How does Viagra work?

Viagra is one of the most recognised medicine names in the world, but the way it actually works inside the body is often misunderstood. It does not create desire, it does not produce an erection on its own, and it is not a stimulant. What it does is something more specific and more mechanical: it amplifies the natural chemical signal that the body already uses to produce an erection during sexual arousal. The rest of this article explains that mechanism in plain English, and sets out the practical points — onset, duration, side effects and safety — that a UK clinician will discuss before prescribing.

The physiology of an erection — in plain English

An erection is, at its core, a blood-flow event. When a man becomes sexually aroused, nerve signals from the brain and from local stimulation cause the release of nitric oxide inside the spongy tissue of the penis, called the corpus cavernosum. Nitric oxide is a tiny gas molecule that diffuses into the smooth-muscle cells lining the small arteries there. Once inside, it triggers the production of a second messenger called cyclic GMP, or cGMP for short.

cGMP is the molecule that does the real work. It tells the smooth muscle in the arterial walls to relax. When those vessels relax, more blood flows into the spongy chambers. The chambers expand, press against the surrounding veins, and slow the outflow of blood. The combination — increased inflow, restricted outflow — is what produces and maintains an erection.

That entire process depends on cGMP staying around long enough to keep the muscles relaxed. And that is where the enzyme PDE5 comes in. PDE5 — phosphodiesterase type 5 — is the body's "off switch" for cGMP. It breaks cGMP down. In healthy arousal, cGMP production outpaces PDE5 breakdown and the erection holds. In erectile dysfunction, that balance can be tipped: less nitric oxide, less cGMP, and PDE5 clears the signal too quickly.

What sildenafil actually does at a cellular level

Sildenafil — the active ingredient in Viagra — is a PDE5 inhibitor. It binds to PDE5 and slows it down. With PDE5 partly switched off, any cGMP the body produces stays around longer, the smooth muscle stays relaxed for longer, and the blood-flow response to arousal is amplified and prolonged.

This is a crucial point that gets missed in pop-culture descriptions of the medicine: sildenafil does not produce cGMP. It does not produce nitric oxide. It does not generate desire or initiate arousal. It simply prevents the body's chemical signal from being broken down too quickly once that signal already exists. Without arousal, there is nothing for sildenafil to amplify — the nitric oxide is never released and the erection does not occur. This is why Viagra is described as a treatment for erectile dysfunction rather than a libido medication.

The same mechanism explains why other PDE5 inhibitors — Cialis (tadalafil), avanafil and vardenafil — produce broadly the same effect. They differ mainly in how quickly they reach peak concentration, how long they stay active, and how strongly they interact with food. The underlying biology is the same.

Why sexual arousal is still required

The reason sildenafil only works "in context" is worth restating because it shapes how the medicine is used in practice. The drug acts on the downstream end of the pathway — the breakdown of cGMP — not the upstream end. If there is no arousal, no nitric oxide is released, no cGMP is produced, and there is nothing for sildenafil to keep in circulation. The tablet quietly does nothing until arousal begins.

This has a practical implication some men find counter-intuitive: a tablet taken on a stressful or distracted evening may seem to "not work", when in fact the medicine is doing exactly what it can — waiting for the natural signal that did not arrive. Stress, fatigue, alcohol, and relationship factors all affect arousal independently of pharmacology, and they affect outcomes with PDE5 inhibitors too. A clinician will often raise these factors during a consultation because they shape what realistic expectations look like.

Onset, duration and what affects timing

Sildenafil is absorbed reasonably quickly, with peak plasma concentration usually reached around one hour after the tablet is taken on an empty stomach. The first effects can begin a little earlier — many men notice a response within 30 to 45 minutes — and the active window is typically around four to five hours. After that, plasma levels fall and the amplifying effect on cGMP gradually fades.

Two factors most often slow the onset:

• Food, especially high-fat food. A heavy meal eaten around the same time as the tablet delays absorption. Onset can be pushed back by 30 to 60 minutes. This is different from tadalafil, which is largely unaffected by food.
• Alcohol. Modest amounts have little impact on the pharmacology, but heavier alcohol intake blunts arousal and impairs the erectile response independently of any medicine.

Sildenafil is licensed in the UK in 25 mg, 50 mg and 100 mg doses. The dose is matched to the individual after clinical assessment — your clinician will advise based on your individual circumstances, including age, other medications and any cardiovascular history. The right starting dose for one patient may be different to another, and adjustments are made after seeing how the first dose performs.

Side effects and contraindications

Because PDE5 is present in blood vessels elsewhere in the body — not only in the penis — the most common side effects of sildenafil reflect mild vasodilation in other places. Headache, facial flushing and nasal congestion sit at the top of the list. Indigestion and a mild blue-tinged or blurred visual disturbance are also recognised. For most men these are short-lived and settle as the dose is eliminated.

The contraindications are more important to understand. The single most dangerous interaction is with nitrate medication. Nitrates — glyceryl trinitrate sprays or tablets for angina, isosorbide mononitrate, and the recreational "poppers" form (amyl nitrite) — also work by raising the same cGMP signal that sildenafil prolongs. Taken together, the result can be a sudden and dangerous drop in blood pressure. The combination is an absolute contraindication, regardless of timing.

Cardiovascular status is the other key area. Sexual activity itself increases heart rate and blood pressure, so before prescribing a PDE5 inhibitor a UK clinician will check whether your heart can comfortably handle that load. Recent heart attack, unstable angina, severe heart failure or very low blood pressure are reasons treatment may be deferred or not offered. The NHS medicines page for sildenafil covers the patient-facing safety summary.

Less common but worth knowing about: a prolonged erection lasting more than four hours (priapism) is a medical emergency and should be treated urgently, because it can damage the tissues of the penis. Sudden vision loss or sudden hearing loss after a dose are also rare but serious and warrant immediate review.

How treatment is reviewed in the UK

In a UK consultation — whether in person or via a regulated online pharmacy — a clinician will check the underlying cause of erectile difficulty as well as deciding on a treatment. Erectile dysfunction can be an early sign of cardiovascular disease, low testosterone, depression, or a side effect of other medications, and identifying the driver matters as much as the prescription. Treatment is reviewed periodically and adjusted: a different dose, a different PDE5 inhibitor, or a different approach altogether if the response is not what was hoped for. For UK patients who want to read further on related topics, our pieces on how long it takes for Cialis to work and on low testosterone and hair loss sit alongside this one.