Sildenafil: Uses, Dosage, Side Effects and How to Get It in the UK
What sildenafil treats, how it works, which dose to take, side effects, who should not take it, and the safe ways to get it in the UK — reviewed by UK clinicians.
When to get urgent help: Call 999 if you get chest pain during or after sex (tell the paramedics you have taken sildenafil, as nitrates such as GTN must not be given), or signs of a serious allergic reaction. Go to A&E or call 999 if an erection lasts more than 4 hours (do not drive yourself). Stop sildenafil and get medical help immediately (contact your GP or NHS 111) if you have a sudden decrease or loss of vision or hearing. See your GP about new erection problems, as they can be an early sign of heart disease or diabetes.
Part of the Erectile Dysfunction Guides.
Key fact: Sildenafil is the active ingredient in Viagra and one of the most widely used treatments for erectile dysfunction in the UK. It starts working in 30 to 60 minutes, lasts about 4 hours, and only works with sexual arousal. It must never be taken with nitrates such as GTN spray or with “poppers”.
Sildenafil Online — 25mg, 50mg and 100mg
Access Doctor provides sildenafil following a GPhC-regulated online consultation with our pharmacist independent prescribers, who check it is safe for you and recommend the right dose.
Order Sildenafil →What is sildenafil and what is it used for?
Sildenafil belongs to a group of medicines called PDE5 inhibitors (phosphodiesterase type 5 inhibitors). It was originally developed by Pfizer as a possible treatment for angina (chest pain caused by heart disease), and its effect on erections was noticed during early clinical trials. It became the first tablet licensed for erectile dysfunction, as Viagra, in 1998, and is now widely available as a much cheaper generic. Generic sildenafil contains the same medicine at the same strengths as Viagra and works in exactly the same way: before a generic medicine can be licensed for use in the UK, it must be shown to be bioequivalent to the original brand, meaning it delivers the same amount of medicine into the bloodstream at the same rate.
In the UK, sildenafil is licensed for:
- Erectile dysfunction (ED) — the main use. Taken as a 25mg, 50mg or 100mg tablet before sex, it helps men get and keep an erection firm enough for sex.
- Pulmonary arterial hypertension (PAH) — a separate, specialist-prescribed use at a lower dose (for example as Revatio, 20mg three times a day) to lower blood pressure in the lungs.
Specialists occasionally prescribe sildenafil “off-label” for other conditions, such as severe Raynaud’s phenomenon. This guide covers sildenafil for erectile dysfunction.
Get checked as well as treated: erection problems can be an early sign of heart disease, high blood pressure, diabetes or low testosterone. Even if sildenafil works for you, see your GP for a review including blood pressure, blood sugar and cholesterol.
How does sildenafil work?
An erection depends on blood flow. During arousal, nerves in the penis release nitric oxide, which triggers production of a messenger chemical called cGMP. cGMP relaxes the muscle in the walls of the penile blood vessels, so blood flows in and is held in the erectile tissue. An enzyme called PDE5 breaks cGMP down. Sildenafil blocks PDE5, so cGMP stays higher for longer and the natural erection response is stronger and easier to keep.
1
Arousal starts the process
Sexual stimulation causes nerves in the penis to release nitric oxide. Without arousal this signal never starts, which is why sildenafil does nothing on its own.
2
cGMP relaxes the blood vessels
Nitric oxide drives production of cGMP, which relaxes the artery walls in the penis and lets blood flow in.
3
Sildenafil blocks PDE5
Sildenafil stops the PDE5 enzyme breaking cGMP down too quickly, keeping the blood vessels relaxed.
4
The erection is firmer and lasts
More blood enters the erectile tissue and stays there, producing an erection that is firmer and easier to maintain.
Important: sildenafil improves the body’s natural response to arousal. It does not cause an erection automatically, it is not a hormone, and it does not increase sex drive.
How long does sildenafil take to work and how long does it last?
30–60
minutes to start working
~1 hr
to peak blood levels on an empty stomach
~4 hrs
window in which an erection is easier
Most men find sildenafil starts working within 30 to 60 minutes. Blood levels peak after about an hour and the effect then gradually wears off, giving a window of about 4 hours in which getting an erection is easier. It does not cause a continuous erection for that time. Sildenafil has a half-life of about 3 to 5 hours, so most of the medicine has left your body within about a day.
