Sildenafil is one of the most widely recognised medicines in the UK. Best known as the active ingredient in Viagra, it has been prescribed for erectile dysfunction (ED) since 1998 and is now available as a generic at a fraction of the original cost. Since the patent expiry, generic sildenafil has become the first-line treatment for ED recommended by NHS guidelines.
This guide covers how sildenafil works, the correct doses, what side effects to watch for, who should not take it, and how to obtain it safely online through a registered UK prescriber.
Sildenafil at a glance
What is sildenafil?
Sildenafil citrate is a phosphodiesterase type 5 (PDE5) inhibitor. It was originally developed by Pfizer researchers investigating treatments for angina and high blood pressure. During clinical trials, participants reported an unexpected side effect: improved erections. The drug was subsequently licensed for erectile dysfunction, launching as Viagra in 1998.
Today, sildenafil is available in the UK under several forms:
- Generic sildenafil — prescription-only in 25mg, 50mg, and 100mg tablets for ED
- Viagra (Pfizer) — branded 25mg, 50mg, and 100mg prescription tablets
- Viagra Connect — 50mg available without prescription (OTC) for men aged 18–64 from pharmacies
- Revatio — 20mg tablets or oral suspension, licensed for pulmonary arterial hypertension (PAH)
Generic sildenafil is clinically equivalent to branded Viagra. It contains the same active ingredient at the same doses and must meet the same MHRA bioequivalence standards before it can be sold in the UK.
How does sildenafil work?
During sexual stimulation, the body produces a signalling molecule called cGMP that relaxes the smooth muscle of the penile arteries, allowing blood to flow in and produce an erection. PDE5 is the enzyme that breaks cGMP down — sildenafil blocks it, allowing the erection response to work correctly.
Sildenafil only works when sexual stimulation is present. It does not cause an automatic erection.
For a detailed explanation of the mechanism — including how it compares to other PDE5 inhibitors — see our full guide: What is Viagra? How sildenafil works.
What is sildenafil used for?
Erectile dysfunction
ED is sildenafil's primary licensed indication. NICE clinical knowledge summary (CKS) recommends offering a PDE5 inhibitor as first-line treatment for men presenting with erectile dysfunction. Sildenafil has demonstrated efficacy in men with ED caused by psychological factors, diabetes, cardiovascular disease, nerve damage, and following prostate surgery. It is effective in approximately 70–80% of men with ED when used correctly.
Pulmonary arterial hypertension (PAH)
At lower doses (20mg three times daily as Revatio), sildenafil dilates the pulmonary vasculature, reducing the pressure the right heart must overcome to pump blood through the lungs. This is a specialist indication, prescribed and monitored by hospital physicians.
Off-label uses
Sildenafil is sometimes used off-label for altitude sickness and in fertility contexts. These uses should only be discussed with a specialist.
Sildenafil dosage
The correct dose depends on your age, other medicines you take, and how you respond to treatment. The prescriber will advise you after a clinical assessment.
| Indication | Starting dose | Maximum dose | Frequency |
|---|---|---|---|
| Erectile dysfunction (standard) | 50mg | 100mg | Once per 24 hours (as needed) |
| ED — older men or kidney/liver impairment | 25mg | 50mg | Once per 24 hours (as needed) |
| Pulmonary arterial hypertension | 20mg three times daily | 20mg three times daily | Daily (specialist-supervised) |
When and how to take sildenafil for ED
- Take approximately 30–60 minutes before anticipated sexual activity
- Avoid high-fat meals beforehand — dietary fat slows absorption and can delay onset by up to an hour
- Swallow the tablet whole with water
- Do not take more than one dose within any 24-hour period
- Effects typically last 4–5 hours
What if 50mg doesn't work?
If the 50mg dose is ineffective after several attempts, your prescriber may increase the dose to 100mg. Before concluding it is ineffective, check: Was your stomach empty or lightly fed? Was sufficient sexual stimulation present? Did you allow 30–60 minutes for onset? Clinical trials show consistent, correct use over several attempts is needed to assess true efficacy.
If 100mg remains ineffective, a prescriber may recommend an alternative PDE5 inhibitor such as tadalafil or avanafil. Speak to your prescriber about which option may suit you best.
Sildenafil side effects
Most side effects are mild, predictable from the drug's vasodilatory mechanism, and resolve as it leaves the body.
