Sildenafil is a widely prescribed medicine for erectile dysfunction (ED) in the UK, available as branded Viagra or generic formulations. Understanding the correct daily dosage is essential for safe and effective treatment. The standard starting dose is 50 mg, taken as needed approximately one hour before sexual activity, with a maximum of 100 mg in any 24-hour period. Sildenafil should not be taken more than once per day. Your GP will tailor the dose to your individual health needs, considering factors such as age, kidney or liver function, and other medications. This article explains recommended dosing, safety limits, and when to seek medical advice.

What Is Sildenafil and How Does It Work?

Sildenafil is a medicine primarily used to treat erectile dysfunction (ED) in adult men. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. In the UK, sildenafil is available under the brand name Viagra as a prescription medicine, and as Viagra Connect 50 mg which can be supplied by pharmacists without a prescription following a consultation. Generic versions are also approved by the Medicines and Healthcare products Regulatory Agency (MHRA).

The mechanism of action involves selective inhibition of the PDE5 enzyme, which is found predominantly in the smooth muscle of the corpus cavernosum of the penis. During sexual stimulation, nitric oxide is released in the erectile tissue, which activates an enzyme that produces cyclic guanosine monophosphate (cGMP). This chemical messenger relaxes smooth muscle and increases blood flow to the penis, facilitating an erection. PDE5 normally breaks down cGMP, so by inhibiting this enzyme, sildenafil allows cGMP levels to remain elevated for longer, thereby enhancing erectile function.

It is important to note that sildenafil does not cause an erection without sexual stimulation — it simply makes it easier to achieve and maintain one when aroused. The medication typically begins to work within 30 to 60 minutes of taking it, with effects lasting for approximately four to five hours. High-fat meals can delay the onset of action, and excessive alcohol can reduce effectiveness and increase the risk of side effects. Sildenafil is also licensed at different doses for the treatment of pulmonary arterial hypertension, though this article focuses on its use for erectile dysfunction.

Sildenafil should only be taken following consultation with a healthcare professional or pharmacist assessment, as it may not be suitable for everyone and can interact with other medications, particularly nitrates used for chest pain. It is not indicated for use in women or children.

Recommended Daily Sildenafil Dosage in the UK

In the UK, the standard starting dose of sildenafil for erectile dysfunction is 50 mg, taken approximately one hour before sexual activity. Your GP or prescriber will assess your individual circumstances before determining the most appropriate dose for you.

Depending on efficacy and tolerability, the dose may be adjusted. If the 50 mg dose is effective but you experience side effects, your doctor may reduce the dose to 25 mg. Conversely, if 50 mg does not produce the desired effect and is well tolerated, the dose may be increased to 100 mg, which is the maximum recommended single dose for erectile dysfunction.

Sildenafil is intended to be taken on an as-needed basis, not as a daily medication for most men with ED. This means you should only take it when you plan to engage in sexual activity, and not more than once in a 24-hour period. Taking sildenafil daily without medical supervision is not recommended and may increase the risk of adverse effects.

For men with certain health conditions — such as severe kidney or liver impairment — or those taking medications that interact with sildenafil, a lower starting dose of 25 mg is usually advised. If you are taking potent CYP3A4 inhibitors (such as ritonavir), co-administration is generally not recommended; if unavoidable, the maximum dose should be restricted to 25 mg within a 48-hour period.

If you are taking alpha-blockers (e.g., doxazosin for prostate enlargement), you should be stable on this treatment before starting sildenafil. A starting dose of 25 mg is recommended, and doses should ideally be separated to reduce the risk of blood pressure dropping too low.

Older adults (over 65 years) may also be started on a lower dose due to altered drug metabolism. Always follow your prescriber's instructions and do not adjust your dose without consulting them first. High-fat meals can delay the absorption of sildenafil, and you should avoid grapefruit juice as it can increase blood levels of the drug.

