What Does Sildenafil Do? How It Works and What It Treats

• Sildenafil helps increase blood flow to the penis by blocking an enzyme called PDE5.
• It is not a stimulant and it does not create desire, it extends a response the body already produces.
• Sildenafil is licensed for erectile dysfunction at 25mg, 50mg and 100mg, and separately for pulmonary arterial hypertension.
• Without sexual stimulation sildenafil does not produce an erection because the signalling pathway needed for an erection is not activated.

Sildenafil helps increase blood flow to the penis so that an erection can be achieved and maintained. It does this by blocking an enzyme called PDE5, which lets a natural signalling molecule stay active for longer.

That is the whole mechanism in one sentence. Everything else on this page follows from it, including the two things people most often get wrong: sildenafil is not a stimulant, and it does nothing on its own without stimulation.

Sildenafil is also not only an erectile dysfunction medicine. It holds a separate UK authorisation to treat pulmonary arterial hypertension (PAH), a serious lung condition, in adults and in children aged 1 to 17. That is a different product at a different dose.

This page covers what sildenafil does in the body step by step, what each licensed strength is used to treat, what it feels like, what to expect the first time, and where the limits of the evidence sit. Results vary between individuals.

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Key things to know

• Sildenafil belongs to a class of drugs called PDE5 inhibitors, which relax smooth muscle cells to widen blood vessels by protecting a signalling molecule.
• The active ingredient is sildenafil citrate, the salt form pressed into the tablet.
• Licensed erectile dysfunction strengths in the UK are 25mg, 50mg and 100mg.
• Sildenafil is also used to treat pulmonary arterial hypertension (PAH).
• It is not indicated for use in women.
• It is not a stimulant, a hormone, or a treatment for desire or drive.
• Sexual stimulation is required for sildenafil to produce an erection.
• MHRA documentation states that sildenafil normally takes 30 to 60 minutes to work, with a terminal half-life of 3 to 5 hours.
• It must never be combined with nitrates, nitric oxide donors such as amyl nitrate or butyl nitrate, or riociguat.
• Combining sildenafil with illegal drugs or street drugs can cause dangerous drug interactions.
• People with cardiovascular disease, liver or kidney problems, or other significant health conditions should discuss sildenafil with a healthcare professional before use.
• Taking too much sildenafil can cause serious side effects, including painful erections and sudden vision loss in one or both eyes.
• A regular dosing schedule is important for those using sildenafil to treat pulmonary arterial hypertension.
• Over the counter medicines and herbal remedies may interact with sildenafil, always inform your healthcare provider about other medications you take.

What does sildenafil do?

Sildenafil helps relax the smooth muscle in blood vessel walls, which widens those vessels and increases blood flow into the erectile tissue of the penis during sexual stimulation.

Its effects are not limited to the penis, which is why some people experience effects such as headache, flushing or nasal congestion.

Put another way, what does sildenafil do to the body is a narrower question than it sounds. The medicine acts on one enzyme, in smooth muscle, and everything you might notice traces back to that.

What sildenafil does not do

Being clear about the negatives is more useful than the positives here, because they are where the misunderstandings live.

• It does not create desire or interest.
• It does not act as a stimulant of any kind.
• It does not change hormone levels.
• It does not produce an erection without sexual stimulation.
• It treats the erectile difficulty itself and does not necessarily address the underlying cause.
• It affects wear off as sildenafil is eliminated from the body.

How does sildenafil work in the body?

Sildenafil works by blocking phosphodiesterase type 5, usually shortened to PDE5. Blocking that enzyme lets cyclic guanosine monophosphate, or cGMP, persist for longer than it otherwise would.

The pathway, step by step

1. Stimulation causes nerve endings and the vessel lining to release nitric oxide.
2. Nitric oxide activates an enzyme called guanylate cyclase.
3. Guanylate cyclase produces cGMP, a signalling molecule.
4. cGMP relaxes the smooth muscle in blood vessel walls, so the vessels widen and blood flows in.
5. PDE5 breaks cGMP down, which ends the response.
6. Sildenafil blocks step 5, so the response from steps 1 to 4 lasts longer.

Why step one matters more than the rest

Sildenafil does not start that sequence. It prevents the sequence being switched off early.

