What Is Viagra? Your Complete Guide to Sildenafil
Mechanism, Viagra vs generic sildenafil, doses, prescription rules, side effects and safe use.
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 is painful or 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 Access Doctor Erectile Dysfunction guide.
Key fact: Viagra is the original brand of sildenafil, a PDE5 inhibitor used to treat erectile dysfunction. It only works when you are sexually aroused, and it must never be taken with nitrates or “poppers”. NICE recommends a PDE5 inhibitor such as sildenafil as the usual first medicine to try for erectile dysfunction.
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Start Consultation →What Does Viagra Do?
Viagra’s active ingredient, sildenafil, is a PDE5 inhibitor (phosphodiesterase type 5 inhibitor). After you take it:
- Sildenafil inhibits the PDE5 enzyme, which normally breaks down cyclic GMP (cGMP) in the smooth muscle of penile blood vessels
- When PDE5 is blocked, cGMP accumulates, causing smooth muscle to relax
- Relaxed smooth muscle allows blood vessels to widen (vasodilation)
- When sexually stimulated, this enables significantly increased blood flow into the erectile tissue of the penis
- The penis fills with blood and becomes erect
- Without sexual stimulation, Viagra has no effect — it enhances the natural response to arousal rather than bypassing it
Viagra vs Generic Sildenafil — Is There a Difference?
No meaningful difference. Branded Viagra and generic sildenafil contain the same active ingredient at the same strengths and work in the same way. Before a generic medicine is licensed, it must show that it is absorbed and acts in the same way as the original (bioequivalence). Generic sildenafil became available in the UK in 2013, after the Viagra patent expired, and is usually lower in price. If you are considering treatment, sildenafil (generic Viagra) offers the same clinical effect as the branded version. If you would prefer the branded medicine, see our Viagra tablets page.
| Branded Viagra | Generic Sildenafil | |
|---|---|---|
| Active ingredient | Sildenafil | Sildenafil |
| Doses | 25mg, 50mg, 100mg | 25mg, 50mg, 100mg |
| How it works | PDE5 inhibitor | PDE5 inhibitor |
| Duration | About 4 hours | About 4 hours |
| Typical price | Higher | Lower |
Do You Need a Prescription for Viagra?
Since 2017, Viagra Connect 50mg has been licensed as a pharmacy medicine. Men aged 18 and over can buy it from a UK pharmacy without a prescription, after a consultation with a pharmacist. All other sildenafil — including Viagra 25mg, 50mg and 100mg tablets and all generic sildenafil — is prescription-only. Access Doctor provides this online: our GPhC-registered pharmacist independent prescribers assess your health history, screen for medicines and conditions that rule sildenafil out, and issue a prescription for the most appropriate dose if it is suitable for you.
How to Take Viagra Correctly
1
Starting dose
50mg is the recommended starting dose for most men. Your prescriber may increase to 100mg if 50mg is insufficient, or decrease to 25mg if side effects occur.
2
Timing
Take Viagra about 1 hour before sexual activity. It can start working from about 30 minutes, but allow an hour for reliable onset.
3
Food
Take on an empty stomach or after a light meal. A large, high-fat meal can delay absorption and reduce peak effectiveness.
4
Frequency
Never take more than one tablet in a 24-hour period.
5
Alcohol
Avoid large amounts of alcohol — it can impair sexual function and worsen dizziness.
6
Grapefruit
Avoid grapefruit juice — it inhibits the enzyme that metabolises sildenafil, increasing exposure and side effect risk.
Who Should Not Take Viagra?
