Sildenafil
PDE5 inhibitor for erectile dysfunction — prescription required
Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor prescribed for the treatment of erectile dysfunction in adult men. It works by relaxing the blood vessels in the penis, allowing increased blood flow during sexual stimulation. Sildenafil requires a prescription and should only be taken under medical supervision. This article provides comprehensive, evidence-based information on how sildenafil works, dosage guidelines, potential side effects, drug interactions, and important safety precautions.
Quick Facts: Sildenafil
Key Takeaways
- Sildenafil is a PDE5 inhibitor that helps achieve and maintain an erection when sexually stimulated — it does not cause spontaneous erections.
- The recommended starting dose is 50 mg, taken approximately one hour before sexual activity. It should not be taken more than once per day.
- Sildenafil must never be combined with nitrates (e.g. nitroglycerin) or nitric oxide donors (e.g. amyl nitrite) due to the risk of life-threatening hypotension.
- Common side effects include headache, facial flushing, nasal congestion, and indigestion — these are generally mild and short-lived.
- Seek immediate medical attention if you experience an erection lasting more than 4 hours, sudden vision loss, or sudden hearing loss.
What Is Sildenafil and What Is It Used For?
Quick Answer: Sildenafil is a prescription medication belonging to a class of drugs called PDE5 inhibitors. It is primarily used to treat erectile dysfunction (ED) in adult men by increasing blood flow to the penis during sexual stimulation.
Sildenafil contains the active substance sildenafil citrate, which belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors. It was originally developed as a treatment for angina pectoris and pulmonary arterial hypertension, but its prominent effect on erectile function led to its widespread use as an erectile dysfunction treatment. Sildenafil has been one of the most extensively studied drugs in history, with decades of clinical data supporting its safety and efficacy profile.
The medication works by inhibiting the enzyme PDE5, which is responsible for the breakdown of cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the corpus cavernosum (the erectile tissue of the penis). During sexual stimulation, nitric oxide is released into the erectile tissue, which activates the enzyme guanylate cyclase. This enzyme increases levels of cGMP, causing the smooth muscles to relax and allowing blood to flow into the penis. By blocking PDE5, sildenafil enhances this natural process, making it easier to achieve and maintain an erection.
It is important to understand that sildenafil only facilitates an erection when the patient is sexually stimulated. It does not increase sexual desire and it does not cause an erection without arousal. The drug has no direct effect on libido and should not be confused with an aphrodisiac. For sildenafil to work, the normal pathway of sexual arousal must be activated — the medication simply enhances the body's natural response.
Sildenafil is approved for the treatment of erectile dysfunction in adult men (aged 18 and older). Erectile dysfunction is defined as the persistent inability to achieve or maintain an erection sufficient for satisfactory sexual performance. This condition affects a significant proportion of men worldwide, with prevalence increasing with age. According to epidemiological studies, approximately 52% of men between the ages of 40 and 70 experience some degree of erectile dysfunction, making it one of the most common sexual health concerns in men.
In addition to its use for erectile dysfunction, sildenafil (under different brand names and dosage forms) is also approved for the treatment of pulmonary arterial hypertension (PAH). However, the formulations and dosing for PAH are different from those used for erectile dysfunction, and this article focuses specifically on the use of sildenafil for ED. Patients should not use sildenafil for ED simultaneously with sildenafil products intended for PAH.
What Should You Know Before Taking Sildenafil?
Quick Answer: Sildenafil is contraindicated with nitrate medications, in patients with severe heart or liver problems, after recent stroke or heart attack, and in those with certain rare eye conditions. Always inform your doctor about all medications you are taking.
Contraindications
There are several important situations in which sildenafil must not be used. Understanding these contraindications is critical for patient safety, as combining sildenafil with certain medications or using it in the presence of specific conditions can lead to serious, potentially life-threatening consequences.
- You are allergic to sildenafil or any of the other ingredients in the formulation
- You take nitrate medications (commonly prescribed for angina/chest pain), such as nitroglycerin, isosorbide mononitrate, or isosorbide dinitrate — the combination can cause a severe and potentially fatal drop in blood pressure
- You use nitric oxide donors such as amyl nitrite (“poppers”) — also risk of dangerous hypotension
- You take riociguat, a medication used for pulmonary arterial hypertension or chronic thromboembolic pulmonary hypertension
- You have severe heart or liver problems
- You have recently had a stroke or heart attack, or if you have low blood pressure
- You have certain rare hereditary eye diseases, such as retinitis pigmentosa
- You have previously experienced non-arteritic anterior ischaemic optic neuropathy (NAION) — a condition involving sudden loss of vision due to interrupted blood flow to the optic nerve
Warnings and Precautions
Before starting treatment with sildenafil, you should discuss your medical history thoroughly with your healthcare provider. Several conditions require special caution or monitoring when using this medication. Your doctor needs to carefully assess whether your cardiovascular system can handle the added physical exertion that sexual activity involves.
