How Does Viagra Work? Sildenafil Explained, and Is It the Same as Viagra?

Short answer: Sildenafil, the active ingredient in Viagra, works by increasing blood flow to the penis when you are sexually aroused. Arousal releases nitric oxide in the penis, which raises a chemical messenger called cGMP; cGMP relaxes the blood vessels so blood can flow in and an erection forms. An enzyme called PDE5 breaks cGMP down, and sildenafil slows that breakdown, so cGMP builds up while you are aroused (Viagra product information, SmPC section 5.1). These tablets only work with sexual stimulation. Take it about 1 hour before sex (anywhere from 30 minutes to 2 hours before). It takes about an hour to work. Generic sildenafil tablets contain the same active ingredient at the same strength and are licensed in the UK.

This guide follows the UK product information for Viagra and a generic sildenafil, the NHS and the British Association of Urological Surgeons (BAUS), as of October 2026. We supply sildenafil 25mg, 50mg and 100mg tablets after a consultation reviewed by a GPhC-registered pharmacist.

How does Viagra work, and what does it do?

Viagra helps men with erectile dysfunction get and keep an erection firm enough for sex. It works by blocking PDE5, an enzyme in the penis that switches an erection off, so the body's own erection signal lasts longer while you are sexually aroused. BAUS describes PDE5 inhibitors such as sildenafil as increasing blood flow into the penis by blocking that enzyme. The product information for Viagra (SmPC section 5.1) sets out the steps:

1. Arousal. When you are sexually stimulated, nitric oxide is released in the erectile tissue of the penis (the corpus cavernosum), the tissue that fills with blood during an erection.
2. A chemical messenger. Nitric oxide switches on an enzyme called guanylate cyclase, which raises the level of a messenger called cGMP (cyclic guanosine monophosphate).
3. Blood flows in. cGMP relaxes the smooth muscle of the erectile tissue, so blood can flow in and the penis becomes erect.
4. The switch-off is slowed. PDE5 breaks cGMP down. Sildenafil blocks PDE5, so cGMP builds up while you are aroused, and an erection is easier to get and keep.

Sildenafil does not relax the erectile tissue by itself: its product information says it has no direct relaxing effect on that tissue, but strongly boosts the relaxing effect of the nitric oxide released during sexual stimulation. That is why sexual stimulation is needed for it to work (SmPC section 5.1).

BAUS lists sildenafil among the commonly used PDE5 inhibitors, with tadalafil (Cialis), vardenafil (Levitra) and avanafil (Spedra). They all work this way; how they differ is in sildenafil vs tadalafil.

Does Viagra work without arousal?

No: Viagra does not cause an erection on its own. These tablets only work with sexual stimulation. Viagra's product information says sexual stimulation is required for it to be effective (SmPC section 4.1), and its leaflet says it helps you get an erection when you are sexually stimulated, and not otherwise. Without arousal, the nitric oxide signal that sildenafil boosts is not switched on, so taking a tablet and waiting is not enough.

What happens when you take Viagra?

You swallow the tablet whole with a glass of water, and once it has been absorbed it helps you get and keep an erection when you are aroused. Take it about 1 hour before sex (anywhere from 30 minutes to 2 hours before). It takes about an hour to work. A heavy or high-fat meal can delay how quickly it works. The leaflet adds that the time it takes varies from person to person.

Sildenafil is short acting. Never take more than one tablet in 24 hours. An erection that lasts longer than 4 hours is a medical emergency (see the safety box below). For how long it lasts and how often you can take it, see how long does sildenafil last; for the strengths and how the dose is chosen, see sildenafil dosage.

Very common / common side effects include headache, dizziness, flushing, nasal congestion, dyspepsia (indigestion), nausea and vision disturbances. Less common side effects are listed in the leaflet in your pack. Please read it before your first dose. Sildenafil also widens blood vessels elsewhere in the body: its product information describes vasodilator properties that cause mild, short-lived falls in blood pressure (SmPC section 4.4).

It acts mainly on PDE5 but also, to a lesser degree, on PDE6, a related enzyme in the retina at the back of the eye. Its product information says a mild, short-lived change in telling blue from green, seen in tests in some men an hour after a 100mg dose and gone by 2 hours, is thought to be linked to this (SmPC section 5.1). Every side effect, and the ones that need help straight away, are in Viagra side effects.

