Everything you need to know about Sildenafil in the UK

DoktorABC editorial team

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

What is sildenafil?

Sildenafil is a prescription medicine belonging to a class of drugs called PDE5 inhibitors. It was originally developed by Pfizer in the late 1980s while researchers were looking for a treatment for angina. During clinical trials, an unexpected effect was observed: the drug consistently produced erections in male participants. This led to its development and approval as a treatment for erectile dysfunction, and it was licensed in the UK and across Europe in 1998 under the brand name Viagra.

Since Pfizer's UK patent expired in 2013, sildenafil has been available as a generic medicine from multiple manufacturers. Generic sildenafil contains the same active ingredient at the same doses as Viagra and is bioequivalent in clinical effect.

Sildenafil is MHRA-approved for two indications in the UK:

• Erectile dysfunction: The primary and most common use, for men who have difficulty achieving or maintaining an erection sufficient for satisfactory sexual activity
• Pulmonary arterial hypertension (PAH): A serious condition involving high blood pressure in the arteries supplying the lungs. Sildenafil is prescribed under the brand name Revatio for this indication, typically at lower doses and on a different dosing schedule from ED treatment

How does sildenafil work?

Understanding the mechanism behind sildenafil helps explain both why it is effective and why it requires sexual stimulation to work.

When a man becomes sexually aroused, the nervous system releases a chemical called nitric oxide into the tissue of the penis (specifically an area called the corpus cavernosum). Nitric oxide triggers the production of another chemical, cyclic guanosine monophosphate, or cGMP. It is cGMP that does the work: it relaxes the smooth muscle in the walls of the blood vessels supplying the penis, allowing them to widen and allowing significantly more blood to flow in. This increased blood flow is what produces an erection.

Under normal circumstances, an enzyme called PDE5 breaks down cGMP, which limits the erection and eventually ends it. Sildenafil works by selectively inhibiting PDE5, preventing the breakdown of cGMP and thereby prolonging smooth muscle relaxation and the blood flow response. 

In plain terms: sildenafil does not create arousal or produce an erection on its own. What it does is make the body's natural erectile response more effective and more sustained by preventing cGMP from being broken down too quickly.

Sildenafil only works in the presence of sexual stimulation. If arousal does not occur, the nitric oxide pathway is not activated, cGMP is not produced, and sildenafil has no effect.

The same mechanism, applied to the blood vessels of the lungs rather than the penis, explains why sildenafil is also used in pulmonary arterial hypertension. PDE5 is present in pulmonary artery smooth muscle, and its inhibition leads to vasodilation and a reduction in pulmonary artery pressure.

How long does sildenafil take to work?

Sildenafil typically begins to work within 30 to 60 minutes with effects lasting approximately four to six hours. Most men find that taking it around an hour before anticipated sexual activity gives the most reliable onset. 

Several factors influence how quickly it takes effect:

• Food: This is the most significant practical factor. High-fat meals delay the absorption of sildenafil and can slow its onset. Taking sildenafil on an empty stomach or after a light meal gives the most predictable and rapid onset. A heavy meal, particularly one high in fat, can delay the effect by an hour or more. 
• Alcohol: Alcohol can lower blood pressure and worsen some of sildenafil's side effects such as dizziness and flushing. Modest alcohol consumption is unlikely to cause a serious problem for most men, but heavy drinking can both reduce effectiveness and increase side effect risk.
• Age: Sildenafil is cleared from the body more slowly in older men, which means it may act for longer and at a higher effective concentration. Starting at a lower dose is often recommended for men over 65.
• Individual metabolism: How quickly your body processes sildenafil varies between individuals and can be influenced by other medicines you are taking, particularly those that affect the CYP3A4 enzyme in the liver.

If sildenafil does not seem to work the first time, it is worth trying it again under different conditions: on a lighter stomach, with less alcohol, and with more time allowed for arousal. 

How long does sildenafil last?

Sildenafil remains active in the body for approximately four to six hours following a standard dose. This refers to the window during which the drug is present at effective levels and can support an erection in response to stimulation. It does not mean an erection lasts for four to six hours.

This duration is related to sildenafil's half-life of approximately three to five hours, meaning the concentration in the bloodstream halves roughly every four hours. By six to eight hours after taking a tablet, most men will find the effects have largely worn off.

Sildenafil should not be taken more than once in any 24-hour period.

