Sildenafil is a widely prescribed treatment for erectile dysfunction (ED) in the UK, available in three standard strengths: 25 mg, 50 mg, and 100 mg. Choosing the appropriate strength depends on multiple factors including your overall health, other medications, and how you respond to treatment. Most men begin with 50 mg, which can be adjusted based on effectiveness and tolerability. Understanding which strength is right for you requires a comprehensive assessment by a qualified healthcare professional who will consider your individual circumstances, medical history, and any potential drug interactions to ensure safe and effective treatment.

What Strengths of Sildenafil Are Available in the UK?

Sildenafil is licensed in the UK for the treatment of erectile dysfunction (ED) and is available in three standard tablet strengths: 25 mg, 50 mg, and 100 mg. These formulations are approved by the Medicines and Healthcare products Regulatory Agency (MHRA) and are available both as the branded product Viagra and as generic sildenafil citrate tablets.

The 50 mg strength is typically considered the standard starting dose for most men with erectile dysfunction. It is important to note that Viagra Connect 50 mg is available as a Pharmacy (P) medicine, meaning you can obtain it from a pharmacy without a prescription following an assessment by a pharmacist. The 25 mg and 100 mg strengths remain prescription-only medicines (POM) and require a consultation with a GP or other qualified prescriber.

The 25 mg dose is generally reserved for patients who may be more sensitive to the medication, those with certain health conditions affecting drug clearance, or those taking other medicines that interact with sildenafil. The 100 mg strength represents the maximum recommended dose and is used when lower doses have proven insufficient.

Sildenafil is also available in a 20 mg strength, but this formulation is specifically licensed for pulmonary arterial hypertension (PAH) under the brand name Revatio, rather than erectile dysfunction. Patients should not substitute PAH formulations for ED treatment without medical guidance, as the dosing regimens and treatment goals differ significantly.

All strengths of sildenafil work through the same mechanism of action—inhibiting phosphodiesterase type 5 (PDE5) to enhance blood flow to the penis during sexual stimulation. The choice of strength primarily affects the likelihood and intensity of response and the risk of side effects, rather than materially altering the duration of action, which is typically around four hours across all doses. A healthcare professional will help determine the appropriate strength for your individual circumstances based on your health status, other medications, and response to treatment.

How to Determine the Right Sildenafil Dose for You

Determining the appropriate sildenafil dose requires a comprehensive assessment by a qualified healthcare professional, typically your GP, a pharmacist (for Viagra Connect 50 mg), or a specialist in sexual health. The selection process begins with a thorough medical history, including details about your erectile dysfunction symptoms, their duration and severity, and any underlying health conditions that may contribute to the problem.

Key factors your healthcare professional will consider include:

• Your overall health status and cardiovascular fitness

• Current medications, particularly nitrates (including GTN, isosorbide mononitrate, and recreational 'poppers'), riociguat, alpha-blockers, or other drugs that may interact with sildenafil

• Presence of conditions such as diabetes, hypertension, or cardiovascular disease

• Liver or kidney function, as these organs metabolise and eliminate the drug

• Previous experience with ED treatments, if applicable

Your healthcare professional will also assess whether sildenafil is safe and appropriate for you. Sildenafil must not be used if you are taking nitrate medicines for angina or chest pain (including GTN, isosorbide mononitrate, or isosorbide dinitrate) or recreational nitrates such as 'poppers' (amyl nitrite), as this combination can cause a dangerous drop in blood pressure. Sildenafil is also contraindicated with riociguat (used for pulmonary hypertension). Men with certain cardiovascular conditions where sexual activity is inadvisable, severe hypotension, recent stroke or heart attack, or severe liver impairment require careful clinical assessment before treatment can be considered.

A careful medication review is essential, as drug interactions can significantly affect both the safety and efficacy of treatment. The NICE Clinical Knowledge Summary on erectile dysfunction recommends that treatment should be part of a holistic approach addressing lifestyle factors such as smoking, alcohol consumption, obesity, and psychological wellbeing. Your healthcare provider may suggest addressing these factors alongside pharmacological treatment.

Your prescriber or pharmacist will explain how to take sildenafil correctly—typically one hour before sexual activity. Sildenafil can be taken with or without food, but high-fat meals can delay the onset of effect and reduce peak response, so it may be advisable to avoid heavy meals close to the time you take the tablet. This individualised approach ensures that the chosen dose balances effectiveness with safety and tolerability.