Common (may affect up to 1 in 10 people)
- Headache
- Facial flushing (warmth and redness)
- Nasal congestion or runny nose
- Indigestion or nausea
- Dizziness or light-headedness
- Visual disturbances — most commonly a blue tinge to vision (cyanopsia), blurred vision, or increased sensitivity to light
Uncommon (may affect up to 1 in 100 people)
- Palpitations or increased heart rate
- Muscle aches
- Dry mouth
- Skin rash
- Priapism — a painful erection lasting more than 4 hours. A medical emergency requiring urgent treatment to prevent permanent damage.
- Sudden loss or deterioration of vision in one or both eyes. Stop sildenafil immediately and call 999 or attend A&E.
- Sudden loss of hearing, ringing in the ears, or dizziness associated with hearing changes.
- Chest pain, irregular heartbeat, or collapse — stop the medication and call 999.
- Severe allergic reaction — swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash.
Who should not take sildenafil?
Sildenafil is contraindicated in certain people. A clinical assessment before prescribing exists to identify these cases. 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 severe and potentially fatal drop in blood pressure.
- Take riociguat (Adempas), used for pulmonary hypertension
- Have been advised to avoid sexual activity due to cardiovascular risk
- Have severe hepatic impairment (liver disease)
- Have a recent history of stroke or myocardial infarction within the last 6 months
- Have severe hypotension (below 90/50 mmHg) or uncontrolled hypertension
- Have a hereditary degenerative eye disease such as retinitis pigmentosa
- Have had non-arteritic anterior ischaemic optic neuropathy (NAION)
Cautions and dose adjustments
Use with caution and at a reduced starting dose (25mg) in men over 65, those with kidney or liver impairment, and those taking CYP3A4 inhibitors such as erythromycin or ketoconazole. Alpha-blockers can potentiate the blood pressure-lowering effect — your prescriber will advise on timing and dose if you take both.
Only purchase sildenafil from a GPhC-registered pharmacy or CQC-registered online clinic. Verify any UK online pharmacy at pharmacyregulation.org.
How to get sildenafil online in the UK
In the UK, sildenafil above 50mg is a prescription-only medicine. Getting it through a regulated online clinic is straightforward:
- Complete an online consultation — a structured questionnaire covering your medical history, medicines, blood pressure, and cardiovascular health
- Clinical review — a GPhC-registered pharmacist independent prescriber reviews your answers and approves or requests further information
- Prescription issued — if appropriate, a prescription is issued and dispensed by a GPhC-registered pharmacy
- Discreet delivery — medication is delivered in plain packaging, typically within 24 hours
At Access Doctor, all consultations are reviewed by our clinical team including Dr Abdishakur M Ali, Clinical Director.
Complete a confidential online consultation and receive sildenafil delivered discreetly to your door.
Getting the best results from sildenafil
Timing
Take sildenafil 30–60 minutes before anticipated sexual activity. Effects last up to 5 hours, so there is no need to rush. Waiting beyond 60 minutes before stimulation is fine — effects build as the drug reaches peak plasma concentration.
Food and drink
High-fat meals significantly slow absorption and can delay onset by up to an hour. Take on an empty stomach or after a light snack for the most predictable results. Alcohol independently impairs erectile function and amplifies the blood-pressure-lowering effect — limit to one or two units.
Give it more than one attempt
Clinical trials consistently show that sildenafil becomes more effective over the first several uses as men grow more comfortable with how it works. If the first dose is disappointing, do not abandon treatment before discussing with a prescriber whether dose or timing can be optimised.
For lifestyle factors that improve erectile function long-term — exercise, weight management, smoking cessation, blood pressure — see our guide: Lifestyle changes for erectile dysfunction →
Frequently asked questions
GPhC-registered prescribers · MHRA-regulated · Discreet next-day delivery
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/
- European Association of Urology. EAU Guidelines on Sexual and Reproductive Health 2024 — Erectile Dysfunction. uroweb.org/guidelines/sexual-and-reproductive-health
- Medicines and Healthcare products Regulatory Agency (MHRA). Sildenafil 50mg film-coated tablets (Viagra Connect): UK summary of product characteristics. medicines.org.uk/emc/
- Fink HA, Mac Donald R, Rutks IR, et al. Sildenafil for male erectile dysfunction: a systematic review and meta-analysis. Archives of Internal Medicine. 2002;162(12):1349–1360. PMID: 12076233.
- Hatzimouratidis K, Salonia A, Adaikan G, et al. Pharmacotherapy for erectile dysfunction: Recommendations from the fourth international consultation for sexual medicine. Journal of Sexual Medicine. 2016;13(4):465–488. PMID: 26953437.