Viagra: What it is, how it works, and everything you need to know 

DoktorABC editorial team

Accurate, up-to-date health information written by our editorial team and reviewed by UK-registered prescribers.

What is Viagra?

Viagra is the original brand name for sildenafil citrate, developed by Pfizer and first approved in the UK in 1998 for the treatment of erectile dysfunction in adult men. It remains one of the most prescribed ED medicines in the world, though generic sildenafil has been available since 2013 and is now the more widely prescribed form in the UK.

Viagra is a prescription-only medicine in the UK at all doses, with the exception of Viagra Connect 50 mg, which has been available over the counter from pharmacies since 2018 following a pharmacist suitability assessment.

What does Viagra do? How it works

Viagra works by inhibiting an enzyme called PDE5, which is found predominantly in the smooth muscle tissue of the penis. During sexual arousal, the body releases nitric oxide into the penile tissue. Nitric oxide triggers the production of a chemical called cyclic GMP (cGMP), which relaxes the smooth muscle in the walls of the blood vessels in the penis. This relaxation allows blood to flow in, producing an erection.

PDE5 is the enzyme responsible for breaking down cGMP, limiting the erection response. Sildenafil is a potent and selective inhibitor of PDE5 in the corpus cavernosum, where PDE5 is responsible for the degradation of cGMP. When the nitric oxide pathway is activated by sexual stimulation, inhibition of PDE5 results in increased cGMP levels, relaxing smooth muscle and increasing blood flow to support an erection. 

In simple terms: Viagra blocks the enzyme that limits the erection response, allowing the body's natural mechanism to work more effectively.

What Viagra can and cannot do 

This is one of the most important and commonly misunderstood aspects of how Viagra works.

Viagra does not cause an erection automatically. It does not increase sexual desire or libido. It does not produce any effect in the absence of arousal. Sildenafil has no direct relaxant effect on the corpus cavernosum. Sexual stimulation is required for sildenafil to produce its intended pharmacological effects. 

What Viagra does is support the physiological process that enables an erection when arousal is already present. For men with ED, where this process is impaired, Viagra helps it work as it should.

How long does Viagra take to work?

For most men, Viagra begins to work within 30 to 60 minutes. It is typically recommended to take it approximately one hour before sexual activity.

Several factors influence how quickly it takes effect:

• Food: Taking Viagra after a large, high-fat meal significantly delays absorption and can reduce peak blood levels. For the most predictable onset, take it on an empty stomach or after a light meal.
• Alcohol: Alcohol can delay onset and worsen side effects such as dizziness and flushing. It also contributes independently to ED, so heavy drinking before taking Viagra is counterproductive.
• Age: Older men clear sildenafil more slowly, meaning onset may be slightly delayed but the drug may also remain active for longer.
• Individual metabolism: Some men notice an effect within 20 to 30 minutes; others may need up to 90 minutes, particularly after food.

How long does Viagra last?

Viagra typically remains active for four to six hours. This is the window during which an erection can be achieved in response to sexual stimulation, not the duration of an erection itself.

Sildenafil has a half-life of three to five hours, meaning blood levels reduce by half roughly every four hours. By six to eight hours after taking it, the effects have worn off for most men.

Viagra and sexual performance: What the evidence shows 

This is a very commonly searched question, and it deserves a direct and honest answer.

No. Viagra does not delay ejaculation or increase stamina in the sense of prolonging sexual activity. It is licensed specifically to help men achieve and maintain an erection, not to address premature ejaculation or ejaculatory control.

That said, for men whose sexual confidence has been significantly affected by ED, the psychological effect of knowing they can achieve and maintain an erection may indirectly improve their overall experience. But this is a confidence effect, not a pharmacological one.

If premature ejaculation is a concern, it is a separate condition with its own treatment options, which a prescriber can discuss with you.

Does Viagra alter physical anatomy?

No. Viagra does not increase penis size in any way. It increases blood flow to the penis during arousal, supporting the natural erectile response. It does not alter anatomy, tissue structure, or resting or erect size.

This claim circulates widely online, sometimes associated with counterfeit or unregulated products making false promises. No licensed medicine increases penis size. Any product marketed as doing so is not credible and may be unsafe.

Can women take Viagra? 

Viagra is not licensed for erectile dysfunction in women, as such a condition doesn’t exist. However, sildenafil does have a licensed use that applies equally to men and women, namely, Pulmonary Arterial Hypertension (PAH).

PAH is a serious condition involving abnormally high blood pressure in the arteries of the lungs, which forces the heart to work harder and can significantly reduce exercise capacity and quality of life. Sildenafil works for PAH through the same mechanism as it does for ED: by inhibiting PDE5 and increasing cGMP levels, it relaxes the smooth muscle in the pulmonary arteries, reducing pressure and improving blood flow through the lungs.

