Sildenafil, sold as Viagra, works for the majority of men who try it, though not as universally as its reputation suggests. Across large meta-analyses, roughly 57% of intercourse attempts succeed on the drug compared with about 21% on placebo, and around 83% of men achieve at least one successful attempt during treatment. Those numbers shift depending on the underlying cause of erectile dysfunction, the dose used, and how the drug is taken. The picture is more layered than a single “success rate” can capture.
What Clinical Trials Actually Found
The largest body of evidence comes from pooled analyses of randomized trials conducted in the late 1990s and 2000s. A systematic review and meta-analysis covering 14 parallel-group trials found that men on sildenafil succeeded in 57% of intercourse attempts versus 21% on placebo, and 83% of sildenafil users had at least one successful intercourse during the treatment period compared with 45% of men on placebo.1JAMA Internal Medicine. Sildenafil for Male Erectile Dysfunction: A Systematic Review and Meta-analysis A separate meta-analysis of clinical trial reports found that with dose optimization, 49% of men achieved a success rate of at least 60% of their intercourse attempts, compared with just 11% on placebo.2PubMed Central. Sildenafil (Viagra) for male erectile dysfunction: a meta-analysis of clinical trial reports
An early dose-escalation trial published in the New England Journal of Medicine reported an even higher figure: 69% of all intercourse attempts were successful on sildenafil versus 22% on placebo, with treated men averaging about six successful attempts per month compared with about one and a half on placebo.3PubMed. Oral sildenafil in the treatment of erectile dysfunction The spread across studies reflects differences in dosing protocols, patient populations, and how “success” was defined. But the direction is consistent: sildenafil reliably improves erectile function for most men, though it does not guarantee success on every attempt.
Why the Drug Works for Some Attempts and Not Others
Sildenafil does not create an erection on its own. It amplifies a natural process. When a man is sexually aroused, nerve signals trigger the release of nitric oxide in the erectile tissue, which sets off a chemical cascade that relaxes smooth muscle and allows blood flow into the penis. Sildenafil blocks the enzyme that breaks down one of the key signaling molecules in that cascade, keeping blood vessels relaxed for longer.4PubMed. Molecular mechanisms of the effects of sildenafil (VIAGRA) This means arousal is still required. If desire, stimulation, or the initial nerve signal is absent or weak, the drug has less to amplify. That partly explains why success rates are reported per attempt rather than as a binary “works or doesn’t” — the same man can succeed on one occasion and not on another, depending on context.
Dose Makes a Real Difference
Sildenafil comes in 25 mg, 50 mg, and 100 mg tablets. The standard starting dose is 50 mg, but many men end up on 100 mg if the lower dose falls short. A study comparing the two found that both dosages improved erectile function and monthly frequency of intercourse, but the 100 mg group had better outcomes in intercourse frequency and sexual enjoyment, even though that group also had more underlying health problems.5PubMed Central. Effects of Two Different Dosages of Sildenafil on Patients With Erectile Dysfunction A separate trial looking specifically at a fixed 100 mg dose versus a flexible dose found that the 100 mg fixed dose was significantly better than 50 mg on measures of overall satisfaction, relationship satisfaction, and sexual experience.6International Journal of Impotence Research. Hardness, function, emotional well-being, satisfaction and the overall sexual experience in men using 100-mg fixed-dose or flexible-dose sildenafil citrate
This matters practically: a man who tries 50 mg and finds it only partially effective should not assume the drug doesn’t work for him. Adjusting to 100 mg, under medical guidance, can push results meaningfully higher.
Food and Timing Can Blunt the Effect
How you take sildenafil matters. A heavy meal, especially one high in fat, slows absorption considerably. One pharmacokinetic study found that a high-fat meal delayed peak blood levels by about an hour and reduced the peak concentration by roughly 29%, though total absorption was only slightly reduced.7PubMed Central. Pharmacokinetics of sildenafil after single oral doses in healthy male subjects: absolute bioavailability, food effects and dose proportionality A study of an orodispersible (dissolving) formulation showed an even more dramatic food effect: peak concentration dropped by 59% after a high-fat meal, and the time to reach peak levels was delayed from under an hour to four hours.8PubMed. Pharmacokinetics of a novel orodispersible tablet of sildenafil in healthy subjects
The practical takeaway is straightforward: taking sildenafil on an empty stomach, or after a light meal, gives it the best chance to work quickly and fully. Taking it right after a large dinner is one of the most common reasons men report the drug “didn’t work.” This is also one of the easiest variables to fix before concluding the medication has failed.
How Results Differ With Diabetes
Diabetes is one of the most common causes of erectile dysfunction, and it tends to make the condition harder to treat because of the vascular and nerve damage the disease causes over time. Still, sildenafil works for the majority of men with diabetes, even if the response rates are somewhat lower than in the general population. A randomized trial of men with type 2 diabetes found that 56% of sildenafil users reported improved erections versus 10% on placebo, and 61% achieved at least one successful intercourse attempt compared with 22% of placebo users.9JAMA. Sildenafil for Treatment of Erectile Dysfunction in Men With Diabetes: A Randomized Controlled Trial Another study found that the percentage of successful intercourse attempts jumped from 6% before treatment to 49% with sildenafil.10PubMed. Efficacy and safety of sildenafil in men with type 2 diabetes mellitus and erectile dysfunction
These are lower figures than the roughly 57% attempt-level success rate seen across broader populations, but they still represent a large improvement over baseline for most diabetic men. The severity of nerve damage and how well-controlled the diabetes is likely play a role in individual variation.
