What Is Sildenafil? Dose, Success Rates and Side Effects Compared

Sildenafil is the active ingredient in Viagra. Every tablet sold as Cenforce, Fildena, Kamagra or plain generic sildenafil contains the same molecule that Pfizer brought to market in 1998. What changes is the maker, the strength and the price.

Most pages about sildenafil list the side effects in one place and the success rates somewhere else, so you never see what a higher dose actually buys you against what it costs. The FDA label contains both sets of numbers, from the same trial programme. This guide puts them side by side.

The short answer

Sildenafil is a PDE5 inhibitor approved for erectile dysfunction. The usual starting dose is 50 mg, taken about an hour before sex, and it can be adjusted up to 100 mg or down to 25 mg, never more than once a day. It works for roughly three quarters of men at 50 mg, against about a quarter on placebo. Effects are strongest in the first two hours and weaker by four, though trials found a smaller effect up to 12 hours in many men. Headache, flushing and indigestion are the common complaints; headache and indigestion get clearly worse as the dose rises. Nitrates are an absolute contraindication.

What sildenafil is, and the names you will see

Sildenafil citrate is the salt form used in tablets. The FDA approved Viagra in March 1998, the patent has long since expired in most markets, and the molecule is now made by dozens of manufacturers.

Name	Where it is sold	Strengths
Viagra	United States, UK, Australia and most markets (brand)	25, 50, 100 mg
Generic sildenafil	Widely available since patent expiry	20, 25, 50, 100 mg
Cenforce	Centurion Laboratories, India	25 to 200 mg
Fildena	Fortune Healthcare, India	25 to 200 mg
Kamagra	Ajanta Pharma, India	50 and 100 mg, plus jelly and chewable forms
Revatio	Same molecule, licensed for pulmonary hypertension	20 mg

Revatio matters for one reason: it is sildenafil under a different name at a lower strength, and taking it alongside an erection tablet means doubling up on the same drug. The label warns against exactly that. What 20 mg tablets are approved for, and how they are dosed in pulmonary hypertension, is set out in our guide to sildenafil 20 mg.

How sildenafil works

An erection depends on nitric oxide released in the penis during sexual stimulation. Nitric oxide triggers an enzyme that raises cyclic GMP, which relaxes smooth muscle and lets blood flow in. An enzyme called PDE5 breaks cyclic GMP down again.

Sildenafil blocks PDE5, so cyclic GMP lasts longer and the erection holds. The label is blunt about the limit of this: sildenafil has no direct relaxant effect on its own, and at recommended doses it does nothing without sexual stimulation. It is not an aphrodisiac and it does not create desire.

Selectivity explains the side effects. Sildenafil is roughly 4,000 times more selective for PDE5 than for PDE3, the enzyme that controls heart contraction, which is why it does not act on the heart directly. But it is only about 10 times more selective for PDE5 than for PDE6, which sits in the retina and handles light signalling. That narrow margin is why colour vision changes appear at higher doses.

What the dose buys you

Four randomised, double-blind, placebo-controlled fixed-dose studies covering 1,797 men asked a simple question at the end: are your erections improved?

Dose	Reported improved erections	Improvement over placebo
Placebo	24%	-
25 mg	63%	39 points
50 mg	74%	50 points
100 mg	82%	58 points

Diary data from about 1,600 men in the same programme is more concrete than a yes or no question. Sildenafil did not change how often men attempted sex, which stayed at roughly twice a week either way. What changed was the outcome: successful attempts per week averaged 1.3 on 50 to 100 mg against 0.4 on placebo, and the overall success rate was about 66% against about 20%.

What the dose costs you

The same fixed-dose programme recorded side effects by strength. These are the reactions reported by at least 2% of men and more often than placebo.

Side effect	25 mg (n=312)	50 mg (n=511)	100 mg (n=506)	Placebo (n=607)
Headache	16%	21%	28%	7%
Flushing	10%	19%	18%	2%
Indigestion	3%	9%	17%	2%
Abnormal vision	1%	2%	11%	1%
Nasal congestion	4%	4%	9%	2%
Back pain	3%	4%	4%	2%
Muscle pain	2%	2%	4%	1%
Nausea	2%	3%	3%	1%
Dizziness	3%	4%	3%	2%
Rash	1%	2%	3%	1%

Abnormal vision on this label means a colour tinge to what you see, usually blue or green, sometimes light sensitivity or blurring. The label describes it as mild to moderate and transient. In the flexible-dose studies, one man out of 734 stopped treatment because of it.

