Sildenafil: What It Is, What It Does and How to Take It

• Sildenafil is licensed in the UK as 25mg, 50mg and 100mg film-coated tablets, taken only when needed
• MHRA documentation records sildenafil as normally taking 30 to 60 minutes to work, with a terminal half-life of 3 to 5 hours
• In the trial data MHRA records, 74% of men on 50mg reported improved erections against 25% on placebo
• Generic sildenafil 50mg is £3.99 for 4 tablets at EveryDayMeds, a standard price

Sildenafil is a medicine that treats erectile dysfunction by increasing blood flow to the penis. It belongs to a group of medicines called PDE5 inhibitors, it is taken only when needed rather than every day, and in the UK it is licensed as 25mg, 50mg and 100mg film-coated tablets.

It is also an active substance in Viagra. Sildenafil and Viagra are the same medicine under two names, which is the single most useful thing to know before comparing prices.

Sildenafil pills are short acting by the standards of this group of medicines. MHRA documentation records a terminal half-life of 3 to 5 hours, which is why sildenafil is taken shortly before it is needed rather than kept at a steady level in the body.

This page covers what sildenafil is, what it does and how, what it is licensed for, the strengths and how they differ, how quickly it works and how long it lasts, side effects, safety, how it compares with the branded version, what it costs and how it is supplied in the UK. Results vary between individuals.

Sildenafil treatment is an effective treatment for erectile dysfunction but does not cure erectile dysfunction or increase desire. It works by enhancing the natural physiological process during stimulation, increasing blood flow to the corpus cavernosum of the penis to produce an erection.

Patients should be aware of the usual dose and prescribed dose, which are typically 25mg, 50mg, or 100mg tablets taken approximately one hour before intended intimate activity. Taking too much sildenafil or exceeding the recommended frequency can lead to serious effects of sildenafil, including painful erection (priapism), stomach discomfort, or allergic reaction.

Certain medications, such as amyl nitrate and decongestant nasal spray, must not be taken with sildenafil due to dangerous interactions. Grapefruit juice can also increase the levels of sildenafil in the bloodstream, raising the risk of side effects.

Elderly patients and those with bleeding disorders, pulmonary arterial hypertension, pulmonary veno occlusive disease, or aortic stenosis should consult a healthcare provider before starting sildenafil treatment. People with conditions such as sickle cell anaemia, multiple myeloma, or retinitis pigmentosa require special consideration due to increased risks.

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Key things to know

• Sildenafil is a PDE5 inhibitor licensed for erectile dysfunction in men aged 18 and over
• UK licensed tablet strengths are 25mg, 50mg and 100mg
• MHRA documentation records the dose as one tablet as needed, approximately 1 hour beforehand, with a maximum of one tablet in 24 hours
• It normally takes 30 to 60 minutes to work and 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 delays absorption by approximately 60 minutes
• Terminal phase half-life is 3 to 5 hours
• Headache is very common, affecting more than 1 in 10 people
• Nitrates, nitric oxide donors and riociguat are absolute contraindications
• Sildenafil does not work on its own, because stimulation is still needed

What is sildenafil?

Sildenafil is a prescription medicine for erectile dysfunction, supplied as a film-coated tablet and taken by mouth only when needed.

It is a phosphodiesterase type 5 inhibitor, usually shortened to PDE5 inhibitor. That is the drug class it belongs to, and it is the same class as tadalafil, vardenafil and avanafil.

Sildenafil is the generic name of the active substance. Viagra is a brand name for the same substance, so a generic sildenafil tablet and a branded tablet at the same strength contain the same active ingredient.

The full chemical name of the ingredient is sildenafil citrate, which is what appears on a pack or a dispensing label. Sildenafil tablets, sildenafil pills and sildenafil citrate tablets all describe the same medicine.

Searches phrased as sildenafil what is, sildenafil what is it and sildenafil what does it do are really asking two separate things: what the medicine is, and what it does in the body. The section below answers the second.

What does sildenafil do?

Sildenafil increases blood flow to the penis so that an erection can be achieved and maintained. It treats the symptom rather than any underlying cause of it.

Erectile dysfunction is defined in MHRA documentation as when a man cannot get or keep an erection hard enough for satisfactory function. Sildenafil addresses that difficulty for a period of hours after it is taken. It does not change anything permanently, and it does not treat whatever is producing the difficulty in the first place.

Sildenafil does not work on its own. MHRA documentation is explicit that stimulation is still needed to produce an erection. This is the most common misunderstanding about the medicine, and it accounts for a large share of people who conclude a tablet has not worked.

