What Is Viagra? Your Guide to Sildenafil
Viagra is the world’s most recognised erectile dysfunction medication. Since its introduction in 1998, it has helped millions of men achieve and maintain erections when other approaches have failed. This guide explains what Viagra is, how it works, how it differs from generic sildenafil, and what to expect when you take it. For dosage, side effects, and contraindications in full, see our clinical reference guide: Sildenafil Dosage & Side Effects. For information on getting sildenafil in the UK, see: Sildenafil UK: How to Get It Safely Online.
Medical disclaimer: This article is for informational purposes only. ED treatments are prescription-only medicines. Always complete a clinical consultation before use. Our prescribers are GPhC-registered pharmacist independent prescribers.
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What Is Viagra?
Viagra is a brand name for sildenafil citrate, a medicine developed by Pfizer and first licensed for erectile dysfunction in 1998. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors, which work by increasing blood flow to the penis when a man is sexually stimulated.
Viagra is manufactured as film-coated tablets in three doses: 25mg, 50mg, and 100mg. Since Pfizer’s patent expired in 2012, generic sildenafil has become available in the UK at significantly lower cost. Generic sildenafil contains the same active ingredient at the same doses and must meet the same MHRA bioequivalence standards as the branded product — the clinical effect is identical.
Note
Sildenafil is also used under the brand name Revatio — at a much lower dose (20mg three times daily) — to treat pulmonary arterial hypertension. This is a completely separate clinical indication from ED treatment and is managed by specialist physicians.
How Does Viagra Work?
When a man is sexually stimulated, nerves in the penis release nitric oxide, which triggers the production of a signalling molecule called cyclic GMP (cGMP). cGMP causes the smooth muscle lining the penile arteries to relax, allowing blood to rush in and fill the erectile chambers of the penis, producing an erection.
PDE5 is the enzyme that normally breaks cGMP down — effectively ending the erection response. Sildenafil blocks PDE5, so cGMP is not broken down as quickly. When you are sexually stimulated, more blood flows in, and achieving and maintaining an erection becomes significantly easier.
Key point: Sildenafil does not cause an automatic erection. Sexual stimulation is still required. The drug enhances the body’s natural erectile response — it does not bypass it.
Viagra vs Generic Sildenafil — Is There a Difference?
No pharmacological difference. Branded Viagra and generic sildenafil contain the same active ingredient (sildenafil citrate) at the same doses, work through the same mechanism, produce the same clinical effect, and have the same side effect profile.
| Branded Viagra | Generic Sildenafil | |
|---|---|---|
| Active ingredient | Sildenafil citrate | Sildenafil citrate |
| Doses | 25mg, 50mg, 100mg | 25mg, 50mg, 100mg |
| Mechanism | PDE5 inhibitor | PDE5 inhibitor |
| Duration | 4–6 hours | 4–6 hours |
| MHRA bioequivalent? | N/A (original) | Yes — required for licensing |
| Typical cost | Higher | Lower |
The only differences are the inactive excipients (fillers, coatings) and the price. Clinically, they are interchangeable.
What Should I Expect When I Take It?
- Sildenafil is typically taken 30–60 minutes before sexual activity
- Effects last 4–6 hours — this is the window in which you are more responsive to stimulation, not a continuous erection
- Take on an empty stomach or after a light meal — a large, high-fat meal delays absorption and can reduce effectiveness
- Avoid grapefruit juice — it interferes with the enzyme that metabolises sildenafil
- Avoid alcohol in large quantities — alcohol impairs erectile function and amplifies the blood-pressure-lowering effect
The most common side effects are headache, facial flushing, nasal congestion, and mild indigestion. These are predictable from the drug’s vasodilating mechanism and typically resolve within a few hours. For a full side effect profile, including serious rare reactions, see: Sildenafil Dosage & Side Effects.
Who Should Not Take Viagra?
Sildenafil is contraindicated in men taking nitrate medicines of any kind — including GTN spray, isosorbide mononitrate tablets, and recreational “poppers” (amyl nitrite). The combination causes a severe and potentially fatal drop in blood pressure. This is an absolute contraindication.
A full list of contraindications and drug interactions is in our clinical reference guide: Sildenafil Dosage & Side Effects.
How Sildenafil Compares to Other ED Medications
Sildenafil is one of four MHRA-approved oral PDE5 inhibitors for ED. The main alternative is tadalafil (Cialis), which lasts up to 36 hours and is not affected by food — making it more suitable for men who want greater spontaneity. For a detailed head-to-head, see: Tadalafil vs Sildenafil.
For a comparison of all four MHRA-approved ED medications, see: Viagra vs Cialis vs Levitra vs Stendra.
How to Get Viagra (Sildenafil) in the UK
Since 2017, sildenafil 50mg (Viagra Connect) has been available without a full prescription from registered UK pharmacies for men aged 18–64, following a pharmacist assessment. All other doses and branded formulations require a prescriber consultation.
For a full guide to prescribing routes, NHS vs private access, and how to get sildenafil safely online, see: Sildenafil UK: How to Get It Safely Online.
For guidance on taking sildenafil for the first time, including dose selection and what to expect, see: Taking Viagra for the First Time.
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Frequently Asked Questions
What is the difference between Viagra and sildenafil?
There is no pharmacological difference. Viagra is the original branded product manufactured by Pfizer. Generic sildenafil — available since the patent expired in 2012 — contains the same active ingredient at the same doses and must demonstrate MHRA bioequivalence before it can be sold in the UK. Both work identically. Generic sildenafil is typically significantly cheaper.
How does Viagra work?
Sildenafil blocks the PDE5 enzyme in the smooth muscle of penile blood vessels. When you are sexually stimulated, this allows more cGMP to accumulate, causing blood vessels to relax and widen. More blood flows into the erectile tissue of the penis, making it significantly easier to achieve and maintain an erection. Sexual stimulation is still required — Viagra does not create an erection automatically.
Does Viagra work for everyone?
Approximately 82% of men who take sildenafil correctly report erections suitable for sexual intercourse. For the remaining men, adjusting the dose, timing, or food intake often improves results. If 100mg consistently fails, a prescriber may recommend switching to tadalafil or another approach.
Is it safe to take Viagra long-term?
Yes. Sildenafil has an excellent long-term safety record with no evidence of harmful cumulative effects at prescribed doses. Regular prescriber review is recommended to ensure the dose remains appropriate and to monitor for any changes in health that might affect suitability.
How is Viagra different from tadalafil (Cialis)?
The key practical difference is duration. Sildenafil lasts 4–6 hours; tadalafil lasts up to 36 hours. Tadalafil is also not affected by food, while sildenafil absorption can be delayed by a fatty meal. For a detailed comparison, see: Tadalafil vs Sildenafil.
What if Viagra doesn’t work the first time?
This is common. Timing, food, alcohol, and anxiety all affect first-use outcomes. For detailed guidance on what to do, see our guide: Taking Viagra for the First Time.
References
- NICE. Erectile dysfunction — management. CKS 2023. cks.nice.org.uk
- NHS. Erectile dysfunction. nhs.uk
- GPhC. Standards for registered pharmacies. pharmacyregulation.org
- Goldstein I et al. Oral sildenafil in the treatment of erectile dysfunction. NEJM 1998. pubmed.ncbi.nlm.nih.gov/9560547
- MHRA. Viagra Connect — product information. medicines.org.uk/emc