What Is Viagra? Your Complete Guide to Sildenafil
Mechanism, Viagra vs generic sildenafil, doses, prescription rules, side effects and safe use.
Part of the Access Doctor Erectile Dysfunction guide.
Key fact: Viagra (sildenafil) has an approximately 82% success rate in clinical trials — making it one of the most effective first-line treatments for erectile dysfunction (Goldstein et al. 1998, PMID 9560547). NICE recommends it as first-line pharmacological treatment for ED.
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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. Branded Viagra and generic sildenafil are in complete bioequivalence — same active ingredient, same dose, same mechanism, same duration, same side effect profile. Generic sildenafil became available in the UK after Pfizer’s patent expired in 2012 and is typically available at a significantly lower price.
| Branded Viagra | Generic Sildenafil | |
|---|---|---|
| Active ingredient | Sildenafil | Sildenafil |
| Doses | 25mg, 50mg, 100mg | 25mg, 50mg, 100mg |
| How it works | PDE5 inhibitor | PDE5 inhibitor |
| Duration | 4–6 hours | 4–6 hours |
| Typical price | Higher | Lower |
Do You Need a Prescription for Viagra?
Since 2017, men can purchase sildenafil 50mg (Viagra Connect) without a prescription from a licensed UK pharmacy following a short pharmacist consultation. All other doses — 25mg and 100mg — require a prescriber consultation. Access Doctor provides this online: our GPhC-registered pharmacist independent prescribers assess your health history, screen for contraindications, and issue prescriptions for the most appropriate dose.
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 30–60 minutes before sexual activity. Allow at least 1 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 (e.g. glyceryl trinitrate, isosorbide mononitrate) — absolute contraindication; risk of severe, potentially fatal blood pressure drop
- Men with severe cardiovascular disease or who have been advised to avoid sexual activity for cardiac reasons
- Men with severe liver or kidney disease
- Men with a known allergy to sildenafil
- Men with hereditary degenerative retinal disorders
Emergency symptoms: An erection lasting more than 4 hours (priapism) requires immediate emergency care — it can permanently damage the penis. Sudden vision loss or sudden hearing loss after taking Viagra requires immediate medical attention. Call 999.
Frequently Asked Questions
What is the difference between Viagra and sildenafil?
There is no pharmacological difference. Viagra is the branded product originally patented by Pfizer; generic sildenafil became widely available after the UK patent expired in 2012. Both have the same effect, safety profile, and efficacy.
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 sexually stimulated. This enables increased blood flow into the erectile tissue. Sexual stimulation is always required.
Do I need a prescription for Viagra in the UK?
Since 2017, men can buy sildenafil 50mg (Viagra Connect) without a prescription from a UK pharmacy. All other doses (25mg, 100mg) require a prescriber consultation. Access Doctor provides this service online.
How long does Viagra take to work and how long does it last?
Viagra typically takes 30–60 minutes to begin working. Effects generally last 4–6 hours. Taking it on an empty stomach results in faster absorption.
What are the side effects of Viagra?
Common side effects include headache, flushing, indigestion, nasal congestion, and visual disturbances. Serious but rare effects include priapism and sudden vision or hearing loss — seek emergency care if these occur.
Is Viagra safe to use long-term?
Yes. Long-term use at prescribed doses has a well-established safety record. Regular prescriber review is recommended.
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.