Does Viagra work, and how effective is it?

Sildenafil, the medicine in Viagra, can help: in a 12-week trial reported in the New England Journal of Medicine in 1998 (Goldstein and colleagues), 69% of attempts at sex were successful for men with erectile dysfunction taking sildenafil, against 22% for men taking a dummy tablet (placebo). The trial included 329 men whose erectile dysfunction had physical, psychological or mixed causes. They took sildenafil or placebo when needed, with the dose raised up to 100mg depending on how well it worked and how well it was tolerated, and the figures count all attempts at intercourse in the last 4 weeks of treatment. Individual results vary.

A trial result is not a promise for any one man. BAUS says tablets like it may not work in severe erectile dysfunction, or where the body's natural erection mechanism has been permanently damaged. Try a tablet 4 to 8 times before deciding it does not work, then tell us.

Things that can get in its way include not being aroused, taking it too close to sex, a heavy meal and drinking alcohol. Drinking less alcohol can help your erections. If it is not working for you, see Viagra not working; for drink, Viagra and alcohol; and for other treatments, alternatives to Viagra.

Viagra does not treat the cause of erection problems, which can be linked to high blood pressure, diabetes, high cholesterol or heart disease (Viagra Connect product information, SmPC section 4.4). More on the causes in what is erectile dysfunction.

Does Viagra make you bigger or hornier?

No: Viagra helps a man with erectile dysfunction get and keep an erection firm enough for sex, but it does not make the penis bigger or increase sex drive, and it does not delay ejaculation. Its licence is for erectile dysfunction (SmPC section 4.1), and BAUS says these tablets have no effect on libido (sexual desire), ejaculation or fertility.

• Bigger or harder. BAUS describes tablets like Viagra as boosting the body's natural way of getting an erection, so that erections are easier to get and keep. For a man with erectile dysfunction, that means help getting an erection firm enough for sex; nothing in Viagra's UK product information says it makes the penis bigger.
• More desire. No. Sildenafil acts on erections in the penis itself (SmPC section 5.1), so you still need to feel desire and be aroused for it to help.
• Lasting longer. It is not a treatment for premature ejaculation. It can help a man with erectile dysfunction keep an erection long enough for sex, and its leaflet says to tell your doctor if your erection does not last long enough to finish (section 3). More in does Viagra make you last longer.
• Without erection problems. The leaflet says you should not take Viagra if you do not have erectile dysfunction (section 2), and BAUS advises against these tablets for men without it, because they can have serious side effects.
• Taking more. More than the maximum dose does not work better: the leaflet says doses above 100mg do not increase the efficacy, and in studies, doses above the 100mg maximum caused more side effects (SmPC section 4.9).

Is sildenafil the same as Viagra?

Sildenafil is the active ingredient in Viagra: Viagra is the brand name of the sildenafil tablets whose UK licence is held by Viatris (SmPC section 7), so sildenafil is Viagra's generic name. Generic sildenafil tablets contain the same active ingredient at the same strength and are licensed in the UK. The pack shows the medicine's name and the name of the company that makes it, instead of the brand name. What is the same and what differs, from the UK product information:

Sources: Viagra's product information and leaflet (emc 7980) and, as an example of a generic, the product information for sildenafil from Amarox Limited (emc 13550). Tablets made by different companies can look different, even when the medicine and the strength are the same, so check the leaflet in your pack. Already a Solira patient? If a new pack looks different and you are not sure it is right, message your pharmacist through your account before you take it.

Viagra Connect contains sildenafil 50mg, the same active ingredient as prescription Viagra. It is a pharmacy medicine, supplied after a pharmacist consultation with no prescription needed, and we supply it to men aged 18 to 64. Erection problems can be an early sign of heart disease, diabetes or high blood pressure. If you use Viagra Connect, book a check-up with your GP as soon as you can, within 6 months of starting it. Less common side effects are listed in the leaflet in your pack. Please read it before your first dose. What else differs is in Viagra Connect vs prescription sildenafil.

What is in Viagra?

