Sildenafil is a widely prescribed medication for erectile dysfunction, but many men wonder whether it will automatically produce an erection. The short answer is that sildenafil does not cause an erection on its own—it requires sexual arousal to work. This phosphodiesterase type 5 (PDE5) inhibitor enhances the body's natural erectile response by improving blood flow to the penis during sexual stimulation. Understanding how sildenafil works, what to expect, and how to use it safely is essential for achieving the best outcomes. This article explains the mechanism of action, realistic expectations, safety considerations, and when to seek further medical advice.
Summary: Sildenafil will not make you hard without sexual arousal—it enhances your body's natural erectile response to sexual stimulation by improving blood flow to the penis.
- Sildenafil is a PDE5 inhibitor that works by blocking the breakdown of cGMP, helping to maintain blood vessel relaxation in the penis during sexual arousal.
- The medication typically begins working within 30 to 60 minutes and remains active for approximately four to five hours, but each erection still requires sexual stimulation.
- Common side effects include headache, facial flushing, indigestion, and nasal congestion, which are usually mild and temporary.
- Sildenafil must never be taken with nitrate medications (including GTN for angina or recreational 'poppers') due to the risk of dangerous blood pressure drops.
- Approximately 7 in 10 men with erectile dysfunction experience improved function with sildenafil, though individual responses vary depending on underlying health factors.
- If sildenafil does not work after 6 to 8 attempts under optimal conditions, consult your GP for dose adjustment or alternative treatments.
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How Sildenafil Works to Support Erections
Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor licensed for the treatment of erectile dysfunction (ED). It does not directly cause an erection on its own—rather, it facilitates the body's natural erectile response to sexual stimulation. Understanding this mechanism is essential for setting realistic expectations about the medication.
When a man becomes sexually aroused, nerve signals trigger the release of nitric oxide in the erectile tissue of the penis. This chemical messenger activates an enzyme called guanylate cyclase, which increases levels of cyclic guanosine monophosphate (cGMP). The cGMP relaxes smooth muscle cells in the blood vessels of the penis, allowing increased blood flow into the corpora cavernosa—the spongy chambers that fill with blood during an erection. Sildenafil works by inhibiting the PDE5 enzyme, which normally breaks down cGMP. By blocking this breakdown, sildenafil helps maintain higher levels of cGMP, thereby prolonging the relaxation of smooth muscle and supporting the erectile response.
It is crucial to note that sildenafil requires sexual stimulation to be effective. Without arousal, the medication will not produce an erection. This distinguishes it from other treatments such as intracavernosal injections, which can cause erections independently of sexual desire. Sildenafil typically begins to work within 30 to 60 minutes of ingestion, though this can vary between individuals. The medication remains active in the body for approximately four to five hours, during which time it can facilitate erections in response to sexual stimulation—each erection still requires arousal during this window.
Important: Do not take sildenafil more than once in a 24-hour period. Taking it more frequently increases the risk of side effects without improving effectiveness.
What to Expect When Taking Sildenafil
When you first take sildenafil, it is important to have realistic expectations about its effects and potential side effects. Clinical trials have demonstrated that sildenafil improves erectile function in around 7 in 10 men with ED, though individual responses vary depending on the underlying cause of erectile difficulties and other health factors.
Common side effects include headache, facial flushing, indigestion, nasal congestion, and visual disturbances such as a blue tinge to vision or increased sensitivity to light. These effects are generally mild to moderate and tend to diminish with continued use. The visual changes occur because sildenafil has some cross-reactivity with PDE6, an enzyme present in retinal photoreceptors, though sildenafil's affinity for PDE6 is much lower than for PDE5. Most men find these side effects tolerable and temporary.
Sildenafil should be taken approximately one hour before anticipated sexual activity, though it can be effective anywhere from 30 minutes to four hours after ingestion. Taking the medication on an empty stomach may lead to faster absorption, whilst a high-fat meal can delay its onset of action. The standard starting dose in the UK is 50 mg, which can be adjusted to 25 mg or 100 mg depending on efficacy and tolerability. Do not take more than one dose in any 24-hour period.
