Sildenafil: Dosage, Side Effects, Indications and Precautions
25mg, 50mg and 100mg doses, timing, food interactions, side effect table, contraindications and drug interactions.
Part of the Access Doctor Erectile Dysfunction guide.
Key fact: Sildenafil 50mg is the recommended starting dose for most men with ED. It should be taken 30–60 minutes before sexual activity, on an empty stomach or with a light meal. Sexual stimulation is always required — sildenafil does not produce an erection on its own.
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Start Consultation →What Is Sildenafil?
Sildenafil citrate is an MHRA-approved PDE5 inhibitor. It was developed by Pfizer researchers investigating treatments for angina, with improved erections noted as an unexpected side effect in clinical trials. It launched as Viagra in 1998 and is now widely available as a generic. Generic sildenafil contains the same active ingredient at the same doses as branded Viagra and is clinically equivalent — both must meet the same MHRA bioequivalence standards before licensing.
What Is Sildenafil Used For?
- Erectile dysfunction (primary licensed indication) — NICE CKS recommends PDE5 inhibitors as first-line pharmacological treatment for ED. Sildenafil is effective in approximately 82% of men with ED when used correctly
- Pulmonary arterial hypertension (PAH) — at 20mg three times daily (licensed as Revatio); a specialist indication managed by hospital physicians, not related to ED dosing
- Off-label uses — altitude sickness prevention; some fertility contexts. These require specialist discussion.
Sildenafil Dosage for Erectile Dysfunction
| 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 |
| Kidney or liver impairment | 25mg | 50mg | Once in any 24-hour period |
| Taking CYP3A4 inhibitors (e.g. clarithromycin, ketoconazole) | 25mg | 50mg | Once per 24 hours — discuss with prescriber |
| Taking ritonavir / HIV protease inhibitors | 25mg | 25mg maximum per 48 hours | Under prescriber guidance only |
How to Take Sildenafil for Best Results
1
Timing
Take 30–60 minutes before anticipated sexual activity. Plan to allow at least 1 hour for reliable onset. Effects last 4–5 hours.
2
Food
Take on an empty stomach or after a light meal. A large high-fat meal can delay onset by up to 1 hour by slowing absorption.
3
Dose
Never take more than one dose in a 24-hour period. Do not take with other PDE5 inhibitors.
4
Stimulation
Sexual stimulation is required — sildenafil enhances your natural erectile response, it does not bypass the need for arousal.
5
Alcohol and grapefruit
Avoid large amounts of alcohol (impairs erections and worsens dizziness). Avoid grapefruit juice (inhibits metabolism, raising drug levels).
If 50mg isn't working: Check timing (1 hour before), food intake (empty stomach), and whether sexual anxiety may be a factor. Some men need several attempts before achieving optimal results. If correctly used at 50mg on multiple occasions without effect, consult your prescriber about increasing to 100mg or switching to tadalafil.
Side Effects
| Side effect | Frequency | Notes |
|---|---|---|
| Headache | Very common (up to 1 in 10) | Caused by cerebral vasodilation; resolves as medication wears off |
| Facial flushing | Very common | Warmth and redness; usually mild and transient |
| Nasal congestion | Common | Temporary; saline spray may help |
| Indigestion / nausea | Common | Take with a light meal next time |
| Dizziness | Common | Sit or lie down; worsened by alcohol and standing quickly |
| Visual disturbances (blue tinge, blurred vision) | Uncommon | Temporary; avoid driving if vision significantly affected |
| Back pain / muscle aches | Uncommon | More common with tadalafil; less so with sildenafil |
| Palpitations | Uncommon | Inform prescriber if persistent |
| Priapism (erection >4 hours) | Rare — medical emergency | Go to A&E immediately; permanent damage can occur without treatment |
| Sudden vision or hearing loss | Very rare — medical emergency | Stop sildenafil; call 999 |
Contraindications and Key Drug Interactions
- Nitrate medicines (all forms) — absolute contraindication. GTN spray, isosorbide mononitrate, recreational “poppers”. Risk of severe, potentially fatal blood pressure drop.
- Riociguat — contraindicated
- Alpha-blockers (tamsulosin, doxazosin) — additive blood-pressure-lowering; start at 25mg
- CYP3A4 inhibitors (clarithromycin, ketoconazole, itraconazole) — increase sildenafil levels; reduce dose to 25mg maximum
- Ritonavir/HIV protease inhibitors — markedly increase sildenafil exposure; maximum 25mg per 48 hours
- Severe liver disease, recent stroke or MI (past 6 months), severe hypotension — contraindicated
- Retinitis pigmentosa or previous NAION — contraindicated
Frequently Asked Questions
What is the starting dose of sildenafil for ED?
50mg for most men, taken 30–60 minutes before sexual activity. Men over 65 or with kidney/liver impairment start at 25mg. Maximum dose is 100mg under prescriber guidance.
What are the most common side effects of sildenafil?
Headache, facial flushing, nasal congestion, indigestion, and dizziness. Visual disturbances (temporary blue tinge) occur in some men. Most are mild and resolve within a few hours.
Can I take sildenafil with alcohol?
Small amounts are generally acceptable. Large amounts impair erectile function and worsen dizziness. Avoid more than 2 standard drinks.
What is sildenafil used for besides erectile dysfunction?
At lower doses (20mg three times daily, as Revatio), sildenafil is licensed for pulmonary arterial hypertension. It is also used off-label for altitude sickness and some fertility contexts under specialist supervision.
What happens if sildenafil 50mg doesn’t work?
Check timing (1 hour before, empty stomach), stimulation, and anxiety. If correctly used multiple times at 50mg without effect, your prescriber may increase to 100mg or switch to tadalafil.
How long does sildenafil stay in your system?
Sildenafil has a half-life of approximately 3–5 hours. It is substantially cleared within 24 hours, which is why no repeat dose is permitted within that window.
References
- National Institute for Health and Care Excellence (NICE). Erectile dysfunction: Clinical Knowledge Summary. Updated 2023. cks.nice.org.uk/topics/erectile-dysfunction
- NHS. Erectile dysfunction (impotence). nhs.uk/conditions/erection-problems-erectile-dysfunction
- Goldstein I et al. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397–1404. PubMed: 9560547
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. ED treatments are prescription-only medicines requiring clinical assessment. In a medical emergency, call 999.