Sildenafil is a prescription PDE5 inhibitor, sold as brand Viagra and as generic tablets, that the FDA has approved to treat erectile dysfunction (ED).1,2 It strengthens the natural blood flow response to arousal and has no effect without sexual stimulation.1 In 21 placebo-controlled trials, it improved erections in every study.1 It must never be combined with nitrates. Your prescriber chooses the dose from your health history, other medicines and how you respond.
What Sildenafil Treats
Sildenafil tablets sold for ED, brand or generic, carry one FDA-approved use: the treatment of erectile dysfunction.1,2 The label states that the ED tablet is not indicated for use in women.1 If cost is part of your decision, ED medication cost compares brand and generic pricing.
The same molecule is also sold under the brand name Revatio for a different disease. Revatio is approved for pulmonary arterial hypertension (PAH), a form of high blood pressure in the lungs. In adults it is used to improve exercise ability and delay clinical worsening, and it is also approved for children aged 1 to 17.3 Revatio is a separate prescription with its own dosing, so an ED prescription does not treat PAH.
Sildenafil belongs to the PDE5 inhibitor class, which also includes tadalafil. The American Urological Association (AUA) guideline asks clinicians to tell every man about all treatments that are not contraindicated and to choose through shared decision-making.4 You have a say in which option you start with, and sildenafil is often on that list.
How Sildenafil Works in the Body
An erection starts when sexual stimulation releases nitric oxide in the erectile tissue. Nitric oxide activates an enzyme that raises a messenger molecule called cyclic GMP (cGMP). Cyclic GMP relaxes smooth muscle and lets blood flow into the penis.1
PDE5 is the enzyme that breaks cGMP down. Sildenafil blocks PDE5, so cGMP stays higher once arousal has started.1 The label states that sildenafil has no direct relaxing effect on erectile tissue and no effect without sexual stimulation.1 It does not create desire. It supports the blood flow response once you are aroused.
Sildenafil is about 4,000 times more selective for PDE5 than for PDE3, an enzyme that helps control how hard the heart contracts.1 It is only about 10 times more selective for PDE5 than for PDE6, an enzyme in the retina.1 The label says that narrow margin is thought to explain the color vision changes some men notice. If you notice a color tinge after a dose, mention it to your prescriber. A sudden loss of vision in one or both eyes is a separate warning that needs medical care right away.1
In one label study, the effect lasted up to 4 hours, though the response was weaker than at 2 hours.1 How long does sildenafil last covers timing, food and wear-off in detail.
What the Trials Found
If you want to know how well sildenafil works, the label summarizes 21 randomized, double-blind, placebo-controlled trials of up to 6 months, in more than 3,000 men aged 19 to 87.1 The men had ED from physical, psychological or mixed causes, and sildenafil improved results compared with placebo in all 21 studies.1
In the fixed-dose trials, 63% to 82% of men reported improved erections across the doses studied, compared with 24% on placebo.1 At the end of the long-term study, 88% of men reported that sildenafil improved their erections.1 Those are group results, and your own response is what your prescriber tracks.
Men with diabetes also responded in a dedicated trial. In a trial limited to 268 men with ED from diabetes complications, 57% on sildenafil reported improved erections compared with 10% on placebo. Diary records showed 48% of intercourse attempts succeeded on sildenafil compared with 12% on placebo.1
An independent meta-analysis included 27 randomized trials with 6,659 men. In its 14 pooled flexible-dose trials, 57% of intercourse attempts succeeded on sildenafil, compared with 21% on placebo.5 In those trials, 83% of men on sildenafil had at least one success during treatment, compared with 45% on placebo.5 Sildenafil improved erectile function in every subgroup evaluated.5 Response varied between groups, which is why your prescriber looks at your cause before setting expectations.
Side Effects of Sildenafil
The side effects you are most likely to notice, according to the label, are headache, flushing, indigestion (dyspepsia), abnormal vision, nasal congestion, back pain, muscle aches, nausea, dizziness and rash.1 In the as-needed trials that match how it is prescribed, headache occurred in 16% of men compared with 4% on placebo. Flushing occurred in 10% compared with 1%, and indigestion in 7% compared with 2%.1
If your vision changes, it is usually a mild, short-lived color tinge, sometimes with light sensitivity or blurring. It occurred in 3% of men compared with none on placebo, and only one man in those studies stopped because of it.1
Few men in the trials stopped because of side effects. In the placebo-controlled studies, 2.5% of men on sildenafil stopped because of side effects, compared with 2.3% on placebo.1 A meta-analysis of 49 randomized trials found that serious adverse events and withdrawals did not differ between sildenafil and placebo. It also found that higher doses carried a greater risk of side effects.6 That dose link is one reason your prescriber starts where the label directs and adjusts from your response.