After Prostate Surgery
Radical prostatectomy, the surgical removal of the prostate for cancer, often damages the nerves responsible for erections. Whether sildenafil helps depends almost entirely on how much nerve tissue was preserved during the surgery. In one study, 72% of men who had both nerve bundles spared responded to sildenafil, 50% of those with one nerve bundle spared responded, but only 15% of men who had no nerve-sparing surgery responded.11PubMed. Role of Viagra after radical prostatectomy A larger study reported a similar pattern: 76% response in bilateral nerve-sparing cases, 54% in unilateral, and 14% in non-nerve-sparing cases.12PubMed. Efficacy and factors associated with successful outcome of sildenafil citrate use for erectile dysfunction after radical prostatectomy
A third study found that overall, about 53% of post-prostatectomy patients had improved erections on sildenafil and 40% regained the ability for intercourse, with nerve-sparing status and baseline erectile function being the strongest predictors.13Journal of Urology. Sildenafil Citrate After Radical Retropubic Prostatectomy The takeaway for post-surgery patients is that sildenafil can still help, but expectations should be calibrated to the type of surgery performed. Men who had nerve-sparing surgery have a good chance of meaningful improvement; men without nerve sparing are much less likely to benefit from the drug alone.
Cardiovascular Disease and ED
Since erectile dysfunction and heart disease share many of the same risk factors (high blood pressure, high cholesterol, smoking, diabetes), many men who need sildenafil also have cardiovascular conditions. A trial focused specifically on men with cardiovascular disease found that 71% of sildenafil users reported improved erections versus 24% on placebo, with no treatment-related cardiovascular adverse events beyond flushing.14International Journal of Clinical Practice. Efficacy and Safety of Sildenafil Citrate for the Treatment of Erectile Dysfunction in Men With Cardiovascular Disease A review of postmarketing safety data confirmed that the overall incidence of cardiovascular adverse events on sildenafil was low and similar to placebo, with no signal of increased cardiovascular death.15PubMed. Viagra and Cardiovascular Disease
The one hard contraindication is nitrate drugs, commonly prescribed for angina. Combining sildenafil with nitrates can cause dangerous drops in blood pressure. A study in men with stable angina showed that sildenafil combined with a long-acting nitrate produced maximum blood pressure drops of 52/29 mmHg, roughly double the drop seen with the nitrate alone.16PubMed. Sildenafil citrate potentiates the hypotensive effects of nitric oxide donor drugs in male patients with stable angina Separate research confirmed that sildenafil combined with nitrate could cause large, prolonged drops in both systemic blood pressure and coronary blood flow in narrowed arteries.17PubMed. Effects of sildenafil citrate (Viagra) combined with nitrate on the heart This interaction is not a theoretical concern — it is the reason sildenafil’s label carries an absolute contraindication with nitrates, and it is also why men who take recreational “poppers” (amyl nitrite) should avoid the drug.
When Sildenafil Doesn’t Work
Despite the overall strong response rates, a meaningful minority of men do not respond adequately to sildenafil. The reasons break down into several categories: the underlying cause of erectile dysfunction may be too severe for the drug to overcome (as seen in non-nerve-sparing prostatectomy cases), the drug may have been taken incorrectly (after a heavy meal, without sufficient arousal time, or at a dose that is too low), or psychological factors like performance anxiety or relationship problems may be interfering.18PubMed Central. Treatment Strategy for Non-Responders to PDE5 Inhibitors
Sildenafil has been specifically shown to work well for erectile dysfunction of psychological or mixed origin, not only for purely physical causes.19PubMed. Sildenafil citrate (Viagra) is effective and well tolerated for treating erectile dysfunction of psychogenic or mixed aetiology But paradoxically, severe performance anxiety can still undercut the drug’s effect because the anxiety itself suppresses the arousal signals the drug needs to work with. Unrealistic expectations also play a role — some men expect the drug to produce an erection without any physical or mental stimulation, which is not how it works.
For men with low testosterone who do not respond to sildenafil alone, combining testosterone therapy with the drug can make a difference. In one study, about a third of hypogonadal men who failed sildenafil alone responded when testosterone replacement was added.20PubMed. Combined use of androgen and sildenafil for hypogonadal patients unresponsive to sildenafil alone Checking testosterone levels is worth considering before labeling someone a sildenafil non-responder.
Does It Lose Effectiveness Over Time?