People with penile deformity, such as Peyronie's disease, should discuss treatment suitability with a healthcare professional. It is important to follow the prescribed starting dose and not to take the same doses more frequently than advised. Combining sildenafil with other medications, especially nitrates or amyl nitrite, can cause serious interactions and should be avoided.

Clinical studies have shown that sildenafil can help improve the ability to engage in intimate activity by enhancing blood flow, but it does not cure erectile dysfunction or increase desire. If symptoms persist or worsen, patients should contact their doctor immediately.

In cases of a missed dose when sildenafil is taken regularly for other licensed uses, the dose should be taken as soon as remembered unless it is near the time for the next dose. Do not double the dose to make up for a missed one.

Sildenafil’s effects typically last about 4 to 6 hours, and it is not recommended to take it more than once within 24 hours. Stopping intimate activity early due to difficulties may be alleviated by sildenafil, improving confidence and performance.

While sildenafil is generally safe, there are rare but serious side effects such as vision loss or prolonged erections lasting more than 4 hours that require urgent medical attention. People with heart disease or stomach ulcer should discuss their condition with a healthcare provider before starting treatment.

Sildenafil can lower blood pressure and may enhance the blood-pressure-lowering effects of some medicines, particularly alpha-blockers, so concomitant treatment requires clinical assessment.

The medication is designed to amplify the body's natural response to arousal by increasing levels of cGMP, which relaxes smooth muscle cells in the blood vessels of the penis, allowing increased blood flow necessary for an erection.

Patients are advised to avoid grapefruit juice while taking sildenafil as it can increase the medication’s concentration in the blood, raising the risk of side effects.

Sildenafil has a terminal half-life of about 3 to 5 hours, while the duration of its erectile-function effect varies between individuals. Food, especially a large meal, can delay its onset by about an hour.

If side effects such as headache or dizziness persist beyond about a week, or if any serious side effects occur, patients should consult their doctor immediately.

How does sildenafil work?

Sildenafil works by blocking an enzyme called PDE5, which allows a natural signalling molecule to last longer and blood vessels to stay relaxed and open.

The sequence runs like this:

1. Stimulation causes nerve endings to release nitric oxide in the tissue of the penis
2. Nitric oxide triggers the production of cyclic guanosine monophosphate, known as cGMP
3. cGMP relaxes the smooth muscle in blood vessel walls, so the vessels widen and blood flows in
4. PDE5 is the enzyme that breaks cGMP down, ending the effect
5. Sildenafil blocks PDE5, so cGMP persists and the blood vessels stay open for longer

The MHRA assessment puts it simply: "By blocking the breakdown of cGMP, sildenafil improves erectile function."

The practical consequence follows directly from step 1. If there is no nitric oxide signal, there is no cGMP for sildenafil to protect. The medicine extends a response rather than creating one.

What is sildenafil used for?

Sildenafil uses in the UK are narrow. It is licensed for erectile dysfunction in men aged 18 and over, and that is the use this page covers.

Sildenafil is also used under different licences at different doses for conditions unrelated to erectile dysfunction. Those are separate products with separate authorisations, and nothing on this page applies to them. A tablet licensed for erectile dysfunction should never be used for another purpose.

What sildenafil is not for

• It is not licensed for women
• It is not licensed for anyone under 18
• It does not benefit anyone who does not have an erection problem, which MHRA documentation states plainly
• It is not a daily treatment, and taking more than one tablet in 24 hours is outside the licence
• It does not treat the underlying causes of erectile dysfunction, which is why assessment matters

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Sildenafil dosage

MHRA documentation records the dose as one tablet taken as needed, approximately 1 hour beforehand, with a maximum of one tablet in any 24 hour period.

Sildenafil is licensed in the UK at three tablet strengths: 25mg, 50mg and 100mg. Which strength is appropriate is a clinical decision. Treatment suitability depends on an individual clinical assessment, and your prescriber will determine the appropriate treatment.

Nothing on this page tells you what to take, and there is no dose escalation plan here. Do not increase your strength or take an extra tablet on your own initiative.

Sildenafil 50mg

Sildenafil 50mg is the strength the published MHRA efficacy data describes, and the strength involved in the 2017 reclassification decision that made a sildenafil product available from pharmacies without a prescription.

In the trial data MHRA records for this strength, 74% of men taking sildenafil 50mg reported an improvement in their erections, against 25% of men on placebo. More than 8,000 participants were enrolled across the studies referred to.

That 74% figure belongs to 50mg. It is not a figure for 25mg or for 100mg, and moving it to another strength would be an attribution error rather than a comparison.