If nitric oxide is not released, no cGMP is produced, and there is nothing for sildenafil to protect. The tablet is present in the bloodstream and simply has no substrate to act on.

That single fact explains most reports of a tablet not working, and it is why the answer to "does sildenafil work without arousal" is no.

An analogy that holds up

Think of cGMP as water filling a bath and PDE5 as the plug hole. Sildenafil blocks the plug hole. If no water is running, blocking the plug hole changes nothing at all.

How does sildenafil affect blood flow?

Sildenafil helps increase blood inflow to erectile tissue by relaxing the smooth muscle in the arteries that supply it. Wider arteries carry more blood.

Where the effect is concentrated

PDE5 is not evenly distributed around the body. It is found in high concentrations in the smooth muscle of the corpus cavernosum, the erectile tissue of the penis, which is why the

Is sildenafil a stimulant?

No. Sildenafil is a vasodilator, meaning it widens blood vessels. It has no stimulant action.

What that distinction means

A stimulant increases activity in the central nervous system, raising alertness, heart rate or energy. Sildenafil does none of that.

It works on smooth muscle by slowing the breakdown of a signalling molecule. It does not act on mood, drive, wakefulness or energy, and it is not related to any stimulant drug class.

Why people assume otherwise

The assumption usually comes from the effects being noticeable, and from cultural framing rather than pharmacology. A medicine that produces a physical effect is not the same as one that stimulates you.

What is sildenafil used for?

Sildenafil is used for two quite separate things in the UK, under two separate authorisations.

Searches phrased as sildenafil, what is it used for, and what are sildenafil tablets used for, are asking the same question. The answer has two halves, and most pages only give one of them.

Is sildenafil only for ED?

No drug safety guidance states that sildenafil "is authorised for the treatment of adults and children aged 1 to 17 years with pulmonary arterial hypertension (PAH)".

The same guidance confirms the erectile dysfunction products "are used in the treatment of men with erectile dysfunction" and that this use "is not indicated for use in women".

Those are different products with different marketing authorisations. A tablet licensed for one is not interchangeable with a tablet licensed for the other.

Sildenafil for pulmonary hypertension

Pulmonary arterial hypertension is high blood pressure in the arteries that carry blood from the heart to the lungs. Because PDE5 is present in the smooth muscle of those vessels too, sildenafil relaxes them and reduces the pressure.

It is the same mechanism acting in different tissues. That is why one molecule ended up with two entirely unrelated licences.

An important boundary

Nothing on this page applies to sildenafil used for pulmonary arterial hypertension. The dosing, frequency and monitoring for that condition are different, and it is a specialist area.

If you have been prescribed sildenafil for a lung or heart condition, follow that prescriber and that leaflet, not general information about erectile dysfunction tablets.

Sildenafil other uses

Sildenafil has been studied in conditions beyond its two licences, and it is worth knowing that studied is not the same as authorised.

Use outside a licence is a prescriber decision made with full knowledge of the evidence. It is not something to pursue independently, and this page does not describe any unlicensed use as an option.

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What is each strength used for?

What is sildenafil 50 mg used for?

Sildenafil 50mg is licensed for erectile dysfunction in men aged 18 and over. It is the strength the published MHRA efficacy data describes.

In the trial data MHRA records, 74% of men taking sildenafil 50mg reported an improvement in their erections, against 25% of men on placebo. More than 8,000 patients received sildenafil across those studies, at doses ranging from 25 to 100mg.

That 74% figure belongs to 50mg. It is not a figure for another strength.

Sildenafil 50 mg uses beyond that

There are none in the UK licence. Sildenafil 50 mg uses are erectile dysfunction, and nothing else.

What is sildenafil 100mg used for?

The same thing. Sildenafil 100mg is a licensed UK strength for erectile dysfunction, and it sits at the upper end of the dose range studied in those trials.

The MHRA sources used here do not publish a separate efficacy percentage for 100mg alongside the 50mg figure, so this page does not quote one. That is an absence in those sources rather than a statement about how the strength performs.

Which strength suits you is decided at assessment. Treatment suitability depends on an individual clinical assessment, and your prescriber will determine the appropriate treatment.