Important safety considerations and contraindications:
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Nitrates: Sildenafil must never be taken with any nitrate medications—including glyceryl trinitrate (GTN) for angina, isosorbide mononitrate or dinitrate, nicorandil, or recreational nitrates such as amyl nitrite ('poppers')—as this combination can cause a dangerous drop in blood pressure.
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Riociguat: Do not take sildenafil if you are taking riociguat (used for pulmonary hypertension), as the combination is contraindicated.
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Alpha-blockers: Men taking alpha-blockers for prostate problems or hypertension should inform their GP. You should be stable on your alpha-blocker therapy before starting sildenafil. Your doctor will usually start you on a lower dose (25 mg) and may advise separating the timing of the two medicines to reduce the risk of postural hypotension (dizziness on standing).
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Other interactions: Certain medicines can increase sildenafil levels in the blood, potentially increasing side effects. These include strong CYP3A4 inhibitors such as ketoconazole, itraconazole, clarithromycin, erythromycin, and HIV protease inhibitors (e.g., ritonavir). Avoid grapefruit juice, as it can also increase sildenafil levels. Always inform your GP or pharmacist about all medications you take, including over-the-counter products and herbal supplements.
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Cardiovascular cautions: Sildenafil should be used with caution (or avoided) in men with severe cardiovascular disease, recent heart attack or stroke (within the last 6 months), severe low blood pressure (below 90/50 mmHg), or unstable angina. Your GP will assess your cardiovascular fitness before prescribing.
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Eye conditions: Men with retinitis pigmentosa or a history of non-arteritic anterior ischaemic optic neuropathy (NAION) should discuss these conditions with their GP before using sildenafil.
Seek immediate medical attention if you experience:
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An erection lasting longer than four hours (priapism)—go to A&E immediately, as this requires urgent treatment to prevent permanent damage
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Chest pain during sexual activity—call 999
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Sudden loss of vision or hearing—stop taking sildenafil and seek urgent medical assessment
If you experience any side effects, you can report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.
Getting the Most from Your Sildenafil Treatment
Maximising the effectiveness of sildenafil involves more than simply taking the tablet—lifestyle factors, timing, and realistic expectations all play important roles in treatment success.
Optimising timing and conditions: For best results, take sildenafil approximately one hour before sexual activity on a relatively empty stomach. Avoid excessive alcohol consumption, as this can impair erectile function independently and may reduce the medication's effectiveness. Some men find that sildenafil works better after they have used it several times, as initial anxiety about performance can interfere with the medication's action. It is worth trying sildenafil on 6 to 8 separate occasions under optimal conditions before concluding that it is ineffective.
Lifestyle modifications can significantly enhance treatment outcomes. NICE guidance emphasises the importance of addressing modifiable risk factors for ED, including:
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Smoking cessation—tobacco use damages blood vessels and impairs circulation
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Weight management—obesity is strongly associated with ED
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Regular physical activity—exercise improves cardiovascular health and erectile function
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Limiting alcohol intake—excessive drinking can worsen ED
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Managing stress and anxiety—psychological factors often contribute to erectile difficulties
Medication interactions and precautions: Inform your GP or pharmacist about all medications you take. Strong CYP3A4 inhibitors (such as ketoconazole, itraconazole, clarithromycin, erythromycin, and HIV protease inhibitors like ritonavir) can increase sildenafil levels and side effects; your doctor may need to adjust your dose. Avoid grapefruit juice whilst taking sildenafil. If you are on an alpha-blocker, ensure you are stable on that treatment before starting sildenafil, and your GP will usually start you on a lower dose (25 mg) with advice on separating the timing of doses to reduce the risk of low blood pressure.
Do not combine sildenafil with other PDE5 inhibitors (such as tadalafil or vardenafil) or other ED treatments (such as alprostadil) without medical supervision.