Maximum Safe Amount of Sildenafil Per Day

The maximum recommended dose of sildenafil for erectile dysfunction is 100 mg in a 24-hour period. This limit is based on clinical trial data and pharmacokinetic studies that have established both the efficacy and safety profile of the medication. Exceeding this dose does not improve effectiveness and significantly increases the risk of adverse effects.

Taking more than 100 mg of sildenafil in one day is not advised and is considered off-label use that lacks supporting evidence for safety or additional benefit. Higher doses have been associated with a greater incidence of side effects, including headaches, flushing, visual disturbances, nasal congestion, and dyspepsia (indigestion). More serious adverse effects, such as priapism (a prolonged and painful erection lasting more than four hours) and cardiovascular events, are also more likely at higher doses.

It is crucial to understand that sildenafil should be taken no more than once per day. The drug has a half-life of approximately three to five hours, meaning it takes time for your body to metabolise and eliminate it. Taking multiple doses within 24 hours increases the risk of side effects and is not more effective.

Sildenafil must never be taken with:

• Nitrates (e.g., glyceryl trinitrate, isosorbide mononitrate) used for angina

• Nicorandil (a potassium channel activator with nitrate-like effects)

• Recreational 'poppers' (amyl nitrite)

• Riociguat (used for pulmonary hypertension)

• Other PDE5 inhibitors

These combinations can cause dangerous drops in blood pressure. If you develop chest pain after taking sildenafil, call 999 immediately and inform the medical team that you have taken sildenafil, as nitrate medications should not be given.

If you feel that the maximum dose of 100 mg is insufficient, do not take additional tablets or split doses to 'top up'. Instead, consult your GP or a specialist in sexual health. There may be underlying factors contributing to erectile dysfunction that require further investigation, or alternative treatments may be more appropriate.

Factors That Affect Your Sildenafil Dose

Several individual factors influence the appropriate dose of sildenafil for each patient. Your GP will consider these when prescribing the medication to ensure both safety and efficacy.

Age is an important consideration. Men over 65 years may metabolise sildenafil more slowly, leading to higher plasma concentrations and a prolonged duration of action. A starting dose of 25 mg is often recommended for older adults to minimise the risk of side effects.

Kidney and liver function significantly affect how sildenafil is processed in the body. Severe renal impairment (creatinine clearance <30 mL/min) or moderate to severe hepatic impairment can reduce drug clearance, necessitating a lower starting dose of 25 mg. Your doctor may request blood tests to assess organ function before prescribing sildenafil.

Concurrent medications can interact with sildenafil, particularly:

• Nitrates (e.g., glyceryl trinitrate, isosorbide mononitrate), nicorandil, and recreational 'poppers' — absolutely contraindicated due to risk of severe hypotension

• Riociguat — contraindicated due to risk of hypotension

• Alpha-blockers (e.g., doxazosin) — may cause additive blood pressure lowering; ensure stability on alpha-blocker before starting sildenafil

• CYP3A4 inhibitors (e.g., erythromycin, ketoconazole, ritonavir) — increase sildenafil levels, requiring dose reduction

• CYP3A4 inducers (e.g., rifampicin, carbamazepine, phenytoin) — may reduce sildenafil effectiveness

• Grapefruit juice — can increase sildenafil blood levels and should be avoided

• Other PDE5 inhibitors — should never be combined with sildenafil

Cardiovascular health is another critical factor. Men with unstable angina, recent heart attack or stroke (within six months), uncontrolled hypertension, or severe heart failure may not be suitable candidates for sildenafil. A cardiovascular assessment may be necessary before starting treatment, as erectile dysfunction can be an early marker of cardiovascular disease.

Other conditions requiring caution include retinitis pigmentosa, anatomical penile deformities, and conditions predisposing to priapism (e.g., sickle cell anaemia, leukaemia, multiple myeloma).

Body weight and overall health status can also influence dosing, though to a lesser extent. Your prescriber will take a comprehensive medical history to determine the safest and most effective dose for your individual circumstances.

