What Is Sildenafil, and How Does It Work?

• Genuine sildenafil supplied through a GPhC-registered UK pharmacy
• Online consultation with UK-registered prescribers before any prescription is issued
• Clear, practical guidance on sildenafil's uses, dosing and safety information
• Straightforward, discreet access to treatment once you are assessed as suitable

Sildenafil is a prescription-only medicine used to treat erectile dysfunction (ED). It is the active ingredient behind one of the most widely used treatments for the condition, and it works by supporting a specific step in the body's natural process for producing an erection.

This guide sets out what sildenafil actually is, how it works, how it is taken, and what to check before starting it. It also covers where its prescribing and safety information comes from, and how to access it safely online in the UK. Each section is written to stand on its own, so you can go straight to the part you need.

Only a UK-registered prescriber can confirm whether sildenafil is suitable for you, based on a full clinical assessment of your health and current medicines.

What Is Sildenafil?

Sildenafil is a treatment for erectile dysfunction. It is also used to treat pulmonary hypertension. It belongs to a group of medicines called PDE5 inhibitors, all of which work in a broadly similar way to support erectile function once a man is aroused.

Sildenafil Citrate: The Active Ingredient

The full name of the active ingredient is sildenafil citrate. Citrate refers to the specific salt form the sildenafil molecule is combined with to make it stable and suitable for use in a tablet. This is a standard pharmaceutical technique, and the "citrate" part of the name does not change how the medicine works. It is the sildenafil component that is pharmacologically active.

Sildenafil is also supplied under a well-known brand name, Viagra, alongside generic sildenafil tablets from various UK-licensed manufacturers in the UK market. Both contain the same active ingredient, sildenafil citrate. Sildenafil is usually cheaper than Viagra.

Medicine Class: PDE5 Inhibitor

Sildenafil belongs to a class of medicines called phosphodiesterase type 5 (PDE5) inhibitors. This class also includes tadalafil and vardenafil, each of which is broken down and cleared by the body at different rates, which is why they differ in how quickly they start working and how long they last. Sildenafil is a potent and selective inhibitor of PDE5.

Who Regulates Sildenafil in the UK

The Medicines and Healthcare products Regulatory Agency (MHRA) is responsible for licensing sildenafil and monitoring its safety in the UK. Every UK-licensed sildenafil product must meet the same regulatory standards for quality, safety and effectiveness before it can be supplied, and genuine sildenafil is only ever dispensed through a pharmacy registered with the General Pharmaceutical Council (GPhC).

How Sildenafil Works

Sildenafil works by increasing blood flow to the penis during sexual arousal by supporting one specific step in the body's natural erection process. It requires sexual stimulation to have any effect and does not work on its own.

The Nitric Oxide-cGMP Pathway

When a man becomes aroused, the body releases a chemical called nitric oxide in the penis. This triggers the production of another chemical, cGMP, which causes smooth muscle relaxation in the corpus cavernosum. That allows blood vessels to widen more easily. As those muscles relax, blood flows in more freely, producing an erection.

An enzyme called PDE5 normally breaks cGMP down fairly quickly, which is part of what ends an erection. Sildenafil works by blocking PDE5, so the nitric oxide signal can keep cGMP elevated for longer, making blood flow easier to achieve and sustain for as long as arousal continues.

Absorption, Onset and Half-Life

Taken on an empty stomach, sildenafil is absorbed fairly quickly, typically reaching peak blood levels somewhere between 30 and 120 minutes after a dose, with a median of around 60 minutes. Its bioavailability, meaning the proportion of the dose that reaches the bloodstream in an active form, is approximately 41%. In other words, about 41% of the administered oral dose reaches the bloodstream in active form.

Sildenafil has a half-life of around 4 hours, meaning it takes approximately 4 hours for the amount of sildenafil in the body to fall by half. Its effects may last for several hours, although this varies between individuals.

Eating a large or high-fat meal before taking sildenafil can delay absorption. A heavy meal can push back the time it takes to reach peak levels by around an hour and reduce the peak amount absorbed by roughly a third. Taking it on an empty stomach or after a lighter meal tends to give more predictable timing.