If sildenafil does not work effectively after several attempts at the appropriate dose, do not simply increase the dose without medical advice. Your GP may need to review your diagnosis, adjust the dosage, consider alternative PDE5 inhibitors (such as tadalafil or vardenafil), or investigate other underlying causes of ED. Approximately 30% of men do not respond adequately to oral PDE5 inhibitors, and alternative treatments may be more appropriate.
Availability: Generic sildenafil is available on NHS prescription where clinically appropriate. Sildenafil 50 mg (Viagra Connect) is also available to purchase from pharmacies following a consultation with a pharmacist, though the same contraindications and precautions apply.
Speaking to Your GP About Erectile Dysfunction
Many men feel embarrassed discussing erectile dysfunction with their GP, but it is a common medical condition affecting approximately 50% of men aged 40–70 to some degree. ED can be an early warning sign of cardiovascular disease, diabetes, or other health conditions, making medical assessment important beyond simply treating the symptom.
What to expect during your consultation: Your GP will take a detailed medical and sexual history, including the duration and severity of erectile difficulties, any associated symptoms, and relevant risk factors. They will ask about cardiovascular health, diabetes, medications, smoking, alcohol use, and psychological factors such as stress, anxiety, or relationship issues. A physical examination may include checking blood pressure, examining the genitals, and assessing secondary sexual characteristics.
Investigations recommended by NICE typically include:
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Blood pressure measurement and cardiovascular risk assessment (such as QRISK score) to ensure it is safe for you to engage in sexual activity and to take sildenafil
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Blood tests—fasting glucose or HbA1c (diabetes screening), lipid profile (cardiovascular risk assessment)
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Testosterone levels—morning total testosterone (measured between 9–11 am) is recommended if you have low sexual desire, symptoms suggestive of hypogonadism (such as reduced energy, mood changes, or loss of muscle mass), or other clinical indicators. If the level is low, the test should be repeated to confirm the result.
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Thyroid function tests—only if clinically indicated by symptoms
Your GP will discuss whether sildenafil is appropriate for you based on your medical history and current medications. They will explain how to use it safely, what to expect, and when to seek further help. Generic sildenafil is available on NHS prescription where clinically appropriate. Sildenafil 50 mg (Viagra Connect) is also available to purchase from pharmacies following a consultation with a pharmacist.
When to seek further review or referral: If sildenafil proves ineffective after multiple attempts (typically 6–8 trials under optimal conditions) at the maximum tolerated dose, if you experience concerning side effects, or if your erectile function deteriorates despite treatment, contact your GP. You may be referred to a specialist urology or sexual health service for further assessment if:
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First-line treatment has failed
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You have penile deformity or Peyronie's disease
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There is suspected endocrine (hormonal) cause requiring specialist investigation
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You have complex comorbidities or significant psychogenic factors that may benefit from psychosexual therapy
Alternative treatment options may include vacuum erection devices, intracavernosal injections, intraurethral therapy, or penile prosthesis surgery in selected cases. Remember that ED is a treatable condition, and effective help is available.
Frequently Asked Questions
Does sildenafil cause an automatic erection?
No, sildenafil does not cause an automatic erection. It requires sexual arousal to work, as it enhances the body's natural erectile response rather than producing an erection independently.
How long does it take for sildenafil to work?
Sildenafil typically begins to work within 30 to 60 minutes of taking it, though this can vary between individuals. It remains active in the body for approximately four to five hours.
Can I take sildenafil with other medications?
Sildenafil must never be taken with nitrate medications (such as GTN for angina or recreational 'poppers') due to the risk of dangerous blood pressure drops. Always inform your GP or pharmacist about all medications you take, including alpha-blockers, antifungals, antibiotics, and HIV medications, as dose adjustments may be necessary.
The health-related content published on this site is based on credible scientific sources and is periodically reviewed to ensure accuracy and relevance. Although we aim to reflect the most current medical knowledge, the material is meant for general education and awareness only.
The information on this site is not a substitute for professional medical advice. For any health concerns, please speak with a qualified medical professional. By using this information, you acknowledge responsibility for any decisions made and understand we are not liable for any consequences that may result.
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