What Happens If You Take Too Much Sildenafil?

Taking more than the recommended dose of sildenafil increases the risk of adverse effects, some of which can be serious. Understanding the potential consequences of overdose is important for patient safety.

Common side effects that become more pronounced with higher doses include:

• Severe headaches

• Facial flushing and warmth

• Dizziness or light-headedness

• Nasal congestion

• Indigestion or dyspepsia

• Visual disturbances (blue-tinted vision, increased light sensitivity, blurred vision)

More serious complications of sildenafil overdose include:

Priapism — a prolonged erection lasting more than four hours that can be painful and may cause permanent damage to penile tissue if not treated promptly. This is a medical emergency requiring immediate attention at your nearest Accident & Emergency department.

Cardiovascular effects — excessive doses can cause significant drops in blood pressure (hypotension), leading to fainting, dizziness, or even cardiovascular collapse in vulnerable individuals. This risk is particularly high in men taking nitrates or those with pre-existing heart conditions.

Visual and auditory disturbances — rare cases of sudden vision loss (non-arteritic anterior ischaemic optic neuropathy or NAION) and sudden hearing loss or tinnitus have been reported in temporal association with PDE5 inhibitors, including sildenafil. While a causal relationship has not been definitively established, an association cannot be excluded. If you experience sudden vision or hearing loss, stop taking sildenafil immediately and seek urgent medical attention.

If you suspect you have taken too much sildenafil, contact NHS 111 immediately or attend your local A&E department. Do not wait for symptoms to develop. If you experience chest pain after taking sildenafil, call 999 and inform emergency services that you have taken sildenafil, as nitrates should not be administered. Bring the medication packaging with you so healthcare professionals know exactly what you have taken.

If you experience any side effects, talk to your doctor, pharmacist or nurse. This includes any possible side effects not listed in the package leaflet. You can also report side effects directly via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

When to Speak to Your GP About Sildenafil Dosing

There are several situations in which you should contact your GP or healthcare provider regarding your sildenafil prescription and dosing.

If sildenafil is not working effectively at your current dose, do not simply increase the amount you take. Your GP can review your treatment, assess whether the dose should be adjusted, or investigate other potential causes of erectile dysfunction. It's worth trying sildenafil on several separate occasions (4-8 attempts) before concluding it's ineffective, ensuring you're taking it correctly (on an empty stomach, with sufficient sexual stimulation). Factors such as uncontrolled diabetes, hormonal imbalances, psychological issues, or relationship difficulties may require additional management.

If you experience troublesome side effects, even at the recommended dose, speak to your doctor. They may suggest reducing the dose, trying a different PDE5 inhibitor, or exploring alternative treatment options. Common side effects often diminish with continued use, but persistent or severe reactions warrant medical review.

Before starting any new medications, inform your GP that you are taking sildenafil. This is particularly important for antibiotics, antifungals, HIV medications, and treatments for prostate problems, as these can interact with sildenafil and require dose adjustments.

If your health status changes — for example, if you develop heart problems, have a stroke, or are diagnosed with kidney or liver disease — you must inform your prescriber. These conditions may affect the safety of continuing sildenafil or necessitate a change in dosing.

If you are considering more regular treatment than occasional use, discuss this with your GP. While sildenafil is not licensed for daily use in erectile dysfunction, tadalafil (another PDE5 inhibitor) is available in 2.5 mg and 5 mg doses specifically licensed for once-daily administration. Your GP can advise whether this might be a more suitable option for you.

Erectile dysfunction can be an early marker of cardiovascular disease, so your GP may recommend a cardiovascular risk assessment (blood pressure, cholesterol, blood glucose) and possibly hormonal tests (such as morning testosterone levels) where appropriate.

NICE guidance emphasises the importance of regular review for men with erectile dysfunction to assess treatment response, monitor for adverse effects, and address any underlying health conditions. Your GP is your partner in managing your sexual health safely and effectively, so maintain open communication about your treatment.

Frequently Asked Questions