Sildenafil 100mg

Sildenafil 100mg is a licensed UK strength, appearing in the MHRA products database as sildenafil 100mg film-coated tablets.

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 these sources rather than a statement about how the strength performs.

A higher strength is not automatically a better outcome, and it is not something to move to because a first tablet is disappointing. The maximum remains one tablet in 24 hours at any strength, and two lower strength tablets are never a route to a higher one.

Food, alcohol and timing

Food delays absorption by approximately 60 minutes, according to MHRA documentation. That does not stop the medicine working, but it does move the timing, and a large meal shortly before taking a tablet is a common reason a tablet seems slow.

Excessive alcohol consumption can make it harder to achieve an erection and may increase the risk of dizziness or low blood pressure when sildenafil is taken.

How long does sildenafil take to work?

Sildenafil normally takes between 30 and 60 minutes to work, and MHRA documentation states it can be taken up to 4 hours before it is needed.

Those are two different numbers doing two different jobs. The dosing instruction gives approximately 1 hour as the usual timing, while the 4 hour figure is the outer limit of how early a tablet can reasonably be taken. It is not a promise that the effect lasts four hours.

Two practical points come before deciding a tablet is slow. Food pushes absorption back by around an hour, and the medicine needs the underlying response to be present before there is anything for it to extend.

How long does sildenafil last?

MHRA documentation records a terminal phase half-life of 3 to 5 hours for sildenafil, which is the clearest published figure on how long it stays active.

A half-life is the time the body takes to clear half of a dose. It is not the same as the duration of a noticeable effect, which varies between individuals, but it is the number that explains why sildenafil is described as short acting. A tablet that starts working in 30 to 60 minutes and clears at that rate is built around a window rather than a whole day.

If an erection lasts longer than 4 hours, that is not the medicine lasting well. It is a condition needing immediate medical attention, and MHRA documentation lists it as rare but urgent.

Sildenafil side effects

Headache is the only side effect of MHRA documentation classes as it is very common, meaning it may affect more than 1 in 10 people.

Several of these trace back to the same mechanism that makes the medicine work. Blood vessels widening in one place widen elsewhere too, which is where headache, flushing and a stuffy nose come from.

The visual effects are worth naming separately because they surprise people. A temporary blue or coloured tinge to vision, blurring or general visual disturbance are listed as common. That is different from sudden loss of vision, which is an emergency and is covered below.

MHRA documentation also advises a check-up with a doctor within the first 6 months of use, which is a reasonable prompt to treat this as ongoing care rather than a one off purchase.

Suspected side effects can be reported through the MHRA Yellow Card scheme.

Is sildenafil safe?

For men who have been properly assessed and are not in one of the excluded groups, sildenafil is a long established medicine with a well characterised side effect profile. The serious risks sit in identifiable groups, which is what an assessment exists to find.

The strongest evidence that the risk is manageable through assessment is regulatory rather than promotional. In November 2017 the MHRA reclassified a sildenafil 50mg product from a prescription only medicine to a pharmacy medicine, on the basis that a pharmacist consultation could identify the people who should not take it.

That decision came with conditions. The reclassified product is not sold to people with severe cardiovascular disorders, those at high cardiovascular risk, those with liver failure or severe kidney failure, or those taking certain interacting medicines.

Safety here is not a property of the tablet on its own. It is a property of the tablet plus the assessment.

Who should not take sildenafil tablets

Sildenafil is contraindicated, meaning it must not be taken, by anyone who:

• Takes nitrates or nitric oxide donors, including medicines for chest pain or heart failure
• Takes riociguat
• Takes ritonavir or other potent CYP3A4 inhibitors, which may require sildenafil to be avoided or used only under specific prescribing restrictions.
• Is allergic to sildenafil or any of the other ingredients
• Has been advised to avoid exertion because of a heart or blood vessel problem, including a heart attack or stroke within the last 6 months, unstable angina or severe cardiac failure
• Has ever had loss of vision from damage to the optic nerve, such as non-arteritic anterior ischaemic optic neuropathy
• Has low blood pressure, or high blood pressure that is not controlled
• Has a severe liver problem
• Has any disease or deformity of the penis, such as Peyronie's disease

The nitrate interaction is the one to be most careful about. Both nitrates and sildenafil lower blood pressure, and together the drop can be severe.

Further caution, and a prescriber conversation before use, applies to anyone taking alpha blockers or CYP3A4 inhibitors, and anyone with mild to moderate liver disease or severe kidney problems. This medicine should only be used under the supervision of an appropriately qualified prescriber.

Sildenafil vs Viagra

Sildenafil and Viagra contain the same active substance at the same strengths. Viagra is a brand name and sildenafil is the generic name for what is inside the tablet.

