At a glance

What it is

How sildenafil works

An erection depends on a signaling chemical called cyclic GMP. It relaxes the smooth muscle in the penis so blood can fill it. An enzyme called PDE5 clears cyclic GMP away. Sildenafil blocks that enzyme, so the signal that sexual stimulation starts lasts longer.

That means the pill does nothing without arousal. The label states that sexual stimulation is required for a response. Sildenafil does not raise testosterone, create desire, or treat the cause of ED. It helps the erection mechanism work.

Sildenafil is absorbed fast. Peak blood levels come 30 to 120 minutes after a dose on an empty stomach, with a median of 60 minutes. The label gives a half-life of about 4 hours for the drug and for its active breakdown product. It is cleared mostly by the liver enzyme CYP3A4, which is why some other medicines change its dose.

Who it is for

Who sildenafil is, and is not, meant for

Sildenafil is approved for ED in men. The label asks that the evaluation include the likely causes of ED, because ED can be an early sign of vascular disease or diabetes. The American Urological Association guideline says all patients should hear about every treatment that is not contraindicated and decide together with their clinician (Burnett, see sources).

• Not for you if you use a nitrate in any form, regularly or now and then. This includes nitroglycerin tablets or spray, isosorbide, and amyl or butyl nitrite poppers.
• Not for you if you take riociguat, a pulmonary hypertension drug, or have had an allergic reaction to sildenafil.
• Think twice if sex is inadvisable for your heart. The label says ED drugs should generally not be used in men for whom sexual activity is a cardiac risk.
• Use caution with a deformed penis or Peyronie’s disease, sickle cell disease, multiple myeloma, leukemia, bleeding disorders, active stomach ulcers, or retinitis pigmentosa. The label says these groups are not well studied.

How it is taken

Label dosing, timing, food and alcohol

• Usual dose: 50 mg taken as needed about 1 hour before sexual activity. The label allows 30 minutes to 4 hours before.
• Range: 25 mg to 100 mg, set by how well it works and how it is tolerated.
• Frequency: no more than once per day.
• Start at 25 mg if you are over 65, have liver impairment such as cirrhosis, have severe kidney impairment (creatinine clearance under 30 mL/minute), take a strong CYP3A4 inhibitor, or are starting while on an alpha-blocker.

Food

The label says sildenafil may be taken with or without food. A fatty meal does slow it. After a high-fat meal, the label reports a 60 minute delay in peak level and a 29 percent lower peak. If a pill seems slow, a heavy dinner is a common reason. A lighter meal or an empty stomach lets it act sooner, though the label does not require that.

How long it lasts

The label does not state a number of hours of effect. It reports a half-life of about 4 hours, so blood levels fall over the evening. How long a given man responds varies, and the label’s dosing window of up to 4 hours before sex is the safest guide.

Alcohol

In a label study of healthy volunteers, sildenafil 50 mg with alcohol at 0.08 percent blood level did not add to alcohol’s effect on blood pressure. That does not mean alcohol helps. Heavy drinking can itself make erections harder and can add to dizziness, so moderate use is the sensible rule.

The evidence

What the trials showed

The label summarizes 21 randomized, placebo-controlled trials lasting up to 6 months in more than 3,000 men aged 19 to 87. Sildenafil beat placebo in all 21.

• Goldstein 1998: in a 24-week dose-response study, 532 men took 25, 50 or 100 mg or placebo. In a separate 12-week flexible-dose study of 329 men, 69 percent of attempts at intercourse succeeded on sildenafil versus 22 percent on placebo. Successful attempts averaged 5.9 per month versus 1.5.
• Pooled fixed-dose studies (1,797 men, 12 to 24 weeks): 63, 74 and 82 percent of men on 25, 50 and 100 mg said their erections improved, versus 24 percent on placebo.
• Diabetes (268 men): 57 percent reported improved erections versus 10 percent on placebo. Diaries showed 48 percent of attempts succeeded versus 12 percent.
• Spinal cord injury (178 men): 83 percent reported improvement versus 12 percent.
• After radical prostatectomy: across all trials, 43 percent improved versus 15 percent on placebo.

These are self-reported improvements in selected trial populations, and results vary man to man. The same label notes that about 1 in 4 men on placebo also reported better erections, which is a real placebo response. Trials in different groups of men are not directly comparable.

Safety

Side effects and the warnings that matter

In the fixed-dose trials, headache occurred in 16, 21 and 28 percent of men on 25, 50 and 100 mg, against 7 percent on placebo. Flushing was 10, 19 and 18 percent against 2. Indigestion was 3, 9 and 17 percent against 2. Nasal congestion, back pain, muscle aches, nausea, dizziness and rash also occurred. At 100 mg, 11 percent reported a vision change, mostly a color tinge, light sensitivity or blur. Only 2.5 percent stopped because of side effects, close to the 2.3 percent on placebo.