How Sildenafil Is Broken Down in the Body

Sildenafil is metabolised in the liver, mainly by an enzyme system called CYP3A4, with a smaller contribution from CYP2C9. This matters because other substances that affect these enzymes can change how much sildenafil is active in the body at any given time.

Grapefruit and grapefruit juice are a well-documented example. They can increase the amount of sildenafil absorbed by a moderate degree, which may increase the risk of side effects. If you regularly consume grapefruit products, it is worth mentioning this to your prescriber during your consultation.

Sildenafil Tablets: Strengths, Dosing and How They are Taken

Available Strengths

Sildenafil tablets are available in three strengths or usual dose options: 25mg, 50mg and 100mg. Viagra and sildenafil are available in the same doses. Which strength is right for you depends on individual factors including your general health, other medicines you take, and how you respond to treatment. Your prescriber determines the appropriate starting point during your assessment and may adjust it based on how treatment goes. This is not something to work out for yourself.

How Sildenafil Tablets Are Taken

A sildenafil tablet is swallowed whole with water and is generally taken approximately one hour before anticipated sexual activity. It can be taken with or without food, although a heavy or fatty meal may delay how quickly it starts working.

No more than one dose should be taken within any 24-hour period. Some interacting medicines require stricter limits. With protease inhibitors, sildenafil may need to be limited to one 25 mg dose every 48 hours under prescriber guidance. If a dose does not seem to have worked, it should not be repeated on the same day. Speak to your prescriber if a particular strength is not giving the result you expected, rather than adjusting the dose yourself and risking too much sildenafil.

"Sildenafil Tablets" or "Sildenafil Pills"? Same Medicine, Different Words

People search using several different terms for the same product, including "sildenafil tablets" and "sildenafil pills". In UK medical terminology, "tablet" is the standard, accurate term, since sildenafil is supplied as a solid, compressed dose form rather than a soft capsule. "Pills" is simply everyday language for the same thing. Whichever word is used, the strengths, dosing and mechanism described on this page are the same.

Who Can Take Sildenafil

Who it is Licensed For

Sildenafil is licensed for adult men who experience erectile dysfunction, meaning a persistent or recurring difficulty achieving or maintaining an erection sufficient for satisfactory sexual performance.

An occasional difficulty, particularly if linked to stress or tiredness, is common and does not automatically mean ongoing treatment is needed. A proper assessment helps establish whether treatment is appropriate for your specific situation. For some men, sildenafil treatment may be ongoing and may need review over time depending on response and tolerability.

Who Should Not Take Sildenafil

Sildenafil is not suitable for everyone. Your prescriber will check for the following during your consultation:

• Anyone taking nitrate medicines for chest pain, nicorandil, or riociguat. Combining these with sildenafil can cause a dangerous drop in blood pressure.
• Anyone using recreational nitrates, sometimes known as "poppers".
• Men with certain cardiovascular conditions or who have recently experienced certain serious cardiovascular events, where treatment or sexual activity may be unsuitable.
• Men with certain severe heart conditions, including unstable angina or severe heart failure, or where sexual activity itself has been advised against because of cardiovascular risk.
• Men with severe liver impairment.
• Men taking another PDE5 inhibitor medicine at the same time. These should never be combined.

This is not an exhaustive list. Your prescriber will review your full medical history, current medicines, and relevant medical conditions, including whether alpha-blocker medicines could contribute to low blood pressure and dizziness when taken with sildenafil, to confirm sildenafil is safe for you specifically.

Interactions to Be Aware Of

Beyond the medicines listed above, sildenafil can interact with anything that strongly affects the CYP3A4 enzyme system responsible for breaking it down, either raising or lowering how much stays active in your body. This is why a full medicines list is reviewed at your consultation, not just a check for the small number of absolute contraindications.

Alcohol can lower blood pressure and may make it harder to achieve or maintain an erection, which can affect how well sildenafil works. Drinking large amounts of alcohol while taking sildenafil may therefore reduce its effectiveness and increase the likelihood of some side effects. Recreational nitrates such as poppers should never be combined with sildenafil.

