Sildenafil in the UK: Uses, Dosage, Side Effects & Online Access
How sildenafil works, the correct dose, NHS vs private prescribing, Viagra Connect, side effects and contraindications.
Part of the Access Doctor Erectile Dysfunction guide.
Key fact: Sildenafil is the UK’s most widely prescribed treatment for erectile dysfunction and the active ingredient in Viagra. NICE recommends it as the first-line pharmacological treatment for ED. It is available on the NHS, privately through a registered online pharmacy, and — at 50mg — without prescription as Viagra Connect.
Get Sildenafil Online
Access Doctor provides sildenafil via a GPhC-regulated online consultation. No GP appointment required. Discreet next-day delivery.
Start Consultation →~82%
success rate in clinical trials for erectile dysfunction (Goldstein et al. 1998)
1998
year sildenafil was first licensed for ED in the UK
3
doses available: 25mg, 50mg and 100mg
How Sildenafil Works
Sildenafil belongs to the class of medicines called phosphodiesterase type 5 (PDE5) inhibitors. During sexual stimulation, nerves in the penis release nitric oxide, which triggers production of cyclic GMP (cGMP) in penile blood vessels. cGMP relaxes smooth muscle, widening penile arteries and enabling blood inflow and an erection. The PDE5 enzyme normally breaks cGMP down — sildenafil blocks it, allowing the erectile response to occur.
Sexual stimulation is required. Sildenafil does not cause an automatic erection. It enhances the natural erectile response when arousal is present. It is neither a hormone nor an aphrodisiac.
How to Get Sildenafil in the UK
| Route | Product | Doses | How to get it |
|---|---|---|---|
| NHS prescription | Generic sildenafil | 25mg, 50mg, 100mg | GP or NHS online service. Subject to NHS prescribing criteria. |
| Private prescription (online) | Generic sildenafil | 25mg, 50mg, 100mg | GPhC-registered online pharmacy e.g. Access Doctor |
| Over the counter | Viagra Connect | 50mg only | GPhC-registered pharmacy — pharmacist assessment required. Men aged 18–64. |
| Branded private prescription | Viagra (Pfizer) | 25mg, 50mg, 100mg | GP or private clinic — clinically equivalent to generic; more expensive |
Kamagra warning: Products such as Kamagra are not licensed for sale in the UK. They are manufactured in unregulated facilities and may contain no active ingredient, dangerous quantities, or harmful contaminants. Only purchase sildenafil from a GPhC-registered pharmacy. For full details, see why Kamagra is dangerous.
Dosage
| Patient group | Starting dose | Maximum dose | Frequency |
|---|---|---|---|
| Standard (adults under 65) | 50mg | 100mg | Once in any 24-hour period |
| Men over 65 or with kidney/liver impairment | 25mg | 50mg | Once in any 24-hour period |
| Taking CYP3A4 inhibitors (e.g. clarithromycin) | 25mg | 50mg | Once per 24 hours — discuss with prescriber |
- Take 30–60 minutes before sexual activity — plan to allow at least 1 hour
- Take on an empty stomach or with a light meal — a large high-fat meal delays onset by up to 1 hour
- Never exceed one dose in 24 hours
- Avoid grapefruit juice — inhibits sildenafil metabolism
- Avoid large amounts of alcohol
Side Effects
Common (up to 1 in 10)
- Headache — caused by cerebral vasodilation
- Facial flushing — warmth and redness, usually mild
- Nasal congestion
- Indigestion or nausea
- Dizziness or light-headedness
- Visual disturbances — temporary blue tinge (cyanopsia), blurred vision, or increased light sensitivity
Seek immediate emergency care for: priapism (erection lasting >4 hours — attend A&E immediately), sudden vision loss, sudden hearing loss, or chest pain. Call 999.
Who Should Not Take Sildenafil
- Men taking nitrate medicines in any form — GTN spray, isosorbide mononitrate, or recreational “poppers”. Absolute contraindication. No exceptions.
- Men taking riociguat (for pulmonary hypertension)
- Men advised to avoid sexual activity for cardiovascular reasons
- Men with severe liver disease, stroke or heart attack within the past 6 months
- Men with very low blood pressure (below 90/50 mmHg) or uncontrolled hypertension
- Men with retinitis pigmentosa or previous NAION
Sildenafil vs Tadalafil at a Glance
| Feature | Sildenafil (Viagra) | Tadalafil (Cialis) |
|---|---|---|
| Duration | 4–5 hours | Up to 36 hours |
| Onset | 30–60 minutes | ~30 minutes |
| Food effect | Yes — avoid fatty meals | No |
| Daily option? | No | Yes (5mg daily) |
| Efficacy | ~82% | ~81% |
For a full comparison, see tadalafil vs sildenafil.
Frequently Asked Questions
Is sildenafil available on the NHS in the UK?
Yes. Generic sildenafil is available on NHS prescription for ED. NICE recommends it as first-line treatment. It can also be obtained privately through a registered online pharmacy without a GP appointment.
Can I get sildenafil without a prescription in the UK?
Sildenafil 50mg (Viagra Connect) is available OTC from registered pharmacies for men aged 18–64 following a pharmacist assessment. All other doses (25mg and 100mg) require a prescription.
What is the correct starting dose of sildenafil for ED?
50mg for most men, taken 30–60 minutes before sexual activity. Men over 65 or with kidney/liver impairment start at 25mg. Maximum dose is 100mg under prescriber guidance.
How quickly does sildenafil work?
Within 30–60 minutes. Effects last 4–5 hours. An empty stomach speeds absorption; a high-fat meal can delay onset by up to 1 hour.
Is generic sildenafil the same as Viagra?
Yes — same active ingredient, same doses, clinically equivalent. The MHRA requires all generics to demonstrate bioequivalence before licensing.
What is the difference between sildenafil and tadalafil?
Sildenafil lasts 4–5 hours and is affected by food. Tadalafil lasts up to 36 hours and is unaffected by food. Both have similar efficacy. See our full tadalafil vs sildenafil comparison.
References
- National Institute for Health and Care Excellence (NICE). Erectile dysfunction: Clinical Knowledge Summary. Updated 2023. cks.nice.org.uk/topics/erectile-dysfunction
- NHS. Erectile dysfunction (impotence). nhs.uk/conditions/erection-problems-erectile-dysfunction
- Goldstein I et al. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397–1404. PubMed: 9560547
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. ED treatments are prescription-only medicines requiring clinical assessment. In a medical emergency, call 999.