Talk to your doctor before taking sildenafil if you have any of the following conditions:
- Blood disorders: Sickle cell anaemia, leukaemia, or multiple myeloma — these conditions may increase the risk of priapism (prolonged erection)
- Penile abnormalities: Anatomical deformation of the penis or conditions such as Peyronie’s disease, which involves fibrous plaques in the erectile tissue
- Heart conditions: Any cardiovascular disease, including coronary artery disease, heart failure, or uncontrolled arrhythmias. Your doctor must determine whether the physical demands of sexual activity are safe for your heart
- Active peptic ulcer or bleeding disorders: Conditions such as haemophilia that may increase the risk of bleeding
- Kidney or liver problems: Impaired function may require dosage adjustment. Your doctor may prescribe a lower starting dose
You experience a sudden decrease or loss of vision while taking sildenafil. Stop the medication immediately and contact your doctor. This could be a sign of NAION, a rare but serious condition that can lead to permanent vision loss if not treated promptly.
Sildenafil should not be used in combination with other treatments for erectile dysfunction, whether oral or topical. It should also not be taken by anyone who does not have erectile dysfunction, and it is not indicated for use in women.
Pregnancy and Breastfeeding
Sildenafil is not indicated for use in women. There are no adequate or well-controlled studies of sildenafil in pregnant or breastfeeding women for the indication of erectile dysfunction. If a female partner becomes accidentally exposed to the medication, she should consult a healthcare provider, although the risk of harm from incidental exposure is considered very low.
Children and Adolescents
Sildenafil for erectile dysfunction should not be given to anyone under 18 years of age. The safety and efficacy of sildenafil for ED have not been established in the paediatric population, and there is no appropriate clinical indication for its use in this age group.
Driving and Using Machines
Sildenafil can cause dizziness and may affect vision (including colour vision changes and blurred vision). You should be aware of how you react to the medication before driving or operating machinery. If you experience any visual disturbances or dizziness, avoid driving and other activities that require alertness until these effects have resolved. These side effects are typically temporary and mild.
How Does Sildenafil Interact with Other Drugs?
Quick Answer: Sildenafil has potentially dangerous interactions with nitrates and nitric oxide donors (absolute contraindication), riociguat, alpha-blockers, HIV protease inhibitors, and CYP3A4 inhibitors. Always tell your doctor about all medications you take.
Drug interactions with sildenafil can range from mildly inconvenient to life-threatening. The most critical interaction is with nitrate-containing medications, which is an absolute contraindication. Sildenafil can affect or be affected by many other medications, so it is essential to inform your healthcare provider about all prescription drugs, over-the-counter medications, and supplements you are currently taking.
Major Interactions (Contraindicated or Dangerous)
| Drug / Drug Class | Risk | Recommendation |
|---|---|---|
| Nitrates (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate) | Severe, potentially fatal hypotension | Absolutely contraindicated — never combine |
| Amyl nitrite (“poppers”) and other nitric oxide donors | Severe hypotension | Absolutely contraindicated — never combine |
| Riociguat (for pulmonary hypertension) | Potentiation of hypotensive effects | Contraindicated — do not combine |
| Other PDE5 inhibitors (tadalafil, vardenafil, avanafil) | Additive vasodilatory effects | Do not combine — use only one PDE5 inhibitor at a time |
Moderate Interactions (Use with Caution)
| Drug / Drug Class | Effect | Recommendation |
|---|---|---|
| Alpha-blockers (doxazosin, tamsulosin, alfuzosin) | Dizziness, lightheadedness, or fainting due to low blood pressure | Start sildenafil at 25 mg; stabilise on alpha-blocker first |
| HIV protease inhibitors (ritonavir, saquinavir, indinavir) | Significantly increased sildenafil plasma levels | Start at 25 mg; do not exceed 25 mg within 48 hours |
| CYP3A4 inhibitors (ketoconazole, itraconazole, erythromycin, clarithromycin) | Increased sildenafil exposure and risk of side effects | Consider dose reduction; start at 25 mg |
| Sacubitril/valsartan (for heart failure) | Potential interaction with cardiovascular effects | Inform your doctor; close monitoring needed |
| Amlodipine and other antihypertensives | Additional blood pressure lowering | Monitor blood pressure; caution advised |
Food and Alcohol Interactions
Sildenafil can be taken with or without food. However, taking it with a heavy, high-fat meal may delay the onset of its effects. The absorption of the drug is reduced when taken after a fatty meal, meaning it may take longer to start working compared to taking it on an empty stomach or after a light meal.