Possible Side Effects

Like all medicines, sildenafil can cause side effects. The most commonly reported include headache, flushing, indigestion, nasal congestion, and visual disturbances such as blurred vision, increased light sensitivity, or a temporary blue tinge to vision. These are generally mild and temporary.

Rare but serious side effects need urgent medical attention. These include sudden loss or deterioration of vision or hearing, signs of an allergic reaction, and a prolonged, often painful erection lasting longer than 4 hours (priapism), which requires immediate medical assessment. Men with certain blood conditions or anatomical deformities of the penis, including Peyronie's disease, may be at higher risk of priapism and should discuss this with their prescriber.

If side effects persist or worry you, seek medical advice. This is not a complete list of possible side effects. The patient information leaflet supplied with your medicine sets out the full picture, and you should read it before starting treatment.

Sildenafil Prescribing Information and Safety Documents

Where Prescribing Information Comes From

Sildenafil's official prescribing information exists in two linked documents. The Summary of Product Characteristics is the detailed technical document written for prescribers and pharmacists, covering dosing, interactions, contraindications and clinical data in full. The Patient Information Leaflet is the plain-language version included in every pack, written for the person taking the medicine.

If you are looking for the full technical prescribing detail behind sildenafil, rather than a simplified summary, the Summary of Product Characteristics is the authoritative source, and your prescriber or pharmacist can talk you through anything in it that is not clear. The prescribing information and supporting clinical trials show that sildenafil is an effective treatment for many men with erectile dysfunction.

Safe Storage and Shelf Life

Sildenafil tablets should be kept out of the sight and reach of children, stored in their original packaging to protect them from moisture, and not used after the expiry date printed on the pack. Most sildenafil products should not be stored above 30°C. Always check the storage instructions on your specific pack, since these can vary slightly between manufacturers.

Getting Sildenafil Online in the UK

Prescription-Only Status

Sildenafil supplied for erectile dysfunction is generally a prescription-only medicine in the UK. However, Viagra Connect 50mg is a specific sildenafil product classified as a Pharmacy medicine and can be supplied directly by a pharmacist following an appropriate consultation.

How to Access Sildenafil Through EveryDayMeds

Genuine sildenafil is available through EveryDayMeds following an online consultation with UK-registered prescribers. You will complete a confidential medical questionnaire covering your health history, current medicines, and cardiovascular health. A prescriber reviews this before deciding whether treatment is appropriate for you.

If you are assessed as suitable, your prescription is dispensed by a GPhC-registered UK pharmacy and dispatched in discreet packaging. Pricing is confirmed during your consultation and at checkout, so you know exactly what you will pay before completing an order.

Recognising a Genuine Online Pharmacy

Erectile dysfunction medicines are a common target for counterfeit and illegally supplied products. The MHRA has reported that around 19.5 million doses of illegal erectile dysfunction medicines were seized in the UK between 2021 and 2025. Buying from an unverified source carries real risk, since there is no way to know what a counterfeit tablet actually contains.

A few signs can help you spot a pharmacy that is not operating properly:

• No consultation or medical questionnaire is required before you can buy.
• Be cautious of websites that advertise prescription-only medicines in ways that do not comply with UK medicines advertising requirements.
• Payment is only accepted in cryptocurrency.
• There is no way to check the pharmacy's registration with the GPhC.

A genuine UK online pharmacy will always require a proper consultation before supplying a prescription-only medicine like sildenafil, regardless of how quick or convenient the process feels.

Sildenafil at a Glance

Medical Disclaimer

This article is provided for general informational purposes only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional. Sildenafil is a prescription-only medicine and should only be used under the supervision of an appropriately qualified prescriber, following an individual clinical assessment. Treatment suitability, dosage, and ongoing use should always be determined by your prescriber based on your personal medical history. If you experience an erection lasting longer than 4 hours, or any sudden change in vision or hearing, seek immediate medical attention. Results vary between individuals, and this content should not be used to self-diagnose or self-treat any condition.