What is Sildenafil?
Sildenafil (sildenafil citrate) is a generic version of Viagra, which is a phosphodiesterase type 5 (PDE5) inhibitor. It is a prescription-only medication to treat erectile dysfunction (ED) and works by relaxing blood vessels and increasing blood flow to the penis. This helps men to achieve and maintain an erection when they are sexually stimulated.
Branded vs generic Viagra
Sildenafil (sildenafil citrate) is the active ingredient in the well-known ED medicine Viagra. Until 2013, Pfizer held exclusive rights to sildenafil. Once the patent expired, pharmaceutical companies were permitted to manufacture generic sildenafil, which contains the same active ingredient in the same strength as the original.
Generic versions of Sildenafil work in exactly the same way as the branded product by inhibiting the PDE-5 enzyme and enhancing blood flow to the penis. Generics provide identical clinical efficacy and safety in the treatment for erectile dysfunction. For this reason, sildenafil tablets are considered a true Viagra equivalent:
- Same active ingredient (sildenafil citrate) and same dose
- Same absorption and concentration profile
- Comparable safety and efficacy
Although tablet shape, colour and packaging vary, the clinical effect of sildenafil citrate remains the same. Ultimately, when it comes to the sildenafil vs. Viagra competition, it's your personal preferences and experience that matter most.
Sildenafil and Viagra - core differences
Is sildenafil the same as Viagra? Yes, pharmaceutically. However, there are still some differences:
- Sildenafil Price: Generic sildenafil tablets are cheaper than branded Viagra because generics do not require investment in research, development or branding.
- Appearance: Generic products differ in colour, shape, and imprint. Viagra is protected as a blue, diamond-shaped tablet. Generics can be round, oval, or other colours.
- Excipients: While the active ingredient is identical, excipients such as binders, colourants, or fillers may differ. Rarely, patients sensitive to certain excipients may experience reactions.
- Manufacturer: Sildenafil is produced by various pharmaceutical companies, and Viagra is exclusively produced by Pfizer.
Some generics may differ slightly in onset time by a few minutes, but overall efficacy in the treatment for erectile dysfunction remains the same.
How does Sildenafil work?
Sildenafil works by supporting the body's natural response to sexual arousal. Rather than triggering an erection directly, it enhances the biological process already set in motion, making it easier to achieve and maintain one when the moment is right.
PDE-5 inhibition mechanism of Sildenafil
When a man is sexually aroused, the body releases nitric oxide in the penis, which triggers the production of a chemical called cGMP. This chemical relaxes the smooth muscles and widens the blood vessels, allowing blood to flow in and create an erection. Normally, an enzyme called PDE5 breaks down cGMP, and ends the erection.
Sildenafil works by blocking PDE5, preventing it from doing so, which keeps cGMP levels elevated and allows the blood vessels to stay relaxed and the erection to be maintained. In simple terms, sildenafil doesn't create arousal on its own – it simply protects the body's natural erection-producing chemical from being broken down too soon, making it easier to achieve and sustain an erection when sexual stimulation is present. This core physiological mechanism is what makes sildenafil an effective treatment for erectile dysfunction of both organic and psychogenic origin.
Onset of action and duration of the Sildenafil effect
Sildenafil tablets are typically taken 30 to 60 minutes before sexual activity, with most men experiencing its effects within this window. The drug reaches peak concentration in the blood around 60 minutes after ingestion, though taking it with a high-fat meal can slow absorption and delay onset.
Once active, sildenafil's effects last for approximately 4 to 6 hours, during which achieving an erection in response to sexual stimulation becomes easier. Though it's worth noting that the erection itself is not continuous throughout this period, but rather responsive to arousal as needed.
Sexual stimulation requirement for erectile dysfunction treatment
Sildenafil is not an aphrodisiac and will not produce an erection on its own; sexual stimulation is absolutely necessary for the drug to be effective. It merely enhances the body's natural arousal response, so without mental or physical sexual stimulation, no erection will occur, regardless of the dose taken. This is because sildenafil only acts on the biochemical pathway that is activated during arousal, sustaining the process more effectively and for a longer period of time once it has been naturally triggered – making desire and stimulation an essential part of the equation.
How effective is Sildenafil for erectile dysfunction?
Sildenafil is one of the most extensively studied and clinically validated treatments for erectile dysfunction, with a strong and consistent evidence base built over more than two decades of real-world use and controlled trials.
Across large-scale clinical studies, sildenafil has demonstrated meaningful improvement in erectile function in approximately 70 to 80% of men with erectile dysfunction across a broad range of ages, underlying causes, and severity levels – making it the benchmark against which newer treatments are measured. [2]
Efficacy varies depending on the underlying cause of erectile dysfunction. In men with psychogenic or mixed-cause ED, response rates tend to be highest, often exceeding 80%. In men with ED secondary to more complex physiological conditions, such as diabetes, radical prostatectomy, or severe vascular disease, response rates are somewhat lower, typically ranging from 40 to 60%, though meaningful improvement is still achievable in the majority of cases. For men with diabetes-related ED specifically, clinical trials have reported success rates of around 57 to 63%, a significant finding given that this population historically responds less well to oral therapies.
Sildenafil dosage for ED: 25mg, 50mg and 100mg
Sildenafil tablets for erectile dysfunction are available in three standard doses: 25mg 50mg and 100mg.
Sildenafil 25mg tablets - starting low:
Recommended for men over 65, those with kidney or liver impairment, or anyone taking medications that may interact with sildenafil. This lower dose minimises the risk of side effects while still providing a therapeutic effect.
Sildenafil 50mg tablets - standard starting dose:
The most commonly prescribed starting dose for otherwise healthy men with erectile dysfunction. It strikes a balance between effectiveness and tolerability, and serves as the baseline from which a doctor may adjust depending on individual response.
Sildenafil 50mg tablets - maximum dose:
Prescribed when the 50 mg dose proves insufficient. This is the highest recommended dose and should only be taken under medical guidance, as it carries a higher likelihood of side effects such as flushing, headache, and visual disturbances.
General Rule:
Regardless of dose, sildenafil should be taken no more than once in a 24-hour period, approximately 30 to 60 minutes before sex.
How to take Sildenafil tablets to treat erectile dysfunction?
- Time of intake: 30-60 minutes before sexual intercourse
- Frequency: Maximum once daily
- Minimum interval: 24 hours between receipts
- Sildenafil dosage: Swallow the tablet whole with a glass of water.
- Meals: Can be taken with or without food, but high-fat meals delay the onset of action.
- Sildenafil and alcohol: While you may drink alcohol in moderation, combining large quantities with the treatment can increase the risk of side effects of sildenafil such as dizziness or low blood pressure.
Follow your doctor's instructions carefully. When used correctly, Sildenafil leads to successful erections in 70-80% of men.
Who can take Sildenafil tablets?
Adult men with erectile dysfunction
Sildenafil is indicated for adult men (above 18 years) who experience difficulty achieving or maintaining an erection sufficient for satisfactory sexual performance. It is suitable for a broad range of men with erectile dysfunction, including those whose condition stems from psychological, vascular, or mixed causes.
Men with diabetes or cardiovascular risk factors
Sildenafil tablets can be prescribed to men with underlying medical conditions such as type 2 diabetes or hypertension (high blood pressure) that contribute to erectile dysfunction, though dosage and safety must be carefully assessed by a doctor, given the potential for drug interactions and cardiovascular effects.
Men with pulmonary arterial hypertension (PAH)
Beyond ED, sildenafil (branded as Revatio) is also approved for men and women diagnosed with PAH. It helps reduce pressure in the lung arteries and improve exercise capacity.
Older men
Sildenafil can be taken by older men, though those aged 65 and above are typically started on the lower dose of Sildenafil 25mg tablets due to slower drug metabolism and a higher likelihood of co-existing health conditions or medications.
Men after prostate surgery
Sildenafil tablets are sometimes prescribed to men who experience erectile dysfunction following radical prostatectomy, as part of penile rehabilitation to support the recovery of erectile function.
When should Sildenafil not be used?
Absolute contraindications:
- Simultaneous intake of nitrates or nitric oxide donors (e.g., glyceryl trinitrate, isosorbide mononitrate, molsidomine, amyl nitrite) - life-threatening due to massive drop in blood pressure
- Concomitant use of guanylate cyclase stimulators such as riociguat (for pulmonary hypertension)
- Known hypersensitivity to sildenafil tablets or other ingredients
- Severe cardiovascular disease (heart disease), when sexual activity is not advisable
- Recently suffered a heart attack (within the last 3 months)
- Recent stroke (within the last 6 months)
- Severe liver failure (Child-Pugh class C)
- Low blood pressure below 90/50 mmHg
- Unstable angina pectoris
- Severe heart failure (NYHA class 4)
- Previous vision loss due to non-arteritic anterior ischemic optic neuropathy (NAION)
- Concomitant use of strong CYP3A4 inhibitors (ritonavir, ketoconazole, clarithromycin), as sildenafil levels can rise to dangerous levels
- Congenital degenerative retinal diseases, such as retinitis pigmentosa
Relative contraindications (caution advised):
- Anatomical deformities of the penis (e.g., angulation, cavernous fibrosis, Peyronie's disease)
- Diseases that can lead to priapism (e.g. sickle cell anaemia, leukaemia, multiple myeloma)
- Active stomach ulcers
- Blood clotting disorders (e.g., haemophilia)
- Severe renal impairment (dose adjustment may be required)
- Concomitant use of alpha-blockers (risk of hypotension unless blood pressure is stable)
Sildenafil is not approved for women or persons under 18 years of age.
What side effects can Sildenafil cause?
As with all medicines, Sildenafil can cause side effects, although not everyone experiences them. Most of these are mild to moderate and temporary, often improving as the body adjusts to the medication. The frequency of side effects is based on clinical studies and post-marketing data.
Most common side effects of Sildenafil
(may affect up to 1 in 10 patients)
The most frequently reported side effects of sildenafil are directly related to its vasodilatory mechanism – the same process that increases blood flow to the penis also affects blood vessels elsewhere in the body. Clinical studies indicate that these effects occur in roughly 10 to 25% of users at standard doses and tend to be dose-dependent, mild, and short-lived.
These include:
- Headache (most common) affects up to 16% of users, caused by increased blood flow and dilation of cerebral vessels.
- Facial flushing occurs in around 10 to 15% of men and presents as a sudden feeling of warmth or redness in the face and neck.
- Dyspepsia – indigestion or an upset stomach – affects around 7% of men, particularly when the drug is taken on a full or fatty meal.
- Nasal congestion (stuffy nose) or a runny nose is reported by approximately 4 to 10% of users due to vasodilation in the nasal mucosa.
- Dizziness and lightheadedness are also reported, typically as a result of a modest drop in blood pressure.
- Visual disturbances, including a bluish tinge to vision, increased light sensitivity, or blurred vision, occur in up to 3% of users and result from sildenafil's mild inhibition of PDE6, an enzyme found in the retina.
Less commonly (up to 1 in 100 patients), some men may experience:
- Nausea or vomiting
- Skin rash
- Muscle aches
- Eye irritation or redness
These effects are usually short-lived and tend to resolve without medical treatment.
Rare and serious side effects
(may affect fewer than 1 in 1,000 patients)
While uncommon, certain side effects of sildenafil require immediate medical attention and should never be dismissed.
Priapism: prolonged painful erections lasting more than four hours unrelated to sexual stimulation, is a urological emergency occurring in less than 1% of users. If left untreated, it can cause permanent damage to erectile tissue.
Sudden hearing loss or tinnitus has been reported in rare cases and may be linked to reduced blood flow to the auditory nerve.
Non-arteritic anterior ischemic optic neuropathy (NAION), a form of sudden vision loss caused by decreased blood flow to the optic nerve, has been associated with PDE5 inhibitor use, though a definitive causal link remains under investigation.
Severe hypotension – a dangerous drop in blood pressure – can occur, particularly when sildenafil tablets are combined with nitrates or alpha-blockers, and in rare cases may lead to fainting or cardiovascular events. Men with pre-existing heart conditions face a higher risk, and any chest pain, irregular heartbeat, or feeling of faintness during or after sex should prompt immediate medical attention.
Severe allergic reactions, including swelling of the face, lips or throat, difficulty breathing or swallowing.
You should seek medical attention if you experience chest pain, severe dizziness, or vision changes after taking Sildenafil.
How to minimise the side effects of Sildenafil tablets
There are several practical steps that can meaningfully reduce the likelihood and severity of sildenafil's side effects when dealing with erectile dysfunction:
Taking the lowest effective dose – starting at 50 mg and adjusting only under medical supervision – limits unnecessary systemic exposure.
Avoiding high-fat meals before taking sildenafil tablets ensures consistent absorption and reduces the risk of gastrointestinal discomfort.
Alcohol should be limited as it intensifies the effects of vasodilation, significantly increasing the risk of dizziness and low blood pressure.
Staying well hydrated can ease headaches and flushing.
Mind co-medication: It is crucial never to combine sildenafil with nitrate medications or recreational nitrates, such as poppers, as this can lead to a potentially fatal drop in blood pressure.
Avoid grapefruit or grapefruit juice, which may affect how the medicine is processed in the body.
The single most important step in ensuring sildenafil is used both effectively and safely is to fully disclose all current medications and health conditions to a prescribing doctor.
If side effects persist, worsen, or become bothersome, consult a healthcare professional for advice or dose adjustment.
Buying Sildenafil in the UK: Regulations and Advice
Prescription
It is illegal to buy sildenafil without a prescription in the UK for medical reasons. This applies to all forms of sildenafil, including branded Viagra and generic versions, and to any purchase made online, regardless of the platform or country of origin. The only exception is Viagra Connect: a 50 mg sildenafil tablet that is available over the counter in UK pharmacies, but only after a mandatory consultation with a pharmacist to assess suitability.
Prescription requirements protect your health: A qualified healthcare provider will assess your symptoms and medical history before deciding whether to prescribe treatment with Sildenafil. A doctor must check before prescribing sildenafil whether it is suitable for you, whether there are any dangerous interactions with other medications, and whether there are any pre-existing conditions that would contraindicate its use. This is the only way to minimise serious side effects and health risks.
Buying Sildenafil over the counter and/or from unverified websites carries a high risk of counterfeit or contaminated products.
Buy Sildenafil online
With a valid prescription from your doctor, you can buy Sildenafil at any pharmacy in the UK. The pharmacist can advise you on its correct use.
When you order Sildenafil online, you can compare the prices of different generics and often benefit from cheaper offers. Whe buying online, larger packs often offer a better price to purchase sildenafil cheaper.
Never buy Sildenafil tablets without a prescription from disreputable online vendors. Such offers are illegal and extremely dangerous.
Sildenafil alternatives - other treatments for erectile dysfunction
When sildenafil doesn't suit every man's lifestyle or medical profile, it's worth knowing that several equally effective PDE5 inhibitors are available – each with its own distinct pharmacological profile, onset time, and duration, allowing men and their doctors to find the option that best fits their individual needs and preferences.
Tadalafil (Cialis generic)
Tadalafil (generic version of Cialis) is widely regarded as the most flexible sildenafil alternative, distinguished primarily by its exceptionally long duration of action – up to 36 hours, earning it the informal nickname "the weekend pill." This extended window removes the need to time intake closely around sexual intercourse, offering a more spontaneous experience.
Tadalafil is available in two dosing formats: an on-demand dose of 10 or 20 mg taken before anticipated sex, and a low dose of Tadalafil Daily 2.5 or 5 mg taken every day regardless of sexual activity, which maintains a consistent level of the drug in the bloodstream.
It is also approved for benign prostatic hyperplasia, making it a particularly suitable option for older men dealing with both erectile dysfunction and urinary symptoms.
Its side effect profile is broadly similar to sildenafil, though back pain and muscle aches are more commonly reported due to its mild inhibition of PDE11.
Vardenafil (Levitra generic)
Vardenafil (a generic of Levitra) has a similar mechanism of action and potency to sildenafil, but some clinicians consider it to have slightly more selective action on PDE5. This may result in it being better tolerated by certain patients.
It typically takes effect within 30 to 60 minutes and remains active for around 4 to 6 hours – comparable to sildenafil.
One notable distinction is that vardenafil is less affected by food intake than sildenafil, offering somewhat greater flexibility around mealtimes.
Vardenafil may be particularly suited to men with diabetes-related erectile dysfunction, as clinical trials have shown consistent efficacy in this population. [1]
Avanafil (Spedra)
Spedra, with the active ingredient avanafil, is the newest of the approved PDE5 inhibitors and is notably distinguished by its rapid onset. It can take effect in as little as 15 minutes, making it the fastest-acting option in its class. This speed is possible because avanafil is highly selective for PDE5 with minimal binding to other PDE enzymes, resulting in not only a quicker onset but also a cleaner side effect profile with lower rates of visual disturbances, back pain, and flushing compared to older alternatives.
Available in doses of 50 mg, 100 mg, and 200 mg, Spedra is taken on demand before having sex and remains effective for approximately 4 to 6 hours.
Avanafil's selectivity and tolerability make it a compelling option for men who have experienced bothersome side effects with other PDE5 inhibitors, or those who value spontaneity without the extended commitment of tadalafil's 36-hour window.
Choosing the right ED medication depends on your individual needs, health conditions and preferences. Your doctor will be happy to advise you on your choice.
Experiences with Sildenafil
Many patients look for Sildenafil cheaper alternatives, as generic versions typically offer the same active ingredient at a lower cost than the branded product:
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References:
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Abdellatif AE, Abdelbary AM, Ragheb AM, et al. Effect of vardenafil in long-standing diabetic patients comparing different doses regarding erectile dysfunction, testosterone level and partner satisfaction. Arab J Urol. 2025;24(1):75-81. Published 2025 Jul 29. doi:10.1080/20905998.2025.2526978
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Hatzimouratidis K. Sildenafil in the treatment of erectile dysfunction: an overview of the clinical evidence. Clin Interv Aging. 2006;1(4):403-414. doi:10.2147/ciia.2006.1.4.403
