Sildenafil is a widely prescribed medication for erectile dysfunction (ED), helping millions of men regain sexual confidence and quality of life. As a phosphodiesterase type 5 (PDE5) inhibitor, it works by enhancing blood flow to the penis during sexual arousal. Whether you're considering sildenafil for the first time or seeking to understand how it fits into your treatment plan, knowing what to expect—from timing and effectiveness to potential side effects—is essential. This guide provides clear, evidence-based information aligned with UK clinical guidance to help you use sildenafil safely and effectively.

What Is Sildenafil and How Does It Work?

Sildenafil is a prescription medicine primarily used to treat erectile dysfunction (ED) in adult men. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. Originally developed for cardiovascular conditions, sildenafil was found to be highly effective for ED and has been licensed for this indication since the late 1990s. It is also prescribed under different brand names for pulmonary arterial hypertension, though this article focuses on its use for erectile dysfunction. Sildenafil is not indicated for women or those under 18 years of age for ED.

The mechanism of action centres on improving blood flow to the penis. During sexual arousal, the body releases nitric oxide in the erectile tissue, which activates an enzyme called guanylate cyclase. This leads to increased levels of cyclic guanosine monophosphate (cGMP), a chemical messenger that relaxes smooth muscle in the blood vessels of the penis, allowing increased blood flow and facilitating an erection. However, another enzyme—PDE5—breaks down cGMP, which can limit this process.

Sildenafil works by selectively inhibiting PDE5, thereby preventing the breakdown of cGMP. This allows cGMP levels to remain elevated for longer, enhancing the natural erectile response to sexual stimulation. It is important to understand that sildenafil does not cause an erection on its own; sexual arousal is still required for the medication to be effective.

In the UK, sildenafil is available on prescription from GPs and specialist clinics. Additionally, sildenafil 50 mg (Viagra Connect) is available as a Pharmacy (P) medicine from community pharmacies without a prescription, following an appropriate assessment by a pharmacist. The Medicines and Healthcare products Regulatory Agency (MHRA) regulates its supply to ensure patient safety. Sildenafil is typically available in tablet form at doses of 25 mg, 50 mg, and 100 mg. According to NICE Clinical Knowledge Summaries (CKS) and the British National Formulary (BNF), the standard starting dose is 50 mg, taken approximately one hour before sexual activity. However, a 25 mg starting dose should be considered for men aged 65 years and over, those with hepatic impairment, severe renal impairment, or those taking potent CYP3A4 inhibitors (such as ritonavir, erythromycin, or ketoconazole). The maximum dose is 100 mg, and sildenafil should not be taken more than once daily.

Sildenafil is contraindicated in men taking nitrate medications (often prescribed for angina) or guanylate cyclase stimulators such as riociguat, as these combinations can cause a severe and potentially life-threatening drop in blood pressure. Men taking alpha-blockers for prostate problems or hypertension should be stable on their alpha-blocker therapy before starting sildenafil; a lower starting dose (25 mg) and dose separation may be advised to reduce the risk of postural hypotension. Always inform your doctor or pharmacist of all medications and supplements you are taking.

What to Expect When Taking Sildenafil for the First Time

When taking sildenafil for the first time, it is natural to have questions about what the experience will be like. Understanding what to expect can help reduce anxiety and improve outcomes. Most men are prescribed an initial dose of 50 mg, to be taken approximately one hour before anticipated sexual activity. Your doctor or pharmacist may adjust this dose based on effectiveness, tolerability, and individual factors such as age, liver or kidney function, and other medications. For example, a 25 mg starting dose is recommended for older adults (aged 65 and over), those with liver or kidney impairment, or those taking potent CYP3A4 inhibitors.

Sildenafil should be swallowed whole with water and can be taken with or without food. However, it is worth noting that high-fat meals may delay absorption, potentially slowing the onset of action. For optimal results, many clinicians advise taking the tablet on an empty stomach or after a light meal. Limiting alcohol intake is also recommended, as alcohol can impair the effectiveness of sildenafil and increase the risk of side effects such as dizziness and low blood pressure. Avoid grapefruit juice, as it may increase sildenafil levels in the blood and raise the risk of side effects.

It is essential to have realistic expectations. Sildenafil facilitates the physiological process of achieving an erection but requires sexual stimulation to work. It will not produce spontaneous erections or increase sexual desire (libido). Some men find that the medication works well on the first attempt, while others may need to try it on several occasions. According to NICE CKS and guidance from the British Society for Sexual Medicine (BSSM), sildenafil should be tried on at least six to eight separate occasions before concluding it is ineffective, as response can improve with repeated use and reduced anxiety.

During your first experience, you may notice some mild side effects such as facial flushing, headache, or nasal congestion. These are common and usually transient. It is important to avoid taking sildenafil more than once in a 24-hour period. Do not combine it with other PDE5 inhibitors or recreational drugs known as 'poppers' (nitrates), as this can cause a dangerous drop in blood pressure.

If you are taking alpha-blockers (for prostate problems or high blood pressure), ensure you are stable on this medication before starting sildenafil. Your doctor may recommend a lower starting dose (25 mg) and advise separating the doses of the two medicines by several hours to minimise the risk of postural hypotension (a sudden drop in blood pressure on standing). If you are taking potent CYP3A4 inhibitors such as ritonavir, erythromycin, or ketoconazole, your doctor will adjust your sildenafil dose accordingly—often to a maximum of 25 mg, and in the case of ritonavir, no more than 25 mg within 48 hours.

If you have any concerns about your first dose—such as pre-existing health conditions, current medications, or unexpected reactions—contact your GP, pharmacist, or NHS 111 for advice. They can provide reassurance and adjust your treatment plan if necessary. In the UK, if you obtain sildenafil as Viagra Connect from a pharmacy, the pharmacist will conduct an assessment and provide tailored counselling.