Alcohol consumption can temporarily impair erectile function independently of medication use. While moderate alcohol intake does not create a direct pharmacological interaction with sildenafil, excessive drinking can counteract the benefits of the medication and increase the risk of side effects such as dizziness, headache, increased heart rate, and low blood pressure. To achieve the best results from sildenafil, it is advisable to avoid consuming large amounts of alcohol before taking the medication.
What Is the Correct Dosage of Sildenafil?
Quick Answer: The recommended starting dose is 50 mg, taken approximately one hour before sexual activity. Based on efficacy and tolerability, the dose may be increased to 100 mg or decreased to 25 mg. Do not take more than one dose per day.
Sildenafil should always be taken according to your doctor’s or pharmacist’s instructions. The dosage will be individualised based on your response to the medication, any concomitant medications, and the presence of any underlying health conditions. Do not adjust your dose without consulting your healthcare provider.
Adults
Standard Dosing
- Starting dose: 50 mg, taken approximately 1 hour before planned sexual activity
- Dose adjustment: Based on efficacy and tolerability, the dose may be increased to a maximum of 100 mg or decreased to 25 mg
- Maximum frequency: Once per day (no more than one dose in any 24-hour period)
- Onset of action: Typically 30–60 minutes; may be delayed by high-fat meals
- Duration of effect: Approximately 4–6 hours
| Patient Group | Starting Dose | Maximum Dose | Notes |
|---|---|---|---|
| Healthy adults | 50 mg | 100 mg | Take ~1 hour before activity |
| Elderly (≥65 years) | 25 mg | 100 mg | Reduced clearance; start low |
| Hepatic impairment | 25 mg | As tolerated | Reduced metabolism; doctor supervision |
| Renal impairment (severe) | 25 mg | As tolerated | Reduced clearance |
| On CYP3A4 inhibitors | 25 mg | 25 mg | Do not exceed 25 mg per 48 hours |
| On alpha-blockers | 25 mg | As tolerated | Stabilise on alpha-blocker first |
How to Take Sildenafil
Swallow the tablet whole with a glass of water. Do not crush, chew, or break the tablet unless your doctor has specifically instructed you to do so. The tablet should be taken approximately one hour before you plan to engage in sexual activity, although the medication can be effective from 30 minutes to 4–6 hours after taking it. The optimal time varies from person to person.
If your doctor has prescribed the 25 mg dose and your tablet is a higher strength with a score line, your doctor or pharmacist can advise you on how to correctly split the tablet. Always follow the specific instructions provided with your medication.
Children
Sildenafil for erectile dysfunction is not indicated for use in individuals under 18 years of age. It should not be given to children or adolescents.
Elderly Patients
In elderly patients (65 years and older), sildenafil clearance is reduced, resulting in higher plasma concentrations of the drug. For this reason, a starting dose of 25 mg is generally recommended. The dose can then be adjusted based on individual response and tolerability, up to a maximum of 100 mg. Elderly patients should be particularly mindful of potential interactions with other medications they may be taking for age-related conditions.
Missed Dose
Since sildenafil is taken on an as-needed basis (not on a regular schedule), there is no concern about missing a dose. Simply take it when needed before planned sexual activity. Do not take a double dose to make up for a missed one, and never take more than one dose within a 24-hour period.
Overdose
If you have taken more sildenafil than prescribed, or if a child has accidentally ingested the medication, contact a doctor, hospital, or poison control centre immediately. Doses above 100 mg do not increase effectiveness but may increase the severity and frequency of side effects. Symptoms of overdose may include severe headache, dizziness, fainting, prolonged erection, visual disturbances, and dangerously low blood pressure.
In cases of overdose, standard supportive measures should be adopted as required. Renal dialysis is not expected to accelerate clearance, as sildenafil is highly bound to plasma proteins and is not significantly eliminated in the urine in its active form.
What Are the Side Effects of Sildenafil?
Quick Answer: The most common side effect is headache (affecting more than 1 in 10 users). Other common effects include facial flushing, indigestion, nasal congestion, dizziness, and visual disturbances. Most side effects are mild to moderate and short-lived.