Reading those two tables together

Subtract the placebo column and the trade at each step becomes clear.

Going from 25 mg to 50 mg buys about 11 extra points of success. It costs about 5 extra points of headache, 9 of flushing and 6 of indigestion. For most men that is a reasonable trade, which is why 50 mg is the standard starting dose rather than 25 mg.

Going from 50 mg to 100 mg buys about 8 extra points of success. It costs about 7 extra points of headache, 8 of indigestion and 9 of abnormal vision. Colour vision changes go from 2% to 11%, which is more than a fivefold jump at the top step. Tablets above 100 mg push that trade further; our guide to sildenafil 150 mg and 200 mg covers what studies found.

Measure	50 mg	100 mg	Change
Improved erections	74%	82%	+8 points
Abnormal vision	2%	11%	+9 points
Indigestion	9%	17%	+8 points
Headache	21%	28%	+7 points
Nasal congestion	4%	9%	+5 points
Flushing	19%	18%	-1 point

A caution on reading this: the benefit figures come from four fixed-dose efficacy studies and the side effect figures from the fixed-dose studies in the label's Table 1. Both are from the same trial programme but not the same set of patients, so treat the comparison as a guide to proportion rather than an exact trade.

So the curve flattens for benefit and steepens for harm. The top dose is worth taking if 50 mg did not work after a fair trial, and worth avoiding if it did. Doubling the dose after a good result on 50 mg is buying the worst part of that trade.

One number puts the side effects in proportion: across the placebo-controlled studies, 2.5% of men on sildenafil stopped because of side effects, against 2.3% on placebo. Headache and flushing are common but they rarely drive anyone off the drug.

How long sildenafil lasts

The half-life is about four hours for sildenafil and its active metabolite. In practice that means the following.

Time after dosing	What to expect
30 to 120 minutes	Peak blood levels, median about 60 minutes on an empty stomach
Up to 2 hours	Strongest effect on erection quality
2 to 4 hours	Still working, but measurably weaker than at 2 hours
About 4 hours	Levels well down from the peak; the label allows dosing up to 4 hours before sex
24 hours	Blood levels around 2 ng/mL, against a peak near 440 ng/mL

That 24 hour figure matters for anyone who might need nitrates. Levels are very low by then, but the label is explicit that it is still unknown when nitrates can be given safely after a dose.

The window is different if you are older, have liver problems, have severe kidney impairment or take a strong CYP3A4 inhibitor. In those groups, 24 hour levels run three to eight times higher than in healthy volunteers.

Studies that tested later time points found a smaller but measurable effect 8 and 12 hours after 100 mg in many men. Our guide on how long sildenafil lasts goes through onset, the 12 hour data, refractory time and how long it stays in the body.

When to take it, and what food does

The label says about one hour before sex, with a usable range of 30 minutes to 4 hours. A high fat meal delays the peak by about an hour and cuts peak concentration by 29%.

That does not make the tablet fail. It makes it slower and somewhat weaker. If you are eating a heavy meal, take the tablet earlier rather than increasing the dose.

Alcohol is a separate question. In a study using 50 mg with alcohol at 0.5 g/kg, reaching a blood alcohol level around 0.08%, sildenafil did not add to alcohol's blood pressure effect. Note the limits: that was 50 mg, not 100 mg, and moderate drinking, not heavy. Alcohol interferes with erections on its own regardless.

How to take sildenafil safely the first time

A first dose is worth treating as a test rather than an event. The full checklist, the alpha blocker and alcohol data and the interacting medicines are in our guide on how to take sildenafil safely.

1. Confirm with a doctor that sexual activity is safe for your heart, and that you take no nitrates in any form.
2. Start at 50 mg unless you fall into one of the 25 mg groups below.
3. Take it on a relatively empty stomach, about an hour before.
4. Expect nothing without arousal. The drug does not work on its own.
5. Note what happened: the erection, the timing and any headache or flushing.
6. Give it more than one attempt before judging it. Trial patients averaged around two attempts a week over weeks of treatment.
7. Only then discuss moving to 100 mg or down to 25 mg.

Who needs to start at 25 mg

The label names four groups where a 25 mg start should be considered, because each one raises blood levels of the drug.

Group	Effect on drug levels
Age over 65	AUC about 84% higher than in younger men
Severe kidney impairment (CrCl under 30 mL/min)	Roughly double the exposure
Liver impairment (Child-Pugh A or B)	AUC 85% higher, peak 47% higher
Taking an alpha blocker	Additive blood pressure drop; start at the lowest dose

Mild and moderate kidney impairment need no adjustment. Severe liver impairment (Child-Pugh C) has never been studied.

Nitrates: the one absolute rule

Sildenafil is contraindicated with any nitric oxide donor. That covers nitroglycerin, isosorbide mononitrate and dinitrate, and recreational amyl or butyl nitrite, the poppers sold in some venues. Both work on the same pathway, and together they can drop blood pressure to a dangerous level.

Riociguat, a guanylate cyclase stimulator used in pulmonary hypertension, is contraindicated for the same reason.

If you get chest pain after taking sildenafil, tell whoever treats you when you took it and at what dose. That information changes what they can safely give you.

Alpha blockers and blood pressure medicines

Alpha blockers such as tamsulosin, doxazosin and terazosin are often prescribed for prostate symptoms. They are vasodilators, and so is sildenafil, so the effects add up.

The label handles this with sequencing rather than a ban. Be stable on the alpha blocker first, then start sildenafil at 25 mg. If you are already on a settled sildenafil dose, start the alpha blocker at its lowest dose.

Ordinary blood pressure medicines are less of an issue. With amlodipine, sildenafil 100 mg added about 8 mmHg systolic and 7 mmHg diastolic. Analysis of the trial safety database found no apparent difference in side effects between men taking blood pressure medication and men not taking it.

Drugs that raise sildenafil levels

Sildenafil is cleared mainly by CYP3A4, so anything that blocks that enzyme pushes levels up.

Drug	Effect on sildenafil exposure	What the label advises
Ritonavir	11-fold increase in AUC	Maximum 25 mg in any 48 hour period
Saquinavir	210% increase in AUC	Consider starting at 25 mg
Erythromycin	182% increase in AUC	Consider starting at 25 mg
Ketoconazole, itraconazole	Expected to exceed saquinavir	Consider starting at 25 mg
Cimetidine	56% increase	Caution
Bosentan	63% decrease in AUC	Reduced effect

Grapefruit juice affects CYP3A4 too, though it is not quantified on this label.

Side effects that need urgent attention

Three things justify stopping the drug and seeking care immediately.

An erection lasting more than four hours. Priapism means painful erections beyond six hours. Untreated, it can permanently damage penile tissue. Men with sickle cell disease, multiple myeloma, leukaemia or an anatomically deformed penis are at higher risk, and the label notes there is no controlled safety data in sickle cell.

Sudden vision loss in one or both eyes. This can signal non-arteritic anterior ischemic optic neuropathy, covered below.

Sudden hearing loss, sometimes with ringing or dizziness.

Chest pain, dizziness or nausea starting during sex is a reason to stop and speak to a doctor, though that reflects the strain of sex on an unwell heart as much as the tablet.

Vision and hearing in more detail

The colour tinge at higher doses is expected pharmacology from that narrow PDE5 to PDE6 margin, and it passes. NAION is a different matter: a rare condition causing loss of vision that can be permanent.

The label gives useful context. Background incidence of NAION is 2.5 to 11.8 cases per 100,000 men aged 50 and over each year. Two observational studies suggested roughly double that risk around the time of PDE5 inhibitor use, with estimates of 2.15 (95% CI 1.06 to 4.34) and 2.27 (95% CI 0.99 to 5.20). The label states plainly that neither the post-marketing reports nor these studies establish that PDE5 inhibitors cause NAION.

Doubling a rare risk still leaves it rare. What matters more is that men who have already had NAION in one eye are at higher risk of it recurring, and men with a crowded optic disc may also be at greater risk.

How well it works if you have diabetes

Diabetes damages the small blood vessels and nerves erections rely on, so it is worth knowing how sildenafil performed in men with diabetic ED specifically.

One flexible-dose trial enrolled 268 such men. On a global question, 57% reported improved erections against 10% on placebo. Diary data showed 48% of attempts succeeded against 12% on placebo.

Those figures are real but lower than the 74% to 82% seen in the general trial population. Sildenafil helps in diabetic ED. It helps less than it does in men without diabetes, and that is worth knowing before you conclude a dose has failed. Our erectile dysfunction guide covers the underlying causes in more detail.

After prostate surgery and spinal cord injury

Response varies sharply by cause, and the label separates these groups out.

Group	Improved on sildenafil	Improved on placebo
Psychogenic ED	84%	26%
Spinal cord injury	83%	12%
Diabetes	57%	10%
After radical prostatectomy	43%	15%

The prostatectomy figure is the lowest in the set, which reflects the nerve damage that surgery can cause. It is still nearly three times the placebo rate.

Sildenafil against the other three tablets

Four PDE5 inhibitors are approved for ED, and the practical differences are timing, duration and food sensitivity rather than how well they work. Every row below comes from the four US prescribing labels.

	Sildenafil	Tadalafil	Vardenafil	Avanafil
Half-life	About 4 hours	About 17.5 hours	About 4 to 5 hours	About 5 hours
When the label says to take it	About 1 hour before (30 minutes to 4 hours)	At least 30 minutes before; effect up to 36 hours	About 60 minutes before	As early as about 15 minutes before (30 minutes for 50 mg)
High fat meal	Peak about 1 hour later and 29% lower	No effect	Peak 18% to 50% lower	Peak about 1 hour later and 39% lower; label allows with or without food
Licensed daily dosing for ED	No	Yes, 2.5 or 5 mg	No	No

Tadalafil's long half-life is the main reason men switch to it, since it removes the planning. Our tadalafil versus sildenafil comparison goes through the trade in detail, and the avanafil guide covers the fastest of the four.

Generic sildenafil against brand Viagra

A generic has to contain the same active ingredient at the same strength and has to be shown to deliver comparable blood levels. Where it can differ is in inactive ingredients, tablet appearance, and which regulator approved the factory. Our generic Viagra review shows what published bioequivalence studies of sildenafil found.

The gap that matters is not brand against generic. It is a manufacturer you can verify against one you cannot. A WHO-GMP certified plant has been inspected against a recognised standard. An anonymous seller has not. Our guide to what WHO-GMP certification means explains what that inspection covers.

What to check before buying sildenafil online

Sildenafil is prescription-only in the United States, the UK, Australia and most other markets. Anything sold as an over-the-counter erection pill is either not sildenafil or not legal where you are.

Before ordering anywhere, check the following.

• The manufacturer is named, and the name is one you can look up.
• The strength is stated in milligrams of sildenafil.
• The seller does not claim a prescription is unnecessary where you live.
• There is a real address and a way to reach a person.
• The site does not promise results the FDA label does not support.

Be careful with supplements marketed for erections. The FDA repeatedly finds undeclared sildenafil in products sold as herbal, which means an unknown dose with no warnings about nitrates. That combination is the dangerous one.

Our guide to checking whether an online pharmacy is legitimate covers verification in more detail.

Common reasons sildenafil looks like it failed

Before concluding the drug does not work for you, rule out the ordinary explanations.

• No sexual stimulation. The label is explicit that sildenafil does nothing without it. This is the single most common reason a first dose disappoints.
• Taken with a heavy meal. An hour's delay and 29% less peak concentration can look like failure.
• Judged on one attempt. The trials ran for weeks.
• Too little time. Thirty minutes is the earliest the label allows, not the typical timing.
• Alcohol. It works against erections independently of the tablet.
• Dose too low for you. 25 mg helps 63% of men; 50 mg helps 74%.

Our ED medicine dosage guide covers adjustment across the class.

When to see a doctor

See a doctor before starting if you have heart disease, have had a heart attack or stroke in the last six months, have very low or uncontrolled high blood pressure, take nitrates or riociguat, have retinitis pigmentosa, or have had NAION.

See a doctor after starting if 100 mg used properly on several occasions does not work, if side effects persist, or if you have had an erection lasting over four hours, sudden vision loss or sudden hearing loss.

Erectile dysfunction is also a symptom. It can be the first sign of cardiovascular disease or undiagnosed diabetes, and it can be caused by medicines you already take. A tablet that works still leaves that question unanswered, which is the strongest argument for having the conversation rather than self-treating indefinitely.

Sources

• VIAGRA (sildenafil citrate) tablets, US prescribing information, sections 2, 4, 5, 6, 7, 8, 12 and 14. DailyMed
• Dose-response efficacy figures: section 14, pooled fixed-dose studies, n=1797.
• Side effect incidence by dose: section 6.1, Table 1, fixed-dose Phase II/III studies.

This article is for information only and is not medical advice. Sildenafil is a prescription medicine. Talk to a doctor or pharmacist before starting, stopping or changing any treatment.