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Sildenafil is a prescription tablet for erectile dysfunction, first authorised across the EU on 13 September 1998. It relaxes the blood vessels in the penis so more blood can flow in when you are sexually aroused — it does not create arousal on its own, so sexual stimulation is still needed for it to work.
Active ingredient: sildenafil · Dosages: 25 / 50 / 100 mg on demand · Onset: 30–60 minutes · Effect window: around 4–6 hours.

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Can I get it?

Yes, if a doctor decides it is right for you. Sildenafil is authorised in the EU and can be prescribed after a medical review of your cardiovascular health, current medication and contraindications.

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What it is

Sildenafil is a phosphodiesterase-5 (PDE5) inhibitor, taken by mouth, for adult men with erectile dysfunction. It does not cause spontaneous erections or change desire — sexual stimulation is still required.

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How it works

Arousal releases nitric oxide

Sexual stimulation triggers the release of nitric oxide (NO) from parasympathetic non-adrenergic, non-cholinergic neurons and endothelial cells in the corpus cavernosum. NO diffuses into adjacent smooth-muscle cells and activates soluble guanylate cyclase, which raises intracellular cyclic guanosine monophosphate (cGMP).

cGMP activates protein kinase G

cGMP binds to and activates protein kinase G (PKG), which phosphorylates ion channels and contractile proteins, lowering intracellular calcium. The smooth muscle of the corpus cavernosum relaxes, sinusoidal spaces expand, and blood flows in under arterial pressure.

Sildenafil blocks PDE5 to sustain cGMP

PDE5 is the main cGMP-hydrolysing phosphodiesterase in corpus cavernosum smooth muscle. Sildenafil competitively inhibits PDE5 (IC50 5.2 nM), preventing cGMP breakdown and prolonging smooth-muscle relaxation during sexual stimulation. Without sexual arousal and the NO signal, sildenafil has no substrate to act on.
Sildenafil also inhibits PDE6 in retinal photoreceptors at roughly one-tenth the potency of its PDE5 inhibition, which accounts for the blue/green colour-discrimination changes reported at higher doses. It has negligible affinity for PDE1–4.

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What the studies reported

Both pivotal studies were short — 12 and 24 weeks — and relied on self-reported questionnaires and diaries, so they describe short-term effect rather than long-term outcomes.
The pivotal NEJM 1998 trial enrolled 861 men across two sequential double-blind, placebo-controlled studies. The first was a 24-week dose-response study (n=532, randomised to 25, 50 or 100 mg sildenafil or placebo); the second was a 12-week flexible dose-escalation study (n=329), after which 225 men entered a 32-week open-label extension. Efficacy was assessed by the International Index of Erectile Function (IIEF) questionnaire, a patient log of sexual attempts, and a global-efficacy question. In the flexible-dose phase, 69% of all intercourse attempts were successful on sildenafil versus 22% on placebo. The trial did not measure effects beyond 12 months, did not compare sildenafil against other PDE5 inhibitors, and excluded men with recent cardiovascular events.

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Who it is not suitable for

Sildenafil is contraindicated in men taking nitrate medicines or nitric-oxide donors, because the combination can cause a serious drop in blood pressure. The mechanism: both sildenafil and nitrates raise cGMP — nitrates by donating NO and stimulating guanylate cyclase, sildenafil by blocking cGMP breakdown — causing synergistic systemic vasodilation. It is also contraindicated in men who have already lost vision in one eye from NAION. The doctor reviews cardiovascular health before prescribing; men with severe cardiovascular disease may not be suitable. A dedicated study found mild, temporary blue/green colour-discrimination changes 1 hour after a 100 mg dose, with no effect remaining 2 hours after the dose; this reflects inhibition of PDE6 in retinal photoreceptors, where PDE6 handles cGMP in the phototransduction cascade.

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What has been reported

Sildenafil is contraindicated with nitrate medicines and nitric-oxide donors, because the combination can cause a serious drop in blood pressure. It is also contraindicated in men who have already lost vision in one eye from NAION.

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Taking it day to day

Timing

Take it 30–60 minutes before sexual activity.

Food

Avoid heavy, high-fat meals right before — they delay onset.

Nitrates

Never combine with nitrates or poppers.

Adjusting the dose

Message your doctor to adjust the dose if needed, rather than exceeding one dose in 24 hours.
Practical day-to-day guidance — storage, and what to do if the medication is not working — is covered in the sildenafil treatment guide.

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Common questions

Sildenafil takes 30 to 60 minutes to start working. The effect lasts around 4 to 6 hours. Sexual stimulation is required — it does not cause an automatic erection.
The EU product information advises that alcohol can worsen dizziness and erectile difficulty. Avoid excess alcohol when taking sildenafil.
Sildenafil is contraindicated with nitrate medicines and nitric-oxide donors because the combination can cause a serious drop in blood pressure. This includes poppers (amyl nitrite). Never combine them.
Visual disturbance, colour distortion and blurred vision are listed as common in the EU product information. A dedicated study found mild, temporary blue/green colour-discrimination changes 1 hour after a 100 mg dose, with no effect after 2 hours. Sudden vision loss in one eye (NAION) is rare.
Take one tablet 30 to 60 minutes before sexual activity. Avoid a heavy, high-fat meal right before — it delays onset. Do not take more than one dose in 24 hours. If the dose is not right, message your doctor rather than increasing it yourself.
Yes. Sildenafil has been authorised across the EU since 13 September 1998 (EMA EPAR Viagra, EMEA/H/C/000202), so a licensed doctor can prescribe it when clinically appropriate.

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How Hi-Doctor handles it

A licensed doctor reviews every sexual health option and recommends what fits your history and goals. See how the consultation works and how we handle safety and eligibility. Pricing is on the pricing page. For a related concern, see premature ejaculation or dapoxetine.

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Start a consultation

Sexual health consultation

Answer the sexual health questionnaire and a registered doctor reviews it. They decide whether treatment is appropriate, which one, and at what dose — and declining is a normal outcome. See what it costs and how the review works.
This page is reference information, not medical advice and not a prescription. Whether a medicine is appropriate for you is decided by a registered doctor after reviewing your consultation.