In the 2002 meta-analysis, the link between sildenafil and serious cardiovascular events or death did not reach statistical significance across its pooled trials.5 The label still lists three warnings to know before starting sildenafil. Seek emergency treatment for an erection lasting more than 4 hours. Stop sildenafil and get medical care for a sudden loss of vision in one or both eyes, or for a sudden decrease or loss of hearing.1 If any of those happen to you, treat them as urgent.
Drug Interactions and Medical Screening
Nitrates are the best-known combination the label forbids, and guanylate cyclase stimulators such as riociguat are the other. Sildenafil strengthens the blood-pressure-lowering effect of nitrates, so it is contraindicated in anyone using organic nitrates or organic nitrites in any form.1 That includes recreational "poppers" such as amyl nitrite.1 The label also states that after sildenafil it is unknown when nitrates can be given safely.1 If you take sildenafil and develop chest pain, tell the emergency team which medicine you took and when.
The label lists other interactions your prescriber screens for:
- Riociguat and other guanylate cyclase stimulators, which must not be combined with sildenafil.1
- Alpha-blockers, which can add to the drop in blood pressure. The label says you should be stable on the alpha-blocker before sildenafil is started.1
- Blood pressure medicines, which call for caution because the effects on blood pressure can add up.1
- CYP3A4 inhibitors such as ritonavir, ketoconazole tablets, itraconazole and erythromycin, which raise sildenafil levels.1
In a label study of healthy volunteers, sildenafil did not strengthen the blood-pressure-lowering effect of alcohol at a blood alcohol level of about 0.08%.1 Your heart health is part of the screen as well. The label states that men for whom sex is inadvisable because of their cardiovascular status should not use sildenafil.1 Bring your full medication list to the visit where sildenafil is discussed, including any supplements.
How a Prescriber Improves Your Response to Sildenafil
Many men who believe sildenafil failed were using it in a way that kept it from working. In a study of 253 men referred after sildenafil failure, 17 were excluded for contraindications, and re-education by a urologist turned 41.5% of the rest into responders.7 Incorrect use accounted for 81% of the failures, the visits averaged 12 minutes, and 94% were still responding at 26 months.7
A second study of 60 men found that 44 had used sildenafil in at least one clearly suboptimal way.8 Of all 60 men, 30% did not know sexual stimulation was needed, and 60% had tried it fewer than four times. After individual instruction, 58.5% of the men who were rechallenged responded.8
The cause of your ED matters too, and it is the part your labs can answer. The label calls for a complete medical assessment to find underlying causes before treatment.1 Low testosterone is one worth measuring. An overview of systematic reviews found that men with ED and hypogonadism (low testosterone) seemed to improve further when testosterone was added to a PDE5 inhibitor.9 If your result is weaker than you hoped, tell your prescriber exactly how and when you took it, because that conversation was enough for many men in these studies.
The Opt Take on Sildenafil
A pill that restores erections does not show what caused the erectile dysfunction. The label itself asks for a complete medical assessment to find what is behind the ED before treatment.1 Blood sugar, cholesterol and testosterone each show up in a lab panel, and none of them show up in how well the pill works.
At Opt Health, every lab we order gets read by a physician before anyone is offered a treatment. Your initial panel covers 55+ biomarkers, including hormones, inflammation and cardiovascular risk. A 1:1 consultation follows with an Opt Health physician, who reviews your existing conditions and current medications. If your plan includes sildenafil or another prescription, it ships from our pharmacy to your door.
Labs and a physician consultation repeat every 3 to 4 months. Your physician compares the new numbers with your baseline, asks how the medicine is working, and adjusts the plan on what changed.
Get started: Begin with Opt Health, where a physician reads your labs, builds your plan, and adjusts it over the loop.