Like all medicines, sildenafil can cause side effects, although not everyone will experience them. The side effects reported with sildenafil are generally mild to moderate in severity and typically transient, resolving within a few hours. Understanding the frequency and nature of potential side effects allows patients and healthcare providers to make informed treatment decisions and to recognise when medical attention may be needed.
- Allergic reaction — sudden wheezing, difficulty breathing, dizziness, or swelling of the eyelids, face, lips, or throat
- Chest pain — do NOT take nitrates to treat chest pain that occurs during or after sexual intercourse
- Prolonged erection (priapism) — an erection lasting longer than 4 hours requires immediate medical treatment
- Sudden vision loss or decrease in one or both eyes
- Sudden hearing loss or decrease, with or without ringing in the ears
- Severe skin reactions — severe peeling or swelling of skin, blistering in the mouth, around the eyes, or on the genitals, fever
- Seizures
Very Common
May affect more than 1 in 10 people
- Headache
Common
May affect up to 1 in 10 people
- Nausea
- Facial flushing (redness)
- Hot flushes (sudden feeling of warmth in the upper body)
- Indigestion (dyspepsia)
- Colour vision changes, blurred vision, visual disturbances
- Nasal congestion (stuffy nose)
- Dizziness
Uncommon
May affect up to 1 in 100 people
- Vomiting, skin rash
- Eye irritation, bloodshot eyes, eye pain, seeing flashes of light, photophobia, watery eyes
- Palpitations, rapid heart rate (tachycardia)
- High blood pressure, low blood pressure
- Muscle pain, drowsiness, reduced sensation of touch
- Vertigo, tinnitus (ringing in the ears)
- Dry mouth, sinus congestion, nasal inflammation
- Upper abdominal pain, gastro-oesophageal reflux
- Blood in urine, nosebleeds, feeling of warmth, fatigue
- Pain in arms or legs
Rare
May affect up to 1 in 1,000 people
- Fainting, stroke, heart attack, irregular heartbeat
- Temporary decreased blood flow to parts of the brain
- Feeling of throat tightness, numbness of the mouth
- Bleeding in the back of the eye, double vision, reduced visual acuity, dilated pupils
- Penile bleeding, blood in semen
- Dry nose, nasal swelling, irritability
- Sudden hearing loss or decrease
After sildenafil became widely available, rare cases of unstable angina and sudden death have been reported. It is important to note that most — but not all — of the men who experienced these events had pre-existing cardiovascular conditions before taking the medication. A causal relationship with sildenafil has not been definitively established. Patients with known cardiovascular risk factors should be carefully evaluated before starting treatment.
If you experience any side effects not listed above, or if any side effects become severe or persistent, consult your doctor or pharmacist. Reporting suspected adverse reactions after the medicine has been authorised is important, as it allows continued monitoring of the benefit-risk balance of the medicine.
How Should You Store Sildenafil?
Quick Answer: Store sildenafil at room temperature, away from moisture and direct sunlight. Keep it out of the reach and sight of children. Do not use after the expiry date on the packaging.
Proper storage of sildenafil helps ensure the medication remains effective and safe throughout its shelf life. Sildenafil tablets do not require any special storage conditions. Store them at room temperature (below 25°C / 77°F) in the original packaging to protect from moisture and light. Keep the blister pack sealed until you are ready to take a tablet.
As with all medications, keep sildenafil out of the sight and reach of children. Accidental ingestion by a child can cause serious adverse effects and constitutes a medical emergency. Do not use sildenafil after the expiry date printed on the carton and blister pack. The expiry date refers to the last day of the stated month.
Do not dispose of medications via wastewater or household waste. Return unused or expired tablets to your pharmacy for safe disposal. This helps protect the environment and prevents accidental exposure to others.
What Does Sildenafil Contain?
Quick Answer: Each tablet contains sildenafil (as citrate) as the active ingredient, along with standard pharmaceutical excipients. The tablets are typically light blue, round, and marked with the dosage strength.
The active substance in sildenafil tablets is sildenafil, present in the form of sildenafil citrate. The tablets are available in strengths of 20 mg, 25 mg, 50 mg, and 100 mg of sildenafil. The exact excipient composition can vary between manufacturers, but commonly used inactive ingredients include:
- Tablet core: Anhydrous calcium hydrogen phosphate, microcrystalline cellulose, copovidone, croscarmellose sodium, magnesium stearate
- Additional excipients: Saccharin sodium, indigo carmine aluminium lake (E132 — responsible for the characteristic blue colour)
Sildenafil tablets contain less than 1 mmol (23 mg) of sodium per tablet, meaning they are essentially sodium-free. This is relevant for patients on a controlled sodium diet.