- Men taking nitrate medicines for chest pain (such as GTN spray, tablets or patches, or isosorbide mononitrate or dinitrate), nicorandil, or recreational “poppers” — together they can cause a severe, potentially fatal drop in blood pressure
- Men taking riociguat (for high blood pressure in the lungs)
- Men who have been advised that sexual activity is not safe for their heart, or who have recently had a stroke or heart attack
- Men with low blood pressure (below 90/50 mmHg) or severe liver problems
- Men with an inherited eye condition such as retinitis pigmentosa, or who have ever lost sight in one eye because of reduced blood flow to the optic nerve (NAION)
- Men with a known allergy to sildenafil
Tell your prescriber first if you have kidney problems or mild to moderate liver problems, take an alpha-blocker or medicines such as ritonavir, clarithromycin or ketoconazole, or have Peyronie’s disease, sickle cell anaemia, leukaemia, multiple myeloma, a bleeding disorder or an active stomach ulcer. For the full list, see our sildenafil guide.
Emergency symptoms: An erection lasting more than 4 hours (priapism) needs emergency care — go to A&E or call 999, as it can permanently damage the penis. If you have a sudden decrease or loss of vision or hearing, stop taking Viagra and get medical help straight away. Call 999 for chest pain during or after sex.
UK safety and prescribing information
- Legal status: Viagra Connect 50mg is a pharmacy (P) medicine that men aged 18 and over can buy after a consultation with a pharmacist. Viagra 25mg, 50mg and 100mg tablets and all generic sildenafil are prescription-only medicines (POM). 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.
- Maximum dose: no more than one tablet in 24 hours. The highest strength is 100mg.
- Interactions: never take sildenafil with nitrates (including GTN spray), nicorandil, “poppers” or riociguat. Tell your prescriber about alpha-blockers, other blood pressure medicines, HIV medicines such as ritonavir, and antifungals or antibiotics such as ketoconazole or clarithromycin.
- Driving: sildenafil can cause dizziness and changes in 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 if it is suitable for you. Branded Viagra is only prescribed on the NHS in limited circumstances. In Wales, NHS prescribing of ED medicines, including generic sildenafil, 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.
Frequently Asked Questions
What is the difference between Viagra and sildenafil?
There is no meaningful difference in how they work. Viagra is the original brand of sildenafil. Generic sildenafil became available in the UK in 2013, after the patent expired. Both contain the same active ingredient at the same strengths and have the same effects and side effects. Generic sildenafil is usually lower in price.
How does Viagra (sildenafil) work?
Sildenafil is a PDE5 inhibitor. It blocks the PDE5 enzyme in the smooth muscle of penile blood vessels, allowing them to relax when you are sexually aroused. This lets more blood flow into the erectile tissue. Sexual stimulation is always needed.
Do I need a prescription for Viagra in the UK?
For most sildenafil products, yes. Viagra Connect 50mg is a pharmacy medicine that men aged 18 and over can buy after a consultation with a pharmacist. Viagra 25mg, 50mg and 100mg tablets and all generic sildenafil need a prescription. Access Doctor provides prescriptions after an online consultation.
How long does Viagra take to work and how long does it last?
Take Viagra about 1 hour before sex. It can start working from about 30 minutes and usually works for about 4 hours. A heavy or high-fat meal can delay it.
What are the side effects of Viagra?
The most common side effect is headache. Others include flushing, indigestion, a blocked nose, dizziness and changes in vision, such as a blue tinge. Get emergency help if an erection lasts more than 4 hours, and stop taking it and get medical help straight away if you have sudden loss of vision or hearing.
Is Viagra safe to use long-term?
Sildenafil has been used since 1998 and is generally safe to keep taking at the prescribed dose for men it suits. Your prescriber should review your treatment regularly, especially if your health or your other medicines change.
References
- National Institute for Health and Care Excellence (NICE). Clinical Knowledge Summaries: Erectile dysfunction. cks.nice.org.uk
- NHS. Erectile dysfunction (impotence). nhs.uk
- NHS. Sildenafil (Viagra). nhs.uk
- Electronic Medicines Compendium. Viagra and generic sildenafil film-coated tablets: Summaries of Product Characteristics. medicines.org.uk
- 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. Viagra and generic sildenafil tablets are prescription-only medicines (Viagra Connect 50mg is a pharmacy medicine), and they need a clinical or pharmacist assessment before they 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.