How Long Does Sildenafil Take to Work and How Long Does It Last?

One of the most common questions about sildenafil concerns timing: how quickly it works and how long its effects last. Understanding the pharmacokinetics of the drug can help you plan appropriately and set realistic expectations.

Onset of action typically occurs within 30 to 60 minutes after taking the tablet, with peak plasma concentrations and maximum effect usually reached at around 60 minutes. However, this can vary between individuals depending on factors such as age, metabolism, whether food has been consumed, and overall health. For some men, effects may be noticed as early as 20 minutes, while for others it may take up to 90 minutes. As mentioned, taking sildenafil with a heavy or high-fat meal can delay absorption and slow the onset. Alcohol may also reduce the effectiveness of the medication.

The duration of action is generally around four to six hours. Sildenafil has a half-life of approximately four hours, meaning the drug is gradually metabolised and eliminated from the body, primarily via the liver and kidneys. This does not mean you will have an erection for this entire period; rather, it is the window during which sildenafil can facilitate an erection in response to sexual stimulation. After this time, the effects diminish.

It is important to note that sildenafil does not need to be taken daily. It is an on-demand treatment, used only when sexual activity is anticipated. This differs from some other ED treatments that are taken regularly. The flexibility of sildenafil allows men to use it as needed, which many find convenient.

If you find that sildenafil is not working as expected after trying it on at least six to eight separate occasions, do not increase the dose without consulting your doctor. Your clinician may consider titrating the dose to 100 mg if the 50 mg dose is ineffective and well tolerated. There may also be underlying factors affecting efficacy, such as psychological issues, hormonal imbalances, or cardiovascular conditions that require further investigation. Erectile dysfunction can be an early marker of cardiovascular disease or diabetes, and NICE CKS recommends a holistic approach to ED management, including cardiovascular risk assessment, lifestyle modifications (such as weight loss, exercise, smoking cessation, and reducing alcohol intake), psychological support, or alternative medications where appropriate.

Should you experience an erection lasting longer than four hours (priapism), this is a medical emergency. Seek immediate medical attention by calling 999 or going to A&E, as urgent treatment is required to prevent permanent damage to the penis.

Common Side Effects and What to Expect During Treatment

Like all medications, sildenafil can cause side effects, although not everyone will experience them. Most side effects are mild to moderate, transient, and resolve without intervention. Being aware of what to expect can help you distinguish between normal reactions and those requiring medical attention.

Common side effects include:

• Headache – often mild and related to the vasodilatory (blood vessel-widening) effects of the drug

• Facial flushing – a warm sensation or redness in the face and neck

• Indigestion or dyspepsia – mild stomach discomfort

• Nasal congestion – a blocked or runny nose

• Dizziness – particularly when standing up quickly; this risk is increased if you are also taking alpha-blockers

• Visual disturbances – such as a blue tinge to vision, increased sensitivity to light, or blurred vision

These effects are generally short-lived and diminish as your body adjusts to the medication. Staying well-hydrated and avoiding alcohol can help minimise some of these symptoms.

Serious side effects require urgent medical attention. You should call 999 or go to A&E immediately if you experience:

• Chest pain, especially during or after sexual activity

• Sudden loss of vision in one or both eyes (this may be a sign of non-arteritic anterior ischaemic optic neuropathy, or NAION, a rare but serious condition). Stop taking sildenafil immediately.

• Severe allergic reaction (anaphylaxis) – symptoms include difficulty breathing, swelling of the face, lips, tongue, or throat, and severe skin rash

Seek urgent same-day medical advice (contact your GP or NHS 111) if you experience:

• Sudden loss or decrease in hearing, which may be accompanied by tinnitus (ringing in the ears) or dizziness. Stop taking sildenafil.

• Severe dizziness or fainting

• Palpitations or irregular heartbeat

If you experience an erection lasting longer than four hours (priapism), this is a medical emergency. Go to A&E immediately, as urgent treatment is needed to prevent permanent damage.

Sildenafil is contraindicated in men taking nitrate medications (often prescribed for angina) or riociguat (a guanylate cyclase stimulator), as these combinations can cause a severe and potentially life-threatening drop in blood pressure. Men taking alpha-blockers should be stable on their medication and may require a lower starting dose of sildenafil and dose separation to reduce the risk of postural hypotension. Potent CYP3A4 inhibitors (such as ritonavir, erythromycin, ketoconazole, and itraconazole) can increase sildenafil levels in the blood, necessitating dose adjustment. Always inform your doctor or pharmacist of all medications and supplements you are taking.

Certain groups require careful monitoring, including those with cardiovascular disease, liver or kidney impairment, anatomical abnormalities of the penis, or conditions that predispose to priapism (such as sickle cell anaemia, multiple myeloma, or leukaemia). Your prescriber will assess your suitability before initiating treatment.

If side effects are troublesome or persistent, stop taking sildenafil and seek medical advice. For serious symptoms, seek urgent care. Because sildenafil is an on-demand medication, it is safe to stop at any time. Your healthcare provider can discuss alternatives, such as dose adjustment or switching to a different PDE5 inhibitor.

You can report suspected side effects via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or by downloading the Yellow Card app. Reporting helps improve the safety monitoring of medicines for everyone.

Overall, sildenafil is well-tolerated and effective for the majority of men with ED. With proper use and medical guidance, it can significantly improve quality of life and sexual confidence. For further information, consult the patient information leaflet, speak to your GP or pharmacist, or refer to trusted UK sources such as the Electronic Medicines Compendium (EMC), the British National Formulary (BNF), NICE Clinical Knowledge Summaries, and NHS medicines A–Z.

Frequently Asked Questions