Sildenafil tablets: strengths and appearance

Sildenafil is available as film-coated oral tablets in three strengths:

• 25 mg: The lowest available dose, typically used in older men or those more sensitive to side effects
• 50 mg: The standard starting dose for most men
• 100 mg: The maximum dose, used when 50 mg is insufficient

The standard starting dose recommended in UK clinical practice is 50 mg, taken approximately one hour before sexual activity. Based on response and tolerability, this can be adjusted up to 100 mg or down to 25 mg. The dose should be taken as needed, not more than once per day.

Branded Viagra tablets are blue, film-coated, and diamond-shaped. Generic sildenafil tablets vary in appearance depending on the manufacturer; they may be a different shape or shade but contain the same active ingredient at the same dose.

What matters is the dose strength and that the product comes from a GPhC-registered pharmacy. If you are unsure about a tablet's appearance, your pharmacist can confirm it is the correct product.

Can women take sildenafil?

Sildenafil is not licensed for use in women for erectile dysfunction or female sexual arousal disorder in the UK. The MHRA-approved indications cover men only for ED, and women for pulmonary arterial hypertension only.

There has been research interest in sildenafil for female sexual arousal disorder, and some studies have investigated its use off-label in this context. The results have been mixed, and there is currently no robust enough evidence base for regulatory approval in this indication in the UK or EU.

A separate medicine, flibanserin (brand name Addyi), is licensed in the US for hypoactive sexual desire disorder in premenopausal women, but it works through a completely different mechanism and is not a PDE5 inhibitor. It is not currently licensed in the UK.

Women who are pregnant or breastfeeding should not take sildenafil for any indication without specific medical advice, as safety data in these populations is limited.

If you are a woman with questions about treatments for sexual dysfunction, a GP or specialist can advise on what is currently available and appropriate for your circumstances.

Who should not take sildenafil?

A medical assessment is required before sildenafil is prescribed, in part because it is contraindicated in certain situations. The following is an overview of the main contraindications; it is not exhaustive.

Sildenafil must not be taken by men who:

• Are taking nitrate medicines in any form, including glyceryl trinitrate (GTN) spray, isosorbide mononitrate, or isosorbide dinitrate. The combination can cause a severe and potentially life-threatening drop in blood pressure
• Are taking riociguat, a medicine used for pulmonary hypertension, as the combination also causes dangerous hypotension
• Have severe cardiovascular disease or have been advised by a doctor to avoid sexual activity on cardiac grounds
• Have severe liver impairment
• Have recently had a stroke or heart attack (timing and individual circumstances determine suitability; always discuss with a prescriber)
• Have a known hypersensitivity or allergy to sildenafil or any of the excipients in the tablet

Sildenafil should be used with caution in men who:

• Are taking antihypertensive medicines, as sildenafil can add to blood-pressure-lowering effects
• Have a history of priapism (prolonged erections) or conditions that may predispose to it
• Have retinitis pigmentosa, a rare inherited eye condition
• Have significant kidney or moderate liver impairment, where dose adjustment may be needed

How to get sildenafil in the UK

Sildenafil is a prescription-only medicine in the UK, with one exception: Viagra Connect 50 mg is available over the counter from pharmacies following a suitability check.

Via your GP: Your GP can prescribe generic sildenafil on the NHS where it meets prescribing criteria. NHS prescriptions are subject to the standard prescription charge in England, and are free in Scotland, Wales, and Northern Ireland for eligible patients.

Via a pharmacist (Viagra Connect only): The 50 mg OTC dose is available from pharmacists who complete a brief health assessment before supplying it. This covers current medicines, medical history, and any contraindications.

Via a regulated online clinic: Private online clinics connect you with UK-registered prescribers who can assess your suitability and issue a private prescription for sildenafil at any licensed dose. DoktorABC connects UK patients with licensed prescribers and GPhC-registered pharmacies, with a medical assessment completed online and discreet delivery to your door.

Report any suspected side effects from sildenafil to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Final thoughts

Sildenafil is a well-established, extensively studied treatment that has helped millions of men manage erectile dysfunction safely and effectively. Understanding how it works, what affects its onset and duration, and who it is and is not suitable for puts you in a much stronger position to use it appropriately and to have an informed conversation with your prescriber.

If you are considering sildenafil for the first time, the right starting point is a medical assessment, whether through your GP, a pharmacist for the OTC option, or a regulated online clinic. A proper assessment is not a barrier; it is what ensures the treatment is right for you and that you can use it safely.

FAQ

The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board