for mild to moderate renal impairment. Sildenafil clearance is reduced in severe renal impairment (creatinine clearance < 30 ml/min) — individuals previously diagnosed with severe renal impairment must be advised to consult their doctor before taking it, since a 25 mg tablet may be more suitable.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Co-administration with nitric oxide donors (such as amyl nitrite) or nitrates in any form — sildenafil potentiates the hypotensive effects of nitrates
Co-administration with ritonavir, and with guanylate cyclase stimulators such as riociguat (risk of symptomatic hypotension)
Men for whom sexual activity is inadvisable, including severe cardiovascular disorders such as recent (6 months) acute myocardial infarction or stroke, unstable angina or severe cardiac failure
Severe hepatic impairment; hypotension (blood pressure < 90/50 mmHg); known hereditary degenerative retinal disorders such as retinitis pigmentosa; loss of vision in one eye because of non-arteritic anterior ischaemic optic neuropathy (NAION); anatomical deformation of the penis (angulation, cavernosal fibrosis or Peyronie's disease); women; men without erectile dysfunction; men under 18 years

Side effects

Headache (very common); dizziness, somnolence, hypoaesthesia; cerebrovascular accident, transient ischaemic attack, seizure and syncope (rare)
Flushing and hot flush (common); nasal congestion (common), epistaxis and sinus congestion; hypertension and hypotension (uncommon)
Visual colour distortions, visual disturbance and blurred vision (common); eye pain, photophobia, photopsia, conjunctivitis (uncommon); NAION, retinal vascular occlusion, retinal haemorrhage and glaucoma (rare)
Nausea and dyspepsia (common); gastro-oesophageal reflux disease, vomiting, upper abdominal pain, dry mouth (uncommon)
Tachycardia and palpitations (uncommon); sudden cardiac death, myocardial infarction, ventricular arrhythmia, atrial fibrillation and unstable angina (rare); rash (uncommon); Stevens-Johnson syndrome and toxic epidermal necrolysis (rare); vertigo, tinnitus and deafness

Interactions

Nitrates and nitric oxide donors — contraindicated; sildenafil potentiates their hypotensive effect
Ritonavir (a highly potent P450 inhibitor) — contraindicated; guanylate cyclase stimulators such as riociguat — contraindicated
Other CYP3A4 inhibitors (ketoconazole, itraconazole, erythromycin, cimetidine) — reduce sildenafil clearance; a 25 mg tablet may be more suitable and patients must consult their doctor first
Alpha blockers (e.g. alfuzosin, doxazosin, tamsulosin) — risk of postural hypotension; patients should be stabilised on alpha blocker therapy before starting sildenafil, and a 25 mg tablet may be more suitable
US labelling adds that sildenafil can potentiate the hypotensive effects of anti-hypertensives generally

Clinical monograph

How it works

It selectively inhibits PDE5, the enzyme that breaks down cyclic GMP, thereby enhancing nitric oxide-mediated smooth muscle relaxation and increasing blood flow into the corpus cavernosum during sexual stimulation.

Prescribing in practice

It is absolutely contraindicated with nitrates or nicorandil because the combination can cause profound, potentially fatal hypotension.
It should be avoided or used with caution in recent stroke or myocardial infarction, significant cardiovascular disease and severe hepatic impairment.
Caution and dose consideration apply with alpha-blockers and potent CYP3A4 inhibitors owing to additive hypotension and increased exposure.

Monitoring

Assess cardiovascular fitness for sexual activity before prescribing and review efficacy and tolerability rather than relying on routine laboratory monitoring.

Counselling the patient

Never take this with nitrate heart medicines or recreational nitrates ('poppers').
Seek urgent medical help for an erection lasting several hours or for sudden loss of vision or hearing.
It works only with sexual stimulation and a high-fat meal can delay its onset.

Evidence & guidelines

Sildenafil is an established first-line PDE5 inhibitor for erectile dysfunction; see NICE guidance and the SPC for indications and cautions.

Reference: NICE NG215 Erectile Dysfunction; ESC PAH Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Sildenafil (CTD-Associated PAH / Raynaud's) · Phosphodiesterase-5 (PDE5) Inhibitor

Pathways