Each Viagra tablet contains sildenafil, as sildenafil citrate, at 25mg, 50mg or 100mg, with inactive ingredients that make up the tablet and its blue coating (leaflet section 6):

• The tablet: microcrystalline cellulose, calcium hydrogen phosphate (anhydrous), croscarmellose sodium and magnesium stearate.
• The coating: hypromellose, titanium dioxide (E171), lactose monohydrate, triacetin and indigo carmine aluminium lake (E132).

A 50mg tablet contains 1.7mg of lactose (SmPC section 2). Because the coating contains lactose, the product information says Viagra should not be given to men with rare inherited problems of galactose intolerance, total lactase deficiency or glucose-galactose malabsorption (SmPC section 4.4). Each tablet has less than 1 mmol (23mg) of sodium, so it is essentially sodium-free.

Generic tablets contain the same active ingredient but can contain different inactive ingredients: the list for the Amarox generic, for example, has no lactose (SmPC section 6.1), so check the leaflet in your pack. Our consultation asks about this for every erectile dysfunction tablet, and we do not supply treatment if this applies to you:

• A rare inherited condition: galactose intolerance, total lactase deficiency or glucose-galactose malabsorption

What was Viagra originally made for?

Sildenafil, the medicine in Viagra, was originally developed as a possible treatment for angina, a kind of chest pain, before it became a treatment for erectile dysfunction. A 2006 review of its history in Nature Reviews Drug Discovery (Ghofrani and colleagues) describes how it came out of a Pfizer research programme in the 1980s that was looking for angina medicines. In studies in healthy volunteers in 1992, some reported erections as a side effect; by 1993 it was looking less promising for angina, and studies in erectile dysfunction followed. Viagra's UK product information dates its authorisation to 14 September 1998 (SmPC section 9).

Sildenafil is still used for another condition: as Revatio, at 20mg three times a day for adults, it is licensed for pulmonary arterial hypertension, high blood pressure in the lungs (Revatio SmPC sections 4.1 and 4.2; NHS). Viagra's leaflet says not to use Viagra with a treatment for it that contains sildenafil, or with any other PDE5 inhibitor (section 2), and our consultation does not supply erectile dysfunction treatment if this applies to you:

• Currently taking another PDE5 inhibitor (sildenafil, tadalafil, vardenafil or avanafil, including for lung blood pressure or prostate symptoms) or any other erectile dysfunction treatment

Who should not take Viagra?

Viagra is not suitable for men who take nitrates in any form (poppers included), riociguat or vericiguat, or certain HIV, antifungal or antibiotic medicines, or for men with severe heart disease. Not suitable if you take:

• Nitrates in any form, including nicorandil or amyl nitrite (poppers)
• Riociguat (Adempas) or vericiguat (Verquvo)
• An HIV protease inhibitor (atazanavir, darunavir, ritonavir, fosamprenavir, lopinavir, saquinavir or tipranavir)
• Ketoconazole, itraconazole, voriconazole or clarithromycin
• Or have severe cardiovascular disease

The reason for the nitrate and riociguat lines comes from how sildenafil works. Nitrates and poppers (amyl nitrite, a nitric oxide donor) act through the same nitric oxide and cGMP pathway, so sildenafil strengthens their effect on blood pressure, and the fall can be dangerous (SmPC section 4.3; leaflet section 2). Riociguat stimulates guanylate cyclase, the next step in that pathway, and the product information warns that it may cause low blood pressure with symptoms when taken with sildenafil (SmPC section 4.3). Our consultation treats vericiguat, and any other medicine that stimulates guanylate cyclase, the same way.

Our consultation also does not supply sildenafil if any of these apply, among other reasons:

• Stroke or heart attack in the last 6 months
• Unstable angina, or angina during intercourse
• A heart condition for which a doctor has told you to avoid sex or strenuous activity
• Very low blood pressure, below 90/50
• Loss of sight in one eye from NAION (a sudden loss of blood supply to the optic nerve), whether or not it happened while taking an erectile dysfunction medicine
• Retinitis pigmentosa or another inherited condition of the retina

The consultation asks about your heart, blood pressure, eyes, liver and kidneys and every other medicine you take, and a GPhC-registered pharmacist reviews your answers before anything is sent. If they cannot approve your order, we refund you in full. The side effects and the longer list are in Viagra side effects, and who should not take it. Do not use unlicensed herbal remedies for erections, and do not stop any prescribed medicine unless your doctor tells you to. See all our erectile dysfunction treatments.