Starting Doses and Adjusting Sildenafil Strength

For most men initiating sildenafil treatment for erectile dysfunction, the recommended starting dose is 50 mg, taken approximately one hour before anticipated sexual activity. This dose should not be taken more than once in a 24-hour period. The 50 mg starting dose represents a balance between efficacy and tolerability for the majority of patients, allowing healthcare providers to assess response and adjust accordingly.

After trying the initial dose on several separate occasions (typically at least four to eight attempts), you and your prescriber can evaluate whether the dose is effective. If the 50 mg dose produces a satisfactory response with minimal side effects, no adjustment is necessary. However, if the effect is insufficient, your doctor may increase the dose to 100 mg—the maximum recommended dose for erectile dysfunction. Conversely, if you experience troublesome side effects at 50 mg, the dose may be reduced to 25 mg.

Dose adjustments may be necessary for specific patient groups:

• Older adults (over 65 years): Consider starting at 25 mg due to higher drug exposure with age-related changes in metabolism

• Patients with hepatic impairment: Typically commence with 25 mg due to reduced drug clearance

• Patients with severe renal impairment: May require dose reduction to 25 mg

• Those taking potent CYP3A4 inhibitors (such as erythromycin, ketoconazole, itraconazole, or ritonavir): Should start with 25 mg due to increased sildenafil levels. If taking ritonavir, do not exceed 25 mg within 48 hours

• Patients taking alpha-blockers for benign prostatic hyperplasia or hypertension: Sildenafil should only be initiated once the patient is stable on alpha-blocker therapy, and treatment should begin with 25 mg to minimise the risk of symptomatic hypotension

It is crucial to allow adequate opportunity—trying the dose on several separate occasions—to properly assess the effect of each strength before concluding that it is ineffective. Patients should never adjust their dose independently or exceed the maximum recommended dose of 100 mg, as this increases the risk of adverse effects without providing additional benefit. If sildenafil proves ineffective at the maximum dose after appropriate trials, your GP may consider alternative treatments or refer you to a specialist for further evaluation.

Factors That Affect Which Sildenafil Strength You Need

Multiple physiological, pharmacological, and lifestyle factors influence the optimal sildenafil strength for individual patients. Understanding these variables helps explain why a standardised approach is inappropriate and why personalised prescribing is essential.

Age and metabolism play significant roles in drug processing. As we age, hepatic and renal function typically decline, reducing the body's ability to metabolise and eliminate sildenafil. This can lead to higher drug concentrations and prolonged effects, which is why a lower starting dose may be considered in older men. The half-life of sildenafil averages four hours but may be extended in elderly patients or those with organ impairment.

Concurrent medications represent a critical consideration. Drugs that inhibit the CYP3A4 enzyme system—including certain antibiotics (erythromycin, clarithromycin), antifungals (ketoconazole, itraconazole), and HIV protease inhibitors (especially ritonavir)—significantly increase sildenafil blood levels, requiring dose reduction to 25 mg. With ritonavir, the maximum dose is 25 mg within 48 hours. Alpha-blockers used for benign prostatic hyperplasia or hypertension can cause additive blood pressure lowering when combined with sildenafil. Sildenafil should only be started once a patient is stable on alpha-blocker therapy, and the lowest dose (25 mg) should be used initially.

Underlying health conditions substantially impact dose selection. Men with cardiovascular disease require careful assessment, as sexual activity itself carries cardiovascular demands. Those with diabetes often have more severe ED due to both vascular and neurological complications. Patients with liver cirrhosis or severe kidney disease require lower doses due to impaired drug clearance and higher drug exposure.

Lifestyle factors including diet, alcohol consumption, and timing of administration affect sildenafil absorption and efficacy. High-fat meals significantly delay absorption and reduce peak concentrations. Rather than increasing the dose, it is advisable to avoid heavy meals close to the time you take sildenafil or to allow more time for the medication to take effect. Excessive alcohol consumption can impair erectile function independently and may reduce sildenafil effectiveness.

Important safety advice: If you experience any of the following, stop taking sildenafil and seek urgent medical attention:

• Chest pain during or after sexual activity

• A prolonged erection lasting more than four hours (priapism)

• Sudden loss of vision in one or both eyes (which may indicate non-arteritic anterior ischaemic optic neuropathy, or NAION)

• Sudden decrease or loss of hearing

Regular review with your healthcare provider ensures your treatment remains appropriate, effective, and safe as your circumstances change. If you experience any side effects, whether listed in the patient information leaflet or not, you can report them via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or by searching for 'Yellow Card' in the Google Play or Apple App Store. Reporting side effects helps provide important safety information about medicines.

Frequently Asked Questions