Sildenafil is available in three tablet strengths in the UK, and many people looking for a dosage guide want to understand which strength is usually prescribed first and how a prescriber decides whether a dose adjustment may be needed.
This guide explains the differences between 25mg, 50mg and 100mg sildenafil, along with when to take it, how food can affect it, and how generic sildenafil compares with branded Viagra.
This information is for general guidance only and does not replace a personal medical assessment. Your prescriber will decide the most suitable strength for you based on your health, any other medicines you take, and how you respond to treatment.
How sildenafil works in the body
Sildenafil belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors .
During sexual stimulation, the body releases nitric oxide, which helps produce a chemical called cyclic guanosine monophosphate (cGMP) in the tissues involved in an erection. Sildenafil works by blocking the enzyme that breaks down cGMP, helping increase blood flow to the penis and supporting an erection when you are sexually stimulated .
Sildenafil does not increase sexual desire or create an erection without sexual stimulation. It supports the body's natural response to arousal, which is why sexual stimulation is still needed for it to work.
Sildenafil is mainly known for treating erectile dysfunction (ED), but the same active ingredient is also used under a different brand name and formulation to treat pulmonary arterial hypertension (PAH), a condition affecting blood pressure in the arteries of the lungs . This is a separate medical use with different dosing instructions and is not covered by the erectile dysfunction doses discussed on this page.
The three sildenafil strengths
Sildenafil citrate is the active ingredient found in both generic sildenafil and Viagra . In the UK, sildenafil for erectile dysfunction is available in three tablet strengths:
25mg
50mg
100mg
The right strength depends on your individual health, other medicines you may take, and how well you respond to treatment. A higher dose is not always more suitable, and your prescriber will recommend the most appropriate option for you.
| Strength | Typical role |
|---|---|
| 25mg | Often used as a lower starting point for those more sensitive to the medication, including some older patients or those on interacting medications |
| 50mg | Commonly used as the standard starting dose for most adult men |
| 100mg | The maximum strength, generally considered if a lower dose hasn't provided an adequate response |
These strengths and their general uses are based on standard product information for sildenafil and apply to both generic sildenafil and branded Viagra, as both contain the same active ingredient at the same licensed strengths.
The right strength for you depends on your individual health, medical history, current medicines, and how you respond to treatment. Your prescriber will assess these factors during a clinical consultation rather than selecting a dose based on a table alone.
How a starting dose is typically decided
For many adult men without significant health concerns that may affect treatment, 50mg is commonly used as a starting strength for sildenafil. This provides a balance between effectiveness and tolerability for many patients.
A lower starting strength, such as 25mg, may be considered for some people, including older adults, those with certain kidney or liver conditions, or people taking medicines that may interact with sildenafil. These factors can affect how the body processes the medicine.
This is general information only and not a personal recommendation. Your prescriber will decide the most suitable starting strength based on your individual circumstances.
Adjusting the dose over time
Whether your sildenafil dose needs to change depends on how well you respond to treatment and whether you experience any side effects. Any dose adjustment should be discussed with your prescriber rather than changed independently.
In general, sildenafil doses may be adjusted within the licensed range if the current dose is not providing the desired effect or if side effects need to be reduced. Sildenafil should not be taken more than once within a 24 hour period, regardless of the strength.
If the prescribed dose is not giving the expected results, speak with your prescriber about your options rather than taking more than recommended.
Timing, food and how absorption is affected
Sildenafil is usually taken before planned sexual activity. Product information recommends taking it around 30 to 60 minutes before sexual activity, although the exact timing and response can vary between individuals.
Food can affect how quickly sildenafil is absorbed. A heavy or high fat meal around the same time may delay how quickly it starts to work. Taking sildenafil on an empty stomach or after a lighter meal may help provide a more predictable response.
Sildenafil does not cause sexual arousal on its own. It works by supporting the natural process involved in achieving an erection when sexual stimulation is present.
This information explains how sildenafil generally works, but it does not replace the instructions provided with your medicine or advice from your healthcare professional.
Generic sildenafil versus branded Viagra
Generic sildenafil and Viagra contain the same active ingredient, sildenafil citrate, at the same licensed strengths. Both must meet UK regulatory standards for quality, safety and effectiveness.
The main differences are usually the brand name and price rather than how the medicine works. Since Viagra's patent protection has ended, generic versions are often available at a lower cost while meeting the same regulatory requirements.
For many patients, choosing between generic sildenafil and Viagra may come down to personal preference and cost. Both require appropriate clinical assessment before being supplied.
Other treatment options worth knowing about
Sildenafil is not the only prescription treatment available for erectile dysfunction, and it may not be suitable for everyone.
Tadalafil, available under brand names including Cialis, works in a similar way but generally lasts longer in the body, which some patients may prefer. Vardenafil and avanafil, available under brands such as Levitra and Spedra, are other treatment options with different timing and dosing considerations.
Viagra Connect is a sildenafil product that may be available from UK pharmacies following a pharmacist suitability assessment for men who meet specific criteria.
Your prescriber will help decide which treatment option is most suitable based on your health, preferences and individual circumstances. This page cannot determine which option is right for you without a clinical assessment.
Who should speak to a prescriber before starting sildenafil
Sildenafil is not suitable for everyone, which is why a clinical assessment is important before starting treatment. It should not be taken with nitrate medicines, which are commonly used for angina, as the combination can cause a significant drop in blood pressure.