For this indication, sildenafil is typically prescribed under the brand name Revatio rather than Viagra, and is managed by a specialist rather than a GP or general prescriber. It is licensed for both adults and children aged one year and older under specialist supervision.

If you are a woman who has been referred to a specialist for pulmonary arterial hypertension, your clinical team will advise on whether sildenafil is appropriate for your circumstances.

Viagra and blood pressure

Viagra's interaction with blood pressure is one of the most clinically important considerations before starting treatment, and one that is frequently misunderstood. This section covers both high and low blood pressure, as the risks and considerations differ significantly between the two.

Viagra and high blood pressure

Sildenafil has vasodilator properties, resulting in mild and transient decreases in blood pressure. Prior to prescribing sildenafil, physicians should carefully consider whether their patients with certain underlying conditions could be adversely affected by such vasodilatory effects, especially in combination with sexual activity. 

For most men with well-controlled high blood pressure, Viagra can be used safely under prescriber supervision. The modest blood pressure reduction it causes is generally well tolerated and does not typically cause problems in men whose hypertension is managed effectively with medication.

However, two specific situations require particular care.

Nitrate medicines: Nitrates should not be administered within 24 hours of sildenafil use. The combination can cause a severe and potentially life-threatening drop in blood pressure. This applies to all nitrate medicines including glyceryl trinitrate (GTN) spray, isosorbide mononitrate, and isosorbide dinitrate. If you take any nitrate medicine for chest pain or heart disease, Viagra is contraindicated.

Antihypertensive medicines: Taking Viagra alongside blood pressure-lowering medicines can produce an additive effect on blood pressure reduction. This does not mean the combination is automatically unsafe, but it does require careful prescriber assessment before starting.

Viagra and low blood pressure

Viagra is contraindicated in men with hypotension, defined as a resting blood pressure below 90/50 mmHg. Because Viagra causes a further reduction in blood pressure through vasodilation, taking it when blood pressure is already low can cause a significant drop, leading to dizziness, fainting, or in serious cases, cardiovascular complications.

If you have low blood pressure, or are unsure of your current blood pressure readings, speak to your GP before taking Viagra. A cardiovascular assessment is an essential part of the prescribing process and will identify whether treatment is appropriate for your circumstances.

Can you take Viagra with blood thinners?

Viagra does not have a direct interaction with most anticoagulant medicines such as warfarin or direct oral anticoagulants (DOACs) such as apixaban or rivaroxaban. However, this does not mean the combination is without risk.

Men taking anticoagulants are often doing so because of underlying cardiovascular conditions, which may themselves affect the safety of Viagra. A prescriber will need to assess the full clinical picture, including your cardiovascular status, before recommending Viagra in this context.

Viagra doses

Viagra is available in three dose strengths, 25 mg, 50 mg, and 100 mg, each of which is appropriate for different circumstances. Dose choice is a clinical decision, not a ranking of strength or quality.

The maximum dose is 100 mg, taken no more than once in any 24-hour period. Doses above 100 mg are not licensed and carry significantly increased side effect risk without any improvement in efficacy.

For a full guide to dosing, including when to adjust and what to do if a dose does not work, see our Viagra dosage guide.

How to use Viagra for best results

Getting the most from Viagra involves a few straightforward practical steps.

• Take it at the right time: Approximately one hour before sexual activity gives the most reliable onset for most men.
• Avoid heavy meals beforehand: A large, high-fat meal delays absorption significantly. A light meal or an empty stomach gives the most predictable effect.
• Limit alcohol: Modest alcohol consumption is unlikely to prevent Viagra from working, but heavy drinking worsens side effects and contributes independently to ED.
• Allow time and avoid pressure: Viagra supports arousal; it works best when you are relaxed and genuinely aroused rather than anxious about whether it will work.
• Be consistent: If the first attempt does not go as expected, review the practical factors above before concluding it is not working. Allow at least two to three attempts under good conditions before discussing a dose review with your prescriber.

Viagra works best as part of an overall approach to health, not as a substitute for it.

Final thoughts

Viagra remains one of the most effective and well-studied treatments for erectile dysfunction available in the UK. For the majority of men who are appropriately assessed and use it correctly, it works reliably, is well tolerated, and has a meaningful positive effect on confidence and quality of life.

The most important things to understand are what Viagra can do and what it cannot: it supports the body's natural response to arousal, it does not create arousal, it does not increase size or stamina, and it is not safe for everyone. A medical assessment before starting is not a formality; it is what makes treatment both safe and effective.

DoktorABC connects UK patients with licensed prescribers and GPhC-registered pharmacies for private ED treatment, with a full medical assessment completed online.

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The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board