Sildenafil tablets are typically light blue, round, and slightly mottled in appearance. They may feature score lines on one side to facilitate splitting and are marked with the dosage strength (e.g. “25”, “50”, or “100”) on the other side. The tablets are supplied in blister packs, with common pack sizes including 1, 2, 4, 8, 12, and 24 tablets. Not all pack sizes may be marketed in every country.
Frequently Asked Questions About Sildenafil
Sildenafil typically begins to work within 30 to 60 minutes after ingestion. However, several factors can affect the onset time. Taking sildenafil on an empty stomach generally allows faster absorption compared to taking it after a heavy, high-fat meal, which may delay the effect. The medication remains effective for approximately 4 to 6 hours, providing a window for sexual activity. Individual responses may vary, and some men find that the drug takes effect more quickly or more slowly based on their metabolism and overall health.
Moderate alcohol consumption is generally compatible with sildenafil use. However, excessive alcohol intake can impair erectile function independently and may increase the risk of side effects such as dizziness, headache, flushing, and low blood pressure. For the best results, limit alcohol consumption when planning to use sildenafil. There is no strict prohibition on alcohol, but moderation is strongly recommended.
Both sildenafil and tadalafil are PDE5 inhibitors used to treat erectile dysfunction. The primary difference lies in their duration of action: sildenafil works for about 4–6 hours, while tadalafil can remain effective for up to 36 hours. Sildenafil is typically taken as needed about one hour before activity, whereas tadalafil is available both as an as-needed dose and as a low daily dose (2.5–5 mg). The choice between them often depends on lifestyle preferences, frequency of sexual activity, and individual response. Your doctor can help determine which option is most suitable for you.
Sildenafil is contraindicated for patients taking nitrate medications and those with severe cardiovascular disease. However, many men with stable, well-managed heart conditions can safely use sildenafil under their doctor’s supervision. The key consideration is whether your heart can handle the physical exertion involved in sexual activity. Your doctor will perform a thorough cardiovascular assessment before prescribing sildenafil. Never use sildenafil with nitrates, as this combination can cause a life-threatening drop in blood pressure.
An erection lasting longer than 4 hours (priapism) is a medical emergency. You should seek immediate medical attention at the nearest emergency department. If left untreated, priapism can damage the erectile tissue of the penis and potentially lead to permanent erectile dysfunction. While priapism is rare, it is more common in individuals with predisposing conditions such as sickle cell disease, leukaemia, or multiple myeloma. Do not attempt to treat priapism at home.
Sildenafil is designed to be taken on an as-needed basis and should not be taken more than once per day. It is not approved for daily use at the standard erectile dysfunction doses (25–100 mg). If you prefer a daily medication for erectile dysfunction, tadalafil at a low dose of 2.5–5 mg per day may be a suitable alternative. Talk to your doctor about which treatment regimen is best for your situation and lifestyle.
References and Sources
This article is based on the following peer-reviewed sources and authoritative medical guidelines:
- European Medicines Agency (EMA). Summary of Product Characteristics — Sildenafil. EMA Product Information. Updated 2025.
- U.S. Food and Drug Administration (FDA). Prescribing Information for Sildenafil Citrate Tablets. FDA Drug Labeling. 2024.
- World Health Organization (WHO). WHO Model List of Essential Medicines — 23rd List, 2023. Geneva: WHO; 2023.
- British National Formulary (BNF). Sildenafil monograph. NICE BNF. Accessed January 2026.
- Goldstein I, Lue TF, Padma-Nathan H, et al. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397-1404. doi:10.1056/NEJM199805143382001
- Hatzimouratidis K, Giuliano F, Moncada I, et al. EAU Guidelines on Erectile Dysfunction. European Association of Urology. 2024.
- Burnett AL, Nehra A, Breau RH, et al. Erectile Dysfunction: AUA Guideline. J Urol. 2018;200(3):633-641. doi:10.1016/j.juro.2018.05.004
- Corona G, Rastrelli G, Burri A, et al. The safety and efficacy of phosphodiesterase type 5 inhibitors: A meta-analysis. Andrology. 2023;11(3):443-456.
- McMahon CG. Current diagnosis and management of erectile dysfunction. Med J Aust. 2019;210(10):469-476. doi:10.5694/mja2.50167
- Kloner RA. Cardiovascular effects of the 3 phosphodiesterase-5 inhibitors approved for the treatment of erectile dysfunction. Circulation. 2004;110(19):3149-3155.
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