Extra care may be needed if you take alpha blockers, which are sometimes prescribed for high blood pressure or prostate conditions, as taking them with sildenafil may also lower blood pressure.
Your prescriber may also need to consider your overall health, including any history of heart conditions, uncontrolled high or low blood pressure, liver or kidney problems, or other medical conditions that may affect how sildenafil works in your body.
Certain blood disorders, including sickle cell anaemia, multiple myeloma or leukaemia, may increase the risk of a prolonged erection, so these conditions should be discussed during your assessment. If you have a history of retinitis pigmentosa, an inherited eye condition, mention this to your prescriber before starting treatment.
This list is not complete and does not replace the full safety information provided with your medicine. It explains why sildenafil should be prescribed following a proper clinical assessment rather than purchased without medical review.
Serious warning signs to know
Most people do not experience serious problems with sildenafil, but some symptoms need urgent medical attention.
Seek medical help immediately if you experience:
Chest pain during sexual activity, as this may be a sign of a heart problem. Do not use nitrate medicines for chest pain while sildenafil is still in your system unless advised by a healthcare professional.
An erection lasting more than 4 hours. This is known as priapism and requires urgent treatment to reduce the risk of lasting damage.
A sudden loss or decrease in vision or hearing. These are rare but serious symptoms and should be checked by a healthcare professional promptly.
If you experience any severe, unexpected, or worrying symptoms after taking sildenafil, seek medical advice rather than taking another dose until you have spoken to a healthcare professional.
Interactions worth knowing about
Sildenafil can interact with some medicines and substances, which is why it is important to tell your prescriber about everything you take, including over the counter medicines and supplements.
Grapefruit juice may affect how sildenafil is broken down by the body and could increase the amount of medicine in your system. It is generally best to avoid grapefruit juice around the time you take sildenafil unless your healthcare professional advises otherwise.
Drinking large amounts of alcohol may affect blood pressure, reduce how well sildenafil works, and may also affect sexual performance separately from the medicine itself.
It is also important to remember that sildenafil and other erectile dysfunction treatments do not protect against sexually transmitted infections (STIs). Usual protection methods are still needed.
Common side effects
Like any prescription medicine, sildenafil can cause side effects. The most commonly reported side effects include headache, flushing, indigestion and a blocked or stuffy nose. These effects are usually mild and are linked to how sildenafil affects blood vessels throughout the body.
Some people may also experience visual changes, such as a temporary blue tint to vision or increased sensitivity to light.
Most side effects are temporary and improve over time. However, seek urgent medical advice if you experience severe symptoms, sudden loss of vision, sudden hearing changes, or an erection lasting more than 4 hours.
How the assessment and ordering process works
Complete an online consultation. This covers your medical history, current medications and general health information, and usually takes only a few minutes to complete.
Clinical review. A UK registered prescriber reviews your information and assesses whether sildenafil is suitable for you, including which strength may be appropriate based on your circumstances.
Choose branded or generic. Both options are available in the strengths listed above and are dispensed through a GPhC registered pharmacy once your assessment has been approved.
Delivery. Your prescription is then dispatched in discreet packaging.
Sildenafil is not suitable for everyone. If a prescriber decides it is not the right option for you, they may discuss alternative treatments that are more appropriate for your health needs.
What sildenafil and other ED treatments typically cost
The cost of erectile dysfunction treatments can vary depending on the medicine, whether you choose a branded or generic option, and the quantity prescribed. The prices below are examples of costs that may be available through UK online pharmacies.
| Treatment | Typical starting price |
|---|---|
| Sildenafil (generic) | From £3.99 |
| Viagra Connect | From £21.99 |
| Cialis Together | From £16.99 |
| Spedra | From £22.99 |
| Viagra (branded) | From £27.99 |
| Vardenafil | From £27.99 |
| Levitra | From £32.99 |
| Tadalafil (generic) | From £34.99 |
| Cialis | From £44.99 |
| Cialis Daily | From £89.99 |
Live pricing at checkout always takes precedence over the figures shown here, and the exact price you pay depends on strength and quantity selected during your consultation.
Why one starting dose does not suit everyone
Standard dosing guidance is based on how sildenafil works for many people, but the right strength can vary from person to person. This is why a clinical assessment is important before starting treatment rather than choosing a dose based on general information alone.
Factors such as age, kidney and liver function, other medicines you take, and your overall health can affect how your body processes sildenafil. This means a strength that works well for one person may not be the most suitable option for someone else.
The different sildenafil strengths exist because individual needs and responses can vary. Your prescriber will consider your circumstances and recommend the most appropriate option for you.
How sildenafil compares with longer acting options
A common question when comparing erectile dysfunction treatments is how sildenafil differs from longer acting options such as tadalafil.
Sildenafil is usually taken before planned sexual activity and generally works for several hours. Tadalafil has a longer duration of action, which some people prefer because it can offer more flexibility around timing. It is also available as a lower strength daily treatment option for some patients.
Neither option is better for everyone. The most suitable treatment depends on your health, preferences, and the advice of your prescriber following a clinical assessment.
Storing and handling your medication
Sildenafil tablets should be stored according to the instructions provided in the patient information leaflet. They are generally kept at room temperature, away from direct heat, moisture and sunlight, and in their original packaging until needed.
Always check the expiry date before use and avoid taking tablets that are out of date. If you have leftover sildenafil that you no longer need, your local pharmacy can advise you on safe disposal.