Sildenafil dosage: 25mg, 50mg and 100mg
Sildenafil tablets for ED come in three strengths. The usual starting dose is 50mg, taken about an hour before sex. Your prescriber may increase it to 100mg or lower it to 25mg depending on how well it works and how you tolerate it. Whatever the dose, take no more than one dose in 24 hours.
| Dose | Who it usually suits |
|---|---|
| 25mg | Men who get side effects at 50mg, and men with liver problems, severe kidney problems or who take certain interacting medicines (see below) |
| 50mg | Most men starting sildenafil — the standard starting dose |
| 100mg | Men who tolerate 50mg but do not get a reliable response — the maximum dose |
When a lower starting dose is used
- Liver problems or severe kidney problems: a 25mg starting dose should be considered.
- Medicines that raise sildenafil levels (CYP3A4 inhibitors such as clarithromycin, erythromycin, ketoconazole or itraconazole): a 25mg starting dose should be considered.
- Ritonavir (an HIV medicine): this greatly increases sildenafil levels. Taking sildenafil with ritonavir is not advised; if it is considered necessary, no more than a single 25mg dose in 48 hours. Other HIV medicines such as saquinavir also raise sildenafil levels, so a 25mg starting dose should be considered.
- Alpha-blockers (such as doxazosin or tamsulosin): you should be stable on the alpha-blocker first, and a 25mg starting dose should be considered.
Older men do not usually need a lower dose because of age alone; your prescriber will decide based on your health and other medicines.
How to take sildenafil for the best results
1
Allow time
Take the tablet about an hour before you plan to have sex, rather than at the last minute.
2
Avoid a heavy meal
Taking it with a large or high-fat meal can delay its effect by about an hour and make it slightly weaker. An empty stomach or a light meal works best.
3
Arousal is still needed
Sildenafil will not produce an erection without sexual stimulation.
4
Keep alcohol low
A small amount of alcohol is unlikely to cause problems, but heavy drinking can make it harder to get an erection.
5
Give it a fair trial
Some men need several attempts before it works reliably, and nerves or performance anxiety can get in the way, especially the first few times. If it has not worked after several tries at the right dose, taken the right way, speak to your prescriber about a higher dose or switching to tadalafil. Our guide to taking Viagra for the first time explains what to expect.
Do not eat grapefruit or drink grapefruit juice while taking sildenafil, as it can raise sildenafil levels and make side effects more likely.
Sildenafil side effects
Side effects are usually mild, short-lived and more likely at higher doses.
| How often | Side effects |
|---|---|
| Very common more than 1 in 10 men | Headache |
| Common up to 1 in 10 men | Facial flushing or hot flushes; blocked nose; indigestion or feeling sick; dizziness; changes to vision, such as a blue or other coloured tinge (cyanopsia) or blurred vision |
| Uncommon up to 1 in 100 men | Examples include palpitations or a fast heartbeat, muscle pain or pain in the arms or legs, eye pain or red eyes, sensitivity to light, nosebleeds, ringing in the ears and feeling tired |
| Rare up to 1 in 1,000 men | Examples include fainting, a sudden decrease or loss of hearing, an erection lasting more than 4 hours, heart attack and stroke |
If a side effect is troublesome, speak to your prescriber about a lower dose.
Get medical help straight away if you have an erection lasting more than 4 hours (go to A&E, as without prompt treatment it can permanently damage the penis), chest pain during or after sex (call 999 and tell them you have taken sildenafil), a sudden decrease or loss of vision or hearing (stop sildenafil and get medical help immediately from your GP or NHS 111), or signs of a serious allergic reaction such as swelling of the face, lips or throat or difficulty breathing (call 999).
Who should not take sildenafil?
Sildenafil is suitable for most men, but some must not take it and others need extra care. This is why a clinical assessment is needed before it is supplied.
Never take sildenafil with nitrates. This includes GTN spray or tablets, isosorbide mononitrate or dinitrate, nicorandil, and recreational “poppers” (amyl or butyl nitrite). Together they can cause a sudden, dangerous drop in blood pressure. Do not take it with riociguat (a pulmonary hypertension medicine) either.
