Sildenafil is the UK’s most widely prescribed treatment for erectile dysfunction (ED). Available as a generic since the Viagra patent expired, it is now recommended by NICE as the first-line pharmacological treatment for ED — and can be obtained quickly and discreetly through a regulated online clinic without needing to visit a GP in person.
This guide explains how sildenafil works, the correct dose for you, what side effects to expect, who should not take it, and exactly how to access it safely online in the UK.
Quick facts — sildenafil UK
How sildenafil works
Sildenafil belongs to a class of medicines called phosphodiesterase type 5 (PDE5) inhibitors. During sexual stimulation, the body releases a signalling molecule called cGMP that relaxes the smooth muscle lining the penile arteries, allowing blood to flow in and produce an erection. PDE5 is the enzyme that breaks cGMP down — sildenafil blocks it, so the erection response can occur normally.
Sildenafil requires sexual stimulation to work. It does not create an automatic erection and has no effect on sexual desire.
How sildenafil is available in the UK
Understanding the different ways to access sildenafil helps you choose the most appropriate route for your situation.
| Route | Product | Doses | How to get it |
|---|---|---|---|
| NHS prescription | Generic sildenafil | 25mg · 50mg · 100mg | GP or NHS online service |
| Private prescription (online) | Generic sildenafil | 25mg · 50mg · 100mg | GPhC-registered online pharmacy e.g. Access Doctor |
| Over the counter (OTC) | Viagra Connect | 50mg only | GPhC-registered pharmacy — pharmacist assessment required. Men aged 18–64 only. |
| Branded private prescription | Viagra (Pfizer) | 25mg · 50mg · 100mg | GP or private clinic — clinically equivalent to generic, significantly more expensive |
Generic sildenafil and branded Viagra contain the same active ingredient at the same doses and are clinically equivalent. The MHRA requires all generic medicines to demonstrate bioequivalence before licensing — meaning the same absorption rate, peak blood concentration, and clinical effect.
Sildenafil dosage: what dose should you take?
The appropriate dose of sildenafil depends on your age, other medicines, and how you respond to treatment. Most men start on 50mg.
| 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. erythromycin, ketoconazole) | 25mg | 50mg | Once in any 24-hour period — discuss with prescriber |
How to take it for best results
- Take 30–60 minutes before anticipated sexual activity
- Take on an empty stomach or with a light meal — a high-fat meal can delay onset by up to an hour
- Swallow the tablet whole with water
- Do not exceed one dose in 24 hours
- Sexual stimulation is required — the tablet alone will not produce an erection
What if sildenafil isn’t working?
Many men try sildenafil once under suboptimal conditions and conclude it does not work. Before changing treatment, check:
- Did you take it on a relatively empty stomach?
- Did you allow 30–60 minutes before stimulation?
- Was there adequate sexual stimulation present?
- Have you tried it more than once? Clinical trials show efficacy improves over the first several attempts.
If 100mg used correctly on multiple occasions is still ineffective, speak to your prescriber. They may suggest an alternative treatment such as tadalafil.
Complete a confidential consultation and receive sildenafil delivered discreetly to your door — from a GPhC-registered prescriber.
Side effects of sildenafil
Most side effects are mild and directly related to the drug’s blood-vessel-widening mechanism. They typically resolve as sildenafil leaves the body over a few hours.
Common (up to 1 in 10 people)
- Headache
- Facial flushing — warmth and redness, often in the face and chest
- Nasal congestion or runny nose
- Indigestion or nausea
- Dizziness or light-headedness
- Visual disturbances — most commonly a temporary blue tinge to vision (cyanopsia), blurred vision, or increased light sensitivity
Uncommon (up to 1 in 100 people)
- Palpitations or increased heart rate
- Muscle aches or back pain
- Dry mouth
- Skin rash
- Priapism — a painful erection lasting more than 4 hours. This is a urological emergency. Without prompt treatment, permanent damage can occur. Go to A&E immediately.
- Sudden loss of vision in one or both eyes. Call 999 or go to A&E — do not delay.
- Sudden loss of hearing or new ringing in the ears.
- Chest pain, irregular heartbeat, or collapse. Call 999.
- Severe allergic reaction — swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash.
Who should not take sildenafil?
A clinical assessment before prescribing exists specifically to identify these contraindications. Do not take sildenafil if you:
- Take nitrate medicines in any form — GTN spray or patches, isosorbide mononitrate or dinitrate tablets, or recreational “poppers” (amyl nitrite). The combination can cause a sudden, severe, and potentially fatal drop in blood pressure.
- Take riociguat (Adempas), prescribed for pulmonary hypertension
- Have been advised by a doctor to avoid sexual activity due to cardiovascular risk
- Have severe liver disease
- Have had a stroke or heart attack within the last 6 months
- Have very low blood pressure (below 90/50 mmHg) or uncontrolled high blood pressure
- Have a hereditary eye condition such as retinitis pigmentosa
- Have previously experienced non-arteritic anterior ischaemic optic neuropathy (NAION)
Use with caution if you take alpha-blockers
Alpha-blockers (prescribed for high blood pressure or benign prostatic hyperplasia) can potentiate sildenafil’s blood-pressure-lowering effect. If you take both, your prescriber will advise on the appropriate starting dose and timing to minimise risk.
Only purchase sildenafil from a GPhC-registered pharmacy that conducts a proper prescribing assessment. You can verify any UK online pharmacy on the GPhC register at pharmacyregulation.org.
How to get sildenafil online in the UK
Accessing sildenafil through a regulated online pharmacy is a straightforward, private process — no GP appointment required. Here is how it works at Access Doctor:
- Complete an online consultation — a structured medical questionnaire covering your medical history, current medicines, blood pressure, and cardiovascular health. Takes around five minutes.
- Clinical review — a GPhC-registered pharmacist independent prescriber reviews your answers and either approves a prescription or requests further information.
- Prescription issued — if clinically appropriate, a prescription is issued and passed to a GPhC-registered dispensing pharmacy.
- Discreet delivery — medication arrives in plain, unbranded packaging, typically next working day.
All consultations at Access Doctor are reviewed by our clinical team, led by Dr Abdishakur M Ali — Clinical Director.
GPhC-registered prescribers · MHRA-regulated · Discreet next-day delivery
Frequently asked questions
Related guides
References
- National Institute for Health and Care Excellence. Erectile dysfunction. NICE clinical knowledge summary. Last revised January 2023. cks.nice.org.uk/topics/erectile-dysfunction/
- General Pharmaceutical Council. Guidance for registered pharmacies providing pharmacy services at a distance, including on the internet. 2015. pharmacyregulation.org
- Medicines and Healthcare products Regulatory Agency. Buying medicines over the internet. gov.uk/guidance/buying-medicines-over-the-internet
- Rendell MS, Rajfer J, Wicker PA, Smith MD. Sildenafil for treatment of erectile dysfunction in men with diabetes: a randomized controlled trial. JAMA. 1999;281(5):421–426. PMID: 9952201.
- Goldstein I, Lue TF, Padma-Nathan H, et al. Oral sildenafil in the treatment of erectile dysfunction. New England Journal of Medicine. 1998;338(20):1397–1404. PMID: 9580646.