Frequently Asked Questions
Sildenafil citrate is the form of sildenafil used in Viagra and Revatio tablets.1,3 It is a PDE5 inhibitor that the FDA has approved for erectile dysfunction, and as Revatio for pulmonary arterial hypertension.1,3 Your prescription label tells you which product and which use your tablets are for.
In the lungs, sildenafil blocks PDE5 in the smooth muscle of the pulmonary blood vessels.3 That raises cGMP and relaxes those vessels, which can widen the pulmonary vascular bed in people with PAH.3 PAH is treated with Revatio as its own prescription, so your ED prescription is a separate decision.
The label describes sildenafil as having systemic vasodilatory properties, meaning it widens blood vessels throughout the body.1 Headache was the most common side effect in the as-needed trials, at 16% compared with 4% on placebo.1 Higher doses carried more side effects in a meta-analysis of 49 trials.6 Tell your prescriber if headaches are frequent, since the dose can be reviewed.
Sildenafil and nitrates both act on the nitric oxide and cGMP pathway, and together they lower blood pressure more than either one alone.1 The label therefore contraindicates sildenafil with organic nitrates or nitrites in any form. It also states that it is unknown when nitrates can be given safely after a dose.1 If you use a nitrate for your heart, your prescriber will look at options outside this class.
The most common correctable reason in one referral study was incorrect use, which caused 81% of failures. Re-education turned 41.5% of those men into responders.7 Missing sexual stimulation, too few attempts and an untreated cause such as low testosterone are other reasons.8,9 Tell your prescriber how and when you took it, so the plan can be reviewed. When tablets do not work, intracavernosal injection therapy is another route your prescriber can weigh.
Sildenafil has not been shown to be better than tadalafil for everyone. An overview of systematic reviews rated sildenafil among the most effective PDE5 inhibitors, but mostly from indirect comparisons. The highest dose also had more side effects and dropouts, and the authors judged the comparative evidence low.9 The two drugs differ in timing and side effects, and sildenafil vs tadalafil lays out how your prescriber weighs them.
Getting the most from an ED prescription starts with knowing why erections changed. An Opt Health membership pairs a 55+ biomarker panel with a 1:1 physician review, then repeats labs and a physician consultation every 3 to 4 months.
References
- Pfizer Laboratories Div Pfizer Inc. VIAGRA (sildenafil citrate) tablets, US prescribing information. DailyMed. Published February 13, 2026. Accessed October 14, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0b0be196-0c62-461c-94f4-9a35339b4501
- TruPharma LLC. Sildenafil tablets, US prescribing information. DailyMed. Published September 14, 2026. Accessed October 14, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=53078688-ad66-4bb2-808c-b76acedd00e3
- Pfizer Laboratories Div Pfizer Inc. REVATIO (sildenafil citrate), US prescribing information. DailyMed. Published June 23, 2026. Accessed October 14, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f158fe10-d5dc-4432-b2c9-fc665401291b
- Burnett AL et al. Erectile dysfunction: AUA guideline. J Urol. 2018;200(3):633-641. https://pubmed.ncbi.nlm.nih.gov/29746858/
- Fink HA et al. Sildenafil for male erectile dysfunction: a systematic review and meta-analysis. Arch Intern Med. 2002;162(12):1349-60. https://pubmed.ncbi.nlm.nih.gov/12076233/
- Tsertsvadze A et al. Oral sildenafil citrate (viagra) for erectile dysfunction: a systematic review and meta-analysis of harms. Urology. 2009;74(4):831-836.e8. https://pubmed.ncbi.nlm.nih.gov/19592078/
- Atiemo HO et al. Salvage of sildenafil failures referred from primary care physicians. J Urol. 2003;170(6 Pt 1):2356-8. https://pubmed.ncbi.nlm.nih.gov/14634415/
- Jiann BP et al. Rechallenge prior sildenafil nonresponders. Int J Impot Res. 2004;16(1):64-8. https://pubmed.ncbi.nlm.nih.gov/14963473/
- Pyrgidis N et al. The effect of phosphodiesterase-type 5 inhibitors on erectile function: an overview of systematic reviews. Front Pharmacol. 2021;12:735708. https://pubmed.ncbi.nlm.nih.gov/34557099/
This content is for informational purposes and does not replace evaluation, diagnosis, or treatment by a qualified medical professional.
This article is part of our guide to erectile dysfunction. See how Opt Health treats ED at the root cause.
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