Interactions

Alpha-blockers, blood pressure drugs and CYP3A4 inhibitors

• Alpha-blockers such as tamsulosin or doxazosin. Both drug types lower blood pressure, and fainting can result. You should be stable on the alpha-blocker first, and sildenafil starts at 25 mg.
• Blood pressure medicines. With amlodipine, a 100 mg sildenafil dose lowered blood pressure by a further 8 mmHg systolic and 7 mmHg diastolic on average.
• Ritonavir raised sildenafil exposure 11-fold. The label limits sildenafil to 25 mg in any 48 hours.
• Other CYP3A4 blockers such as ketoconazole, itraconazole, saquinavir and erythromycin raise levels about 3-fold. The label suggests a 25 mg start.
• Other PDE5 pills or ED treatments. The label does not recommend combining them. That includes Revatio, which is sildenafil sold for pulmonary arterial hypertension.

Heart

Cardiac cautions

Sex raises heart demand, so the question is whether your heart is ready for it. In healthy volunteers sildenafil lowered supine blood pressure by an average maximum of 8.4/5.5 mmHg. The label advises caution in men with aortic stenosis or hypertrophic obstructive heart disease and in men with severely impaired autonomic control of blood pressure.

The label says there are no controlled data in men who had a heart attack, stroke or life-threatening arrhythmia in the last 6 months, in men with resting blood pressure under 90/50 or over 170/110, or in men with heart failure or unstable angina. The Princeton III consensus panel advises gauging exercise capacity, with stress testing where needed, before treating ED, and treats ED itself as a prompt to look at cardiovascular risk (Nehra, see sources). Read more about heart risk in this post on cardiac inflammation.

If chest pain, dizziness or nausea start during sex, stop and tell your physician. Do not use a nitroglycerin tablet you may have on hand without telling emergency staff you took sildenafil.

If it does not work

What to try when sildenafil falls short

A first dose that fails does not close the question. The label points to several fixable reasons:

• There was no sexual stimulation, which the drug needs.
• A heavy, fatty meal delayed absorption.
• The dose was too low. The label allows a step from 50 mg to 100 mg.
• A drug interaction or liver or kidney problem changed the level you actually got.

If a properly timed, adjusted dose still fails, your physician looks at causes sildenafil does not treat: low testosterone, diabetes control, sleep apnea, depression, other medicines and relationship stress. Low testosterone is real but not always the explanation. In a trial of 140 men with ED and low testosterone, optimizing sildenafil first and then adding testosterone gel did not beat placebo gel on erectile function scores (Spitzer, see sources). If labs show low testosterone, see testosterone or the low T overview. Switching to another PDE5 inhibitor is a common next step.

Compared

How sildenafil sits among the four pills

All four are PDE5 inhibitors and share the same warnings. They differ in timing, food effects and dosing details.

• Sildenafil: about 1 hour before sex, a fatty meal slows it, half-life about 4 hours.
• Vardenafil: about 1 hour before. It also comes as a tablet that dissolves on the tongue.
• Avanafil: labeled for use as early as 15 minutes before sex.
• Tadalafil: longer acting, and the only one of the four also approved for an enlarged prostate. It can be taken daily.

A 2013 network meta-analysis of 118 randomized trials (31,195 men) found that every oral PDE5 inhibitor beat placebo, that tadalafil and vardenafil ranked highest on effectiveness, and that safety profiles did not differ much (Yuan, see sources). That is an indirect comparison across different trials, so it does not tell any one man which pill will suit him.

Summit’s role

What Summit does with sildenafil

Dr. Miranda decides which erectile dysfunction medicine fits you after your history and labs. Summit prescribes tadalafil in Summit Men and covers sildenafil here so you can understand the whole class. Summit does not sell or dispense Viagra or any other brand-name product.

Summit is telehealth only. The review covers your medicine list, with a direct question about nitrates and riociguat, plus your blood pressure and heart history. Nothing here promises that any medicine will be prescribed. See pricing for what is included.

Keep reading

Related medications

Sources

Where this comes from

All retrieved November 10, 2026.

1. Viagra (sildenafil citrate) prescribing information. Pfizer. DailyMed. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=0b0be196-0c62-461c-94f4-9a35339b4501
2. Goldstein I et al. Oral sildenafil in the treatment of erectile dysfunction. Sildenafil Study Group. N Engl J Med 1998. https://pubmed.ncbi.nlm.nih.gov/9580646/
3. Yuan J et al. Comparative effectiveness and safety of oral phosphodiesterase type 5 inhibitors for erectile dysfunction: a systematic review and network meta-analysis. Eur Urol 2013. https://pubmed.ncbi.nlm.nih.gov/23395275/
4. Nehra A et al. The Princeton III Consensus recommendations for the management of erectile dysfunction and cardiovascular disease. Mayo Clin Proc 2012. https://pubmed.ncbi.nlm.nih.gov/22862865/
5. Burnett AL et al. Erectile Dysfunction: AUA Guideline. J Urol 2018. https://pubmed.ncbi.nlm.nih.gov/29746858/
6. Spitzer M et al. Effect of testosterone replacement on response to sildenafil citrate in men with erectile dysfunction: a parallel, randomized trial. Ann Intern Med 2012. https://pubmed.ncbi.nlm.nih.gov/23165659/