Do not take sildenafil if you
- are allergic to sildenafil or any of the other ingredients in the tablets
- take nitrates, nicorandil or riociguat, or use poppers
- have been advised that sexual activity is not safe for your heart, for example with severe heart disease
- have severe liver problems
- have had a stroke or heart attack recently
- have low blood pressure (below 90/50 mmHg)
- have ever lost sight in one eye because of reduced blood flow to the optic nerve (NAION)
- have an inherited eye condition such as retinitis pigmentosa
Tell your prescriber first if you
- take an alpha-blocker (such as doxazosin, tamsulosin or alfuzosin), as the combination can lower blood pressure
- have high blood pressure that is not well controlled, or another heart condition such as heart valve disease, heart failure, cardiomyopathy or an irregular heartbeat, or you get chest pain or feel very breathless with light or moderate activity
- take a medicine that raises sildenafil levels, such as clarithromycin, erythromycin, ketoconazole, itraconazole or HIV medicines including ritonavir
- have kidney or liver problems
- have a physical deformity of the penis (such as Peyronie’s disease), or sickle cell disease, leukaemia or multiple myeloma, which increase the risk of a prolonged erection
- have a stomach ulcer or a bleeding disorder
- take another treatment for erectile dysfunction — do not combine sildenafil with other PDE5 inhibitors
How to get sildenafil in the UK
| Route | What you get | How it works |
|---|---|---|
| NHS prescription | Generic sildenafil 25mg, 50mg or 100mg | From your GP, or a sexual health clinic, after a health check (in Wales, only for men who meet certain criteria). NHS prescription charges apply in England unless you are exempt; prescriptions are free in Scotland, Wales and Northern Ireland. |
| Private prescription online | Generic sildenafil or branded Viagra, 25mg, 50mg or 100mg | Through a GPhC-registered online pharmacy, such as Access Doctor, after an online consultation reviewed by a prescriber. |
| Pharmacy medicine | Sildenafil 50mg only (for example Viagra Connect) | Sold by pharmacies to men aged 18 and over after a consultation with a pharmacist, without a prescription. If you buy it this way, see your GP for a health check within 6 months. |
Avoid unlicensed sellers. Products such as Kamagra are not licensed for sale in the UK. Medicines bought from unregulated websites or social media may contain the wrong dose, no active ingredient or harmful contaminants. Only buy sildenafil from a GPhC-registered pharmacy. See why Kamagra is dangerous.
Sildenafil vs tadalafil
Both medicines work in the same way. The main difference is how long they last: tadalafil (the active ingredient in Cialis) works for up to 36 hours, compared with about 4 hours for sildenafil.
| Sildenafil (Viagra) | Tadalafil (Cialis) | |
|---|---|---|
| Starts working | 30 to 60 minutes | 30 minutes to 2 hours |
| How long it lasts | About 4 hours | Up to 36 hours |
| Effect of food | A heavy meal can delay it | Largely unaffected by food |
| Daily option | No — as needed only | Yes — a low dose once a day |
Neither is “stronger”: studies show they work about as well as each other, and the best choice depends on whether you prefer to plan sex or want more spontaneity. See our full tadalafil vs sildenafil comparison. You can request tadalafil through the same online consultation. Sildenafil does not treat premature ejaculation; see our premature ejaculation treatment page.
UK safety and prescribing information
- Legal status: prescription sildenafil (25mg, 50mg and 100mg) is a prescription-only medicine (POM). Viagra Connect 50mg is a pharmacy (P) medicine that men aged 18 and over can buy after a consultation with a pharmacist.
- Maximum dose: no more than one dose in 24 hours. The maximum prescription dose is 100mg; Viagra Connect is limited to one 50mg tablet a day.
- Licensed supply: Access Doctor supplies MHRA-licensed UK medicines, dispensed by our GPhC-registered pharmacy (#9011198) only after a prescriber has assessed your consultation. It is illegal to sell or supply prescription-only medicines without a prescription in the UK.
- Interactions: never take sildenafil with nitrates (including GTN spray), nicorandil, “poppers” or riociguat. Tell your prescriber if you take ritonavir or other HIV medicines, antifungals such as ketoconazole, some antibiotics such as clarithromycin, or an alpha-blocker.
- Driving: sildenafil can cause dizziness and changes to vision. Check how it affects you before driving or using machinery.
- NHS: in England and Scotland, your GP can prescribe generic sildenafil on the NHS for erectile dysfunction; in Wales, NHS prescribing is currently limited to men who meet certain criteria, and the rules may differ in Northern Ireland. NHS prescription charges apply in England unless you are exempt; prescriptions are free in Scotland, Wales and Northern Ireland.
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Complete a short, confidential online consultation. Our prescribers review every order, and can help you change dose or switch to tadalafil if sildenafil does not suit you.