One common concern is whether the body builds tolerance to sildenafil, requiring higher doses or rendering it useless over time. The evidence here is mostly reassuring, with a caveat. A large open-label extension study followed over a thousand men for up to a year and found that after 52 weeks, 89% of patients still reported that sildenafil improved their erections, with no signs that long-term use diminished the drug’s effect.21International Journal of Impotence Research. Assessment of the efficacy and safety of Viagra (sildenafil citrate) in men with erectile dysfunction during long-term treatment
However, a two-year follow-up study painted a slightly more cautious picture: 20% of patients needed to increase their dose to maintain the same effect, and 17% discontinued use entirely because the drug stopped working for them.22PubMed. Long-term efficacy of sildenafil and tachyphylaxis effect Whether that represents true pharmacological tolerance or simply progression of the underlying erectile dysfunction is unclear. As men age and their vascular health changes, the disease itself can worsen even while treatment remains the same. For most users, the drug continues to work well for years, but a gradual decline in response over a long time horizon is possible for some.
Side Effects in Practice
Sildenafil’s side effects are generally mild and short-lived. In a clinical practice study, about a third of patients experienced at least one side effect. The most common were:
- Flushing: reported by about 31% of users
- Headache: about 25%
- Nasal congestion: about 19%
- Heartburn: about 11%
No patients in that study withdrew because of the severity of side effects, and higher doses were associated with more frequent side effects.23Urology. Side-effect profile of sildenafil citrate (Viagra) in clinical practice Visual disturbances — a blue tint to vision — are occasionally reported at higher doses but are temporary. These side effects are all downstream of the same mechanism that produces the therapeutic effect: widening blood vessels. The headache and flushing come from vasodilation in places other than the intended target.
How Sildenafil Compares to Tadalafil and Vardenafil
Sildenafil was the first drug in its class, but tadalafil (Cialis) and vardenafil (Levitra) followed. A comparative meta-analysis of fixed-dose trials found that all three drugs improved erectile function by 7 to 10 points on a standard questionnaire versus placebo. Sildenafil showed a pooled improvement of about 9.7 points, tadalafil about 8.5 points, and vardenafil about 7.5 points. The difference between sildenafil and vardenafil reached statistical significance, though the authors cautioned that direct head-to-head trials were still lacking.24International Journal of Impotence Research. Efficacy of PDE-5-inhibitors for erectile dysfunction
In practice, the biggest difference between these drugs is not raw efficacy but pharmacokinetics. Tadalafil lasts much longer (up to 36 hours versus about 4 to 6 for sildenafil), which appeals to men who prefer not to time their dose precisely before sex. Vardenafil’s profile is similar to sildenafil’s. The choice between them usually comes down to lifestyle preference and individual side-effect experience rather than which one is “stronger.”
Effects Beyond the Individual
Erectile dysfunction is not solely the patient’s problem — it affects partners and relationships. A randomized trial that measured outcomes in both men and their female partners found that sildenafil treatment improved sexual function and satisfaction for both members of the couple. Partners of treated men reported significantly greater satisfaction, and improvement in one partner correlated with improvement in the other.25PubMed. Sexual function and satisfaction in heterosexual couples when men are administered sildenafil citrate (Viagra) for erectile dysfunction: a multicentre, randomised, double-blind, placebo-controlled trial Research in men with spinal cord injuries similarly found that sildenafil improved overall satisfaction with sex life and the quality of the sexual relationship with a partner.26PubMed. Quality of life in patients with spinal cord injury receiving Viagra (sildenafil citrate) for the treatment of erectile dysfunction The psychological burden of erectile dysfunction extends well beyond the bedroom, and effective treatment can shift dynamics in ways that clinical outcome scores do not fully capture.
The Counterfeit Problem
Sildenafil’s fame and the embarrassment some men feel about seeking a prescription have created a thriving market for counterfeit pills, sold online and in person. This is not a minor quality-control issue. An analysis of counterfeit pills found that only about 10% of tested samples contained within 10% of the labeled dose of active ingredient. Some pills contained zero active ingredient, while others had more than double the labeled amount. Contaminants detected in counterfeit tablets included talcum powder, commercial paint, and printer ink.27PubMed Central. Counterfeit phosphodiesterase type 5 inhibitors pose significant safety risks Separate testing of illicit sildenafil raw material found batches that did not meet pharmacopeial quality standards, with contaminants at milligram-scale levels and the wrong chemical salt form used, both of which increase the risk of unpredictable dosing and side effects.28PubMed. The quality of sildenafil active substance of illegal source
A pill that contains zero active ingredient explains some anecdotal reports of “Viagra not working.” A pill that contains double the dose explains some emergency room visits. Neither has anything to do with the drug’s actual efficacy when obtained legitimately. Since sildenafil is now available as an inexpensive generic in most countries, there is less financial incentive than ever to turn to unregulated sources, but the counterfeit market persists.
Uses Outside Erectile Dysfunction
Sildenafil’s mechanism of action — blocking the same enzyme in smooth muscle tissue — turns out to be useful beyond the penis. The drug was originally developed as a potential treatment for angina (chest pain from restricted coronary blood flow), and while it did not pan out for that purpose, it found a second approved use: treating pulmonary arterial hypertension, marketed under the brand name Revatio.29PubMed Central. Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