Important dosing information for pulmonary arterial hypertension

For the treatment of pulmonary arterial hypertension (PAH), sildenafil is used at different doses and frequency, typically taken three times a day. For PAH, sildenafil is typically taken three times daily as prescribed.

How to take sildenafil safely

Sildenafil tablets should be swallowed whole with water. Avoid taking sildenafil with too much alcohol, as excessive drinking can reduce its effectiveness and increase side effects. Sildenafil must not be combined with nitrates or nitric oxide donors. If you take an alpha blocker, tell your prescriber, as the combination can lower blood pressure and may require a lower sildenafil starting dose.

Missed dose guidance

If you miss a dose of sildenafil prescribed for PAH, take it as soon as you remember unless it is almost time for your next dose. Do not take a double dose to make up for a missed one. For erectile dysfunction, sildenafil is usually taken as needed, so missed doses are less relevant.

Potential side effects and when to seek help

Common side effects include headache, flushing, indigestion, and nasal congestion. Some people may experience dizziness or visual disturbances, including blurred vision or changes in colour perception.

Serious side effects, although rare, can include sudden hearing loss or sudden vision loss. If you experience any sudden loss of hearing or vision, seek emergency medical care and contact a doctor immediately.

Drug interactions to be aware of

Sildenafil’s effects can be increased by certain medications such as protease inhibitors and erythromycin, which prolong its plasma half-life. This can increase the risk of side effects. Always inform your healthcare provider about any other medicines or supplements you are taking.

Who should avoid sildenafil

People with cardiovascular diseases, low blood pressure, certain heart conditions, pulmonary veno-occlusive disease, or other significant health conditions should discuss sildenafil with a doctor before use. It is not suitable for individuals taking nitrates or nitric oxide donors.

Generic sildenafil and brand name Viagra

Generic sildenafil contains the same active ingredient as Viagra and is used for the same licensed indication when supplied for erectile dysfunction. Both are prescription-only medicines and should be used under medical supervision.

Role of health system pharmacists

Pharmacists can help with safe dispensing of sildenafil, checking for drug interactions and advising about potential side effects.

What is sildenafil 20mg used for?

20mg is not one of the erectile dysfunction strengths. The UK erectile dysfunction licence covers 25mg, 50mg and 100mg.

Sildenafil 20 mg is a licensed strength used for pulmonary arterial hypertension and is not one of the licensed erectile dysfunction strengths in the UK.

If you are holding a 20mg tablet, the dispensing label and the patient information leaflet in your pack tell you what it is licensed for. Do not assume it is an erectile dysfunction tablet because the active ingredient is familiar.

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What does sildenafil feel like?

For most people, sildenafil does not feel like much at all. There is no rush, no high and no sense of the medicine working.

What you may physically notice

Many of the noticeable effects are related to its effects on blood vessels.

A feeling of warmth in the face, or a blocked nose an hour after taking it, are the two most commonly noticed.

What you should not feel

You should not feel stimulated, energised, altered in mood, or different in any mental sense. If you do, that is worth mentioning to a prescriber or pharmacist, because it is not the expected profile.

The honest summary

The absence of sensation is normal and is not evidence the tablet has failed. Judge it by what happens with stimulation, not by whether you can feel it circulating.

Sildenafil what to expect first time

A first tablet is where most of the avoidable disappointment happens, almost always for practical reasons rather than clinical ones.

Timing

MHRA documentation records the dose as one tablet taken approximately 1 hour beforehand, and states it normally takes 30 to 60 minutes to work. It also states it can be taken up to 4 hours in advance.

Peak plasma concentration is reached in 30 to 120 minutes, with a median of 60 minutes.

Food and drink

A high-fat meal can make it take a little longer to start working, so a relatively empty stomach avoids that delay.

MHRA documentation also says plainly not to take sildenafil with grapefruit or grapefruit juice, because this can affect how the medicine works. Alcohol appears among the cautions, and excessive alcohol works against the underlying response the tablet depends on.

The expectation to set

Stimulation is required. A first tablet taken in a tense, clock-watching situation with no stimulation will do nothing, and that outcome tells you nothing about whether the medicine suits you.