Start your consultation →Frequently Asked Questions
Is sildenafil the same as Viagra?
Yes. Sildenafil is the active ingredient in Viagra. Generic sildenafil contains the same medicine at the same strengths and works in exactly the same way. The main differences are the name on the packet and the price, with generic sildenafil usually costing much less.
What is the usual starting dose of sildenafil?
For most men the starting dose is 50mg, taken about an hour before sex. Your prescriber may increase it to 100mg if it does not work well enough, or lower it to 25mg if you get side effects, have liver or severe kidney problems, or take certain other medicines. Never take more than one dose in 24 hours.
How long does sildenafil take to work?
Most men find it starts working within 30 to 60 minutes. The effect lasts about 4 hours, during which it is easier to get an erection when you are aroused. A heavy or fatty meal can delay the effect.
How long does sildenafil stay in your system?
Sildenafil has a half-life of about 3 to 5 hours, so the amount in your blood roughly halves every 3 to 5 hours. Its effect on erections usually wears off after about 4 hours, and most of the medicine has left your body within about a day. Levels stay higher for longer in men over 65 and in men with liver or severe kidney problems. Never take more than one dose in 24 hours.
What are the most common side effects of sildenafil?
Headache is the most common side effect. Others include facial flushing, a blocked nose, indigestion, dizziness and temporary changes to vision such as a blue or coloured tinge. They are usually mild and wear off within a few hours.
What should I do if sildenafil does not work?
Check that you took it about an hour before sex, not straight after a heavy meal, and that you were aroused. Some men need several attempts before it works reliably. If it still does not work after several tries, your prescriber may increase the dose or suggest tadalafil instead.
Can I take sildenafil every day?
You should not take more than one dose of sildenafil in 24 hours. It is designed to be taken as needed before sex. If you would prefer a daily treatment, low-dose daily tadalafil may be more suitable, so discuss this with your prescriber.
Does sildenafil work without arousal?
No. Sildenafil only works alongside sexual stimulation. It does not cause an erection on its own and does not increase sexual desire.
Can I drink alcohol with sildenafil?
A small amount of alcohol is unlikely to cause problems. Drinking too much alcohol can make it harder to get an erection, so it is best to avoid large amounts of alcohol before sex.
Can I get sildenafil without a prescription in the UK?
Viagra Connect 50mg can be bought from a pharmacy by men aged 18 and over after a consultation with a pharmacist. Other strengths of sildenafil (25mg and 100mg) need a prescription, which you can get from your GP or a registered online pharmacy.
Can I get sildenafil on the NHS?
Yes. In England and Scotland, your GP can prescribe generic sildenafil on the NHS for erectile dysfunction after checking your general health. In Wales, NHS prescribing is currently limited to men who meet certain criteria, and the rules may differ in Northern Ireland, so ask your GP. NHS prescription charges apply in England unless you are exempt; prescriptions are free in Scotland, Wales and Northern Ireland.
What else is sildenafil used for?
Under a separate licence and at a lower dose, sildenafil is used to treat pulmonary arterial hypertension, a type of high blood pressure in the lungs. This is managed by specialists and is different from its use for erectile dysfunction.
References
- NHS. Sildenafil (including Viagra). nhs.uk
- NHS. Erectile dysfunction (impotence). nhs.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summaries: Erectile dysfunction. cks.nice.org.uk
- Electronic Medicines Compendium. Viagra film-coated tablets and Viagra Connect 50 mg: Summaries of Product Characteristics. Viagra SmPC; Viagra Connect SmPC
- Feldman HA, Goldstein I, Hatzichristou DG, et al. Impotence and its medical and psychosocial correlates: results of the Massachusetts Male Aging Study. Journal of Urology. 1994. pubmed.ncbi.nlm.nih.gov
- Ghofrani HA, Osterloh IH, Grimminger F. Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond. Nature Reviews Drug Discovery. 2006;5:689–702. nature.com
- Medicines and Healthcare products Regulatory Agency. Yellow Card scheme: report a suspected side effect or a fake medicine. yellowcard.mhra.gov.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Prescription sildenafil is a prescription-only medicine and needs a clinical assessment before it can be supplied. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.
Reporting side effects: If you think a medicine has caused a side effect, you can report it to the MHRA through the Yellow Card scheme at yellowcard.mhra.gov.uk or the Yellow Card app. You can also report suspected fake or unlicensed medicines. Reporting helps keep medicines safe for everyone in the UK.