Sildenafil: Dosage, Side Effects, Contraindications & Drug Interactions
This is the clinical reference guide for sildenafil — covering dosage by indication, how to take it for best results, the full side effect profile, contraindications, and drug interactions. For an introduction to what sildenafil is and how it works, see: What Is Viagra? Sildenafil Explained. For information on getting sildenafil in the UK, see: Sildenafil UK: How to Get It Safely Online.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Sildenafil is a prescription-only medicine (POM) in most doses. Always consult a qualified prescriber before starting or changing any medication. In a medical emergency, call 999 or go to your nearest A&E.
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Sildenafil Dosage for Erectile Dysfunction
Sildenafil for ED is available in three doses: 25mg, 50mg, and 100mg. The correct dose depends on your age, other medicines, liver and kidney function, and how you respond to treatment.
| Patient Group | Starting Dose | Maximum Dose | Frequency |
|---|---|---|---|
| Standard (adults under 65) | 50mg | 100mg | Once in any 24-hour period |
| Men over 65 | 25mg | 50mg | Once in any 24-hour period |
| Significant kidney impairment (CrCl <30 ml/min) | 25mg | 50mg | Once in any 24-hour period |
| Significant liver impairment | 25mg | 50mg | Once in any 24-hour period |
| Taking CYP3A4 inhibitors (e.g. erythromycin, ketoconazole) | 25mg | 50mg | Once in any 24-hour period — discuss with prescriber |
| Taking ritonavir or other potent CYP3A4 inhibitors | 25mg | 25mg | Maximum once in 48 hours |
Never exceed one dose in a 24-hour period. There is no clinical benefit to taking more than 100mg, and doing so significantly increases the risk of side effects.
How to Take Sildenafil for Best Results
Timing
Take sildenafil approximately 30–60 minutes before anticipated sexual activity. The drug reaches peak blood concentration 60–120 minutes after ingestion. Effects persist for 4–5 hours — this is a window of enhanced responsiveness, not a continuous erection.
Food
Avoid high-fat meals before taking sildenafil. Dietary fat significantly delays gastric emptying and slows sildenafil absorption. A high-fat meal can delay onset by up to 60 minutes and reduce peak plasma concentration by up to 29%. Take on a completely empty stomach or after a light, low-fat meal for the most predictable results.
Alcohol
Small amounts of alcohol (1–2 units) are unlikely to cause significant problems, but alcohol independently impairs erectile function and amplifies sildenafil’s blood-pressure-lowering effect — increasing the risk of dizziness and flushing. Limit alcohol when taking sildenafil.
Grapefruit Juice
Avoid grapefruit juice. It inhibits the intestinal CYP3A4 enzyme that metabolises sildenafil, unpredictably increasing plasma concentration and the risk of adverse effects.
Sexual Stimulation
Sildenafil requires sexual stimulation to work. It does not cause an automatic erection in the absence of arousal. The drug enhances the physiological response to stimulation — it does not replace it.
What If Sildenafil Is Not Working?
- Was the stomach reasonably empty (not after a large or fatty meal)?
- Was adequate sexual stimulation present?
- Was 30–60 minutes allowed between taking the tablet and sexual activity?
- Has it been tried on more than one occasion? Efficacy improves over the first several attempts.
- Was the dose adequate? If 50mg has consistently failed under good conditions, your prescriber may increase to 100mg.
If 100mg used correctly on multiple occasions under good conditions is still ineffective, your prescriber may recommend an alternative PDE5 inhibitor (tadalafil or vardenafil) or investigate other contributing causes. For a comparison, see: Tadalafil vs Sildenafil.
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Side Effects of Sildenafil
Most side effects are mild, predictable from the drug’s vasodilatory mechanism, and resolve as sildenafil clears the body over 4–6 hours.
Common (may affect up to 1 in 10 people)
- Headache — the most frequently reported side effect; caused by cerebral vasodilation
- Facial flushing — warmth and redness, typically in the face and chest; usually brief
- Nasal congestion or rhinorrhoea — nasal blood vessel dilation
- Dyspepsia (indigestion) — particularly if taken close to a meal; antacids may help
- Dizziness or light-headedness — blood-pressure-lowering effect; worsened by alcohol and by standing quickly
- Visual disturbances — most commonly a blue-green tinge to vision (cyanopsia), blurred vision, or increased sensitivity to light. Related to mild inhibition of retinal PDE6. Usually brief and resolves fully.
Uncommon (may affect up to 1 in 100 people)
- Palpitations or tachycardia (increased heart rate)
- Myalgia (muscle aches)
- Dry mouth
- Skin rash or urticaria
- Hypotension (low blood pressure) — particularly in combination with alpha-blockers or antihypertensives
Rare but Serious — Seek Emergency Care Immediately
Stop Sildenafil and Call 999 or Attend A&E Immediately If
Priapism — a painful erection lasting more than 4 hours. This is a urological emergency. Without prompt treatment (within 4–6 hours of onset), irreversible damage to erectile tissue can occur. Do not wait — attend A&E immediately.
Sudden loss of vision in one or both eyes. Sudden loss of hearing with or without tinnitus. Chest pain, irregular heartbeat, or collapse — call 999 immediately. Severe allergic reaction — swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash.
Contraindications — Who Should Not Take Sildenafil
Do not take sildenafil if you:
- Take nitrate medicines in any form — GTN spray or patches, isosorbide mononitrate or dinitrate tablets, or recreational “poppers” (amyl nitrite, butyl nitrite). The sildenafil–nitrate combination causes a potentially fatal, acute drop in blood pressure. This is an absolute contraindication with no exceptions.
- Take riociguat (Adempas), used for pulmonary hypertension
- Have been advised by a doctor to avoid sexual activity for cardiovascular reasons
- Have severe hepatic impairment (liver disease — Child-Pugh class C)
- Have had a stroke or myocardial infarction within the past 6 months
- Have severe hypotension (resting blood pressure below 90/50 mmHg)
- Have uncontrolled hypertension (resting blood pressure above 170/110 mmHg)
- Have a hereditary degenerative retinal disorder such as retinitis pigmentosa
- Have previously experienced non-arteritic anterior ischaemic optic neuropathy (NAION)
Drug Interactions
| Drug/Class | Interaction | Management |
|---|---|---|
| Nitrates (all forms) | Severe hypotension — potentially fatal | Absolute contraindication. Do not combine. |
| Riociguat | Severe hypotension | Contraindicated |
| Alpha-blockers (e.g. tamsulosin, doxazosin) | Additive blood-pressure-lowering effect | Start sildenafil at 25mg; allow 4+ hours between doses |
| CYP3A4 inhibitors (erythromycin, clarithromycin, ketoconazole, itraconazole) | Increase sildenafil plasma levels | Reduce sildenafil dose to 25mg; monitor for adverse effects |
| Ritonavir and other HIV protease inhibitors | Markedly increase sildenafil levels | Maximum 25mg per 48 hours |
| Antihypertensives (beta-blockers, ACE inhibitors, calcium channel blockers) | Additive blood-pressure-lowering effect | Generally manageable; monitor blood pressure. Inform prescriber. |
| Alcohol | Additive vasodilation; worsens dizziness, flushing | Limit to 1–2 units maximum |
| Grapefruit juice | Inhibits CYP3A4; raises sildenafil levels unpredictably | Avoid |
Dosage for Pulmonary Arterial Hypertension (PAH)
Sildenafil is licensed for pulmonary arterial hypertension under the brand name Revatio at a substantially lower dose: 20mg three times daily (approximately every 8 hours). This indication is managed by specialist physicians and is entirely separate from ED treatment. Revatio tablets are not interchangeable with ED formulations — do not substitute one for the other.
Frequently Asked Questions
What is the correct starting dose of sildenafil for erectile dysfunction?
The recommended starting dose for most men is 50mg, taken approximately 30–60 minutes before sexual activity. If 50mg is well tolerated but insufficient, it can be increased to 100mg. Men over 65, or those with kidney or liver impairment, should start at 25mg. Never exceed 100mg or take more than one dose in 24 hours.
What are the most common side effects of sildenafil?
The most common side effects are headache, facial flushing, nasal congestion, indigestion, and dizziness. A temporary visual disturbance (slight blue tinge or increased light sensitivity) is less common but well recognised. Most side effects are mild and resolve within a few hours.
Can I take sildenafil with my blood pressure medication?
It depends on the medication. Sildenafil is absolutely contraindicated with nitrate medicines (GTN spray, isosorbide mononitrate). With alpha-blockers (tamsulosin, doxazosin), sildenafil should be started at 25mg with careful timing. Other antihypertensives (ACE inhibitors, calcium channel blockers, beta-blockers) are generally manageable — inform your prescriber of all medicines you take.
How long do the effects of sildenafil last?
The effects typically last 4–5 hours, though this varies between individuals based on age, dose, metabolism, and other factors. This does not mean a continuous erection — it means enhanced responsiveness to sexual stimulation during that window.
Can I take sildenafil every day?
Sildenafil for ED is an on-demand medicine — not a daily tablet. Do not exceed one dose in any 24-hour period. If you want continuous ED treatment without on-demand dosing, discuss daily tadalafil 5mg with your prescriber, which is specifically licensed for daily use.
What should I do if I get a prolonged erection after taking sildenafil?
If an erection lasts more than 4 hours, go to an A&E immediately. Priapism is a medical emergency — without prompt treatment, permanent damage to erectile tissue can occur. Do not wait to see if the erection resolves.
Is sildenafil safe long-term?
Yes. Sildenafil has been in clinical use since 1998 and has an excellent long-term safety record at prescribed doses. There is no evidence of harmful cumulative effects. Regular prescriber review is recommended to ensure the dose remains appropriate.
References
- NICE. Erectile dysfunction. CKS 2023. cks.nice.org.uk
- European Association of Urology. EAU Guidelines on Sexual and Reproductive Health 2024. uroweb.org
- MHRA. Sildenafil 50mg film-coated tablets (Viagra Connect): summary of product characteristics. medicines.org.uk/emc
- Fink HA et al. Sildenafil for male erectile dysfunction: a systematic review and meta-analysis. Arch Intern Med. 2002.
- Hatzimouratidis K et al. Pharmacotherapy for erectile dysfunction. J Sex Med. 2016.
- GPhC. Standards for registered pharmacies. pharmacyregulation.org