What not to do if it disappoints

Do not take a second tablet, and do not increase the strength yourself. The maximum is one tablet in any 24 hour period, and two tablets of a lower strength are never a route to a higher one.

Raise it at review instead. There are practical explanations worth ruling out before anyone concludes a strength is wrong.

How long does the effect last?

MHRA documentation records a terminal half-life of 3 to 5 hours. A half-life is the time the body takes to clear half a dose.

What that means practically

Sildenafil is short acting compared with some other medicines in its class. As sildenafil is eliminated from the body, its effects gradually wear off.

An erection lasting longer than 4 hours is not the medicine working well. It is a condition needing immediate medical attention.

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Who should not take it

Certain conditions and medicines rule sildenafil out entirely, which is what any assessment is checking for.

Contraindications and important precautions

• Nitrates or nitric oxide donors, including medicines for chest pain or heart failure.
• Riociguat. Ritonavir requires specific precautions with sildenafil and is contraindicated with sildenafil used for PAH.
• Allergy to sildenafil or any other ingredient.
• Being advised to avoid exertion because of a heart or blood vessel problem.
• A heart attack or stroke within the last 6 months, unstable angina or severe cardiac failure.
• Previous loss of vision from damage to the optic nerve.
• Low blood pressure, or high blood pressure that is not controlled.
• Severe liver problems.
• Known hereditary degenerative retinal disorders such as retinitis pigmentosa.
• A stomach ulcer or a bleeding disorder such as haemophilia.
• Conditions that can cause a prolonged erection, including sickle cell anaemia, leukaemia and multiple myeloma.

When to seek help urgently

• An erection lasting longer than 4 hours
• A sudden decrease or loss of vision, or of hearing
• Chest pain during or after use, which is an emergency
• Fainting or collapse
• Signs of an allergic reaction, which MHRA lists as sudden wheeziness, difficulty breathing or dizziness, and swelling of the eyelids, face, lips or throat

If you have chest pain, say that you have taken sildenafil, because nitrates must not be given.

Getting assessed

Sildenafil is a prescription medicine, so a clinical assessment comes before any supply. That assessment exists to find the exclusions listed above.

Medical Disclaimer

This article is for general information only and is not medical advice. It does not replace an assessment by a qualified healthcare professional. Treatment suitability depends on an individual clinical assessment. Your prescriber will determine the appropriate treatment. This medicine should only be used under the supervision of an appropriately qualified prescriber. Results vary between individuals.

Nothing here instructs you to take a particular strength, quantity or frequency, or to change an existing regimen, and there is no dose escalation plan on this page. The guidance here does not replace the instructions on your dispensing label or in the patient information leaflet supplied with your pack, and where they differ you should follow your prescriber and your leaflet.

This page describes sildenafil as used for erectile dysfunction. Sildenafil is separately authorised for pulmonary arterial hypertension as a different product at a different dose, and none of the dosing, timing or practical information here applies to that use. Anyone prescribed sildenafil for a lung or heart condition should follow their own prescriber and leaflet. The statement about the 20mg strength describes what the erectile dysfunction licence covers and what appears in the MHRA products database, it is not a statement about which product any individual tablet is, and your own label and leaflet are the authority on that.

Sildenafil has been studied in conditions outside its licences. Studied is not authorised, and no unlicensed use is described here as an option. Prescribing outside a licence is a clinical decision, not a patient one.

Dosing, timing, food, interaction, contraindication and side effect information is drawn from MHRA documentation relating to sildenafil for erectile dysfunction. The efficacy figure of 74% against 25% on placebo relates to sildenafil 50mg in the studies MHRA records and is not transferable to another strength. Contraindication lists are illustrative of published exclusions rather than exhaustive. Licensed indication statements are drawn from published Government drug safety guidance.

Do not start, stop or change the strength or frequency of any prescribed medicine because of this page. For sildenafil used for erectile dysfunction, do not take more than one dose in any 24-hour period unless specifically instructed otherwise by your prescriber. Never combine sildenafil with a nitrate, a nitric oxide donor or riociguat. Suspected side effects can be reported through the MHRA Yellow Card scheme. Seek immediate medical help for an erection lasting longer than 4 hours, chest pain, fainting, or sudden loss of vision or hearing.