A UK generic must contain the same active substance as the reference medicinal product, in the same pharmaceutical form, and must be shown to be bioequivalent to it. What differs is the name, the manufacturer, the tablet's appearance, the inactive ingredients and the price.

At the 4 tablet pack in 25mg, the branded product costs about 7.3 times as much per tablet as the generic. That multiple is arithmetic from the two standard prices shown, not a claim about either product.

One limitation belongs with that table. The branded product is stocked here at 25mg only, so a like for like brand and generic comparison at 50mg or 100mg cannot be made from these listings.

Which of the two is supplied follows from the assessment and from what is appropriate for you, not from the price difference. Prices are standard prices and may change.

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Sildenafil price

These are standard prices at EveryDayMeds, quoted before any code or offer. The per tablet figure is calculated from each pack price.

Two things in those tables are worth pointing out, because they contradict what people assume.

The per tablet price does not fall steadily as packs get bigger. At 50mg, the 32-tablet pack works out at £0.76 per tablet, while the 48-tablet pack is £0.87 per tablet. At 100mg, the 32-tablet pack is £0.84 per tablet, while the 48-tablet pack is £0.87. Buying the largest pack does not automatically produce the lowest per-tablet price here, so it is worth doing the arithmetic on the pack you are actually being supplied.

The 50mg and 100mg prices also do not increase proportionally with strength. For example, the 4-tablet packs are £3.59 for 50mg and £4.04 for 100mg, while the 8-tablet packs are £6.97 and £7.64, respectively. Cost therefore should not be used to decide which strength is appropriate; that is a clinical decision.

Which quantity or strength you are supplied follows from the clinical assessment rather than from these tables.

Sildenafil prescription UK: what the law says

Sildenafil is a prescription only medicine in the UK, with one narrow exception at a single strength.

In November 2017 the MHRA reclassified a sildenafil 50mg product from prescription only medicine to pharmacy medicine, making that specific product available without a prescription for men over 18 with erectile dysfunction, after a pharmacist consultation and subject to the exclusion conditions described above.

That exception is narrower than it is often reported to be. It applies to one product at one strength, not to sildenafil generally.

The practical rule for anyone buying online is unchanged. A registered UK pharmacy assesses before it supplies and can decline.A legitimate UK online pharmacy should require the appropriate clinical assessment and prescription or pharmacist consultation required for the specific product before supplying it.

Buy sildenafil online

You can buy sildenafil online in the UK from a registered pharmacy, after a clinical assessment. The assessment comes first and determines whether there is anything to dispense.

1. You complete an online consultation covering your medical history, current medicines, blood pressure and heart health
2. A UK registered prescriber reviews it and decides whether treatment is appropriate, and at which strength and quantity
3. If approved, the pharmacy dispenses and dispatches the medicine

How to check a pharmacy is genuinely registered

• A registered pharmacy displays its GPhC registration number and names its superintendent pharmacist
• That registration can be checked on the regulator's own register rather than taken on trust
• A legitimate service assesses before it supplies, and is able to decline
• Prescription-only sildenafil products require a prescription; the specific 50mg product reclassified as a pharmacy medicine is supplied after the required pharmacist consultation.

EveryDayMeds is a GPhC registered pharmacy. Treatment is supplied only after clinical assessment, and all medication is genuine and UK sourced.

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When to seek urgent help

• An erection lasting longer than 4 hours needs immediate medical attention
• Sudden loss of vision in one or both eyes, or sudden loss or reduction of hearing, means stopping the medicine and being seen urgently
• Chest pain during or after use is an emergency, and you must say that you have taken sildenafil, because nitrates must not be given
• Fainting or collapse needs assessment

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.

The efficacy figure of 74% against 25% on placebo relates to sildenafil 50mg in the studies MHRA records, with more than 8,000 participants enrolled across them, and it is not transferable to 25mg or 100mg. No matching percentage for those strengths appears in the sources used here, which is an absence rather than a statement about how they perform. Pharmacokinetic figures, adverse effect frequencies, contraindications and cautions are taken from MHRA documentation relating to sildenafil for erectile dysfunction, and do not apply to sildenafil products licensed for other conditions.

Prices quoted are standard prices at the time of writing, shown per pack and per tablet. They are not promotional or first order prices, every per tablet figure is calculated from the pack price shown, and prices may change. Cost is not a basis on which to choose a strength.

Do not start, stop or change the strength or frequency of any prescribed medicine because of this page. Never take more than one tablet in any 24 hour period, and never take two tablets of a lower strength to make a higher one. 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.