Sildenafil is the active ingredient in Viagra and in generic versions such as Cenforce, Fildena and Kamagra. It treats erectile dysfunction by relaxing blood vessels so more blood reaches the penis during arousal. It does not create desire and does nothing without sexual stimulation.
This guide covers dosage, side effects, how long it lasts, what it must never be combined with, and why generic price differences are so wide.
Short answer: Sildenafil is usually taken 30–60 minutes before sex, at 25mg, 50mg or 100mg, once daily at most. It lasts around 4–6 hours. Common side effects are headache, flushing, indigestion and a blocked nose. It must never be taken with nitrate medicines for chest pain — that combination can cause a dangerous drop in blood pressure. It is prescription-only.
How Sildenafil Works
Arousal causes the release of nitric oxide, which triggers a chemical signal telling penile blood vessels to relax. An enzyme called PDE5 breaks that signal down. Sildenafil blocks PDE5, so the signal lasts longer and blood flow is sustained.
Two consequences follow, and both are commonly misunderstood:
- It requires arousal. Without stimulation there is no signal to amplify, and nothing happens.
- It does not increase desire. Libido is a separate mechanism entirely.
Dosage
| Dose | Typically for |
|---|---|
| 25 mg | Older men, kidney or liver impairment, or where interactions require a lower dose |
| 50 mg | The usual starting dose |
| 100 mg | Where 50mg is insufficient and tolerated |
Key points on timing and use:
- Take 30–60 minutes before sexual activity
- Maximum once in 24 hours
- A heavy or high-fat meal delays absorption and slows onset
- Alcohol worsens side effects and can itself impair erection
- It may take several attempts before it works reliably — a single failure is not proof it does not work
Side Effects
Common
- Headache — the most frequent
- Facial flushing
- Indigestion or reflux
- Blocked or runny nose
- Dizziness
- Blue tinge to vision or light sensitivity, usually at higher doses
Seek Urgent Medical Help For
- An erection lasting more than 4 hours (priapism) — this is an emergency and can cause permanent damage
- Sudden loss of vision in one or both eyes
- Sudden hearing loss or ringing in the ears
- Chest pain during or after sex
- Fainting
The MedlinePlus entry on sildenafil lists the complete profile.
The Interaction That Can Kill
Never combine sildenafil with nitrates. Nitrate medicines — glyceryl trinitrate, isosorbide mononitrate or dinitrate, and amyl nitrite or “poppers” — are used for angina. Both drugs lower blood pressure through the same pathway, and together they can cause a catastrophic drop.
Also requiring medical review before use:
- Alpha-blockers for blood pressure or prostate symptoms — risk of fainting; doses must be separated
- Riociguat for pulmonary hypertension — contraindicated
- Some antifungals and antibiotics (ketoconazole, itraconazole, erythromycin) — raise sildenafil levels
- HIV protease inhibitors, particularly ritonavir — substantially raise levels; a lower dose is required
- Grapefruit juice — increases absorption unpredictably
Who Should Avoid It
- Anyone taking nitrates in any form
- Men who have had a recent heart attack or stroke
- Severe heart failure or unstable angina
- Very low blood pressure
- Severe liver impairment
- A history of non-arteritic anterior ischaemic optic neuropathy (NAION)
- Certain inherited retinal conditions such as retinitis pigmentosa
Why Generic Prices Vary So Widely
Sildenafil’s patent has expired, so many manufacturers produce it. Generics containing the same active ingredient at the same dose are therapeutically equivalent when properly manufactured — the price difference reflects branding, manufacturing scale and distribution, not effectiveness.
What does vary is manufacturing quality. The safeguard is buying from a pharmacy that sources from regulated, inspected facilities, and requiring a prescription. A seller willing to supply prescription medicines without one is not a safeguard against anything.
Sildenafil products are grouped under Sildenafil, including Cenforce 100 Mg, Fildena 100 Mg and Kamagra Oral Jelly. The wider range sits under Erectile Dysfunction.
Sildenafil vs Tadalafil
| Sildenafil | Tadalafil | |
|---|---|---|
| Onset | 30–60 minutes | 30–120 minutes |
| Duration | 4–6 hours | Up to 36 hours |
| Food effect | Delayed by fatty meals | Minimal |
| Daily low dose | Not typical | Available (2.5–5mg) |
Neither is better in general — tadalafil suits people who prefer spontaneity, sildenafil those who prefer a defined window. Tadalafil products are under Tadalafil.
Frequently Asked Questions
What are the side effects of sildenafil?
Most commonly headache, flushing, indigestion, blocked nose and dizziness. Rarely, priapism, sudden vision loss or sudden hearing loss — all of which need urgent care.
What dose of sildenafil should I take?
50mg is the usual starting dose, adjusted to 25mg or 100mg by a doctor based on response, age, kidney and liver function and other medicines. Never exceed one dose in 24 hours.
How long does sildenafil last?
Around 4–6 hours, though the strongest effect is in the first two.
How long before sex should I take it?
30–60 minutes. A heavy or fatty meal delays this.
Can I drink alcohol with sildenafil?
Small amounts are usually tolerated, but alcohol lowers blood pressure and impairs erections in its own right, so heavy drinking works against the medicine and worsens side effects.
Is generic sildenafil as effective as Viagra?
A properly manufactured generic with the same active ingredient and dose is therapeutically equivalent. What matters is the manufacturing standard and the supply chain.
Does sildenafil increase desire?
No. It affects blood flow only. If the problem is low desire rather than erection, it is the wrong treatment.
What if it does not work the first time?
That is common. Several attempts are often needed. If it still fails, see a doctor — the dose may be wrong, or an underlying cause such as diabetes, low testosterone or vascular disease may need treating.
Health and Medical Disclaimer
This article is general information, not medical advice, and does not replace consultation with a qualified healthcare professional. Sildenafil is a prescription-only medicine. It must never be taken with nitrate medicines for chest pain, including amyl nitrite, as the combination can cause a life-threatening fall in blood pressure. Seek emergency care for an erection lasting more than four hours, sudden vision or hearing loss, or chest pain. Do not take sildenafil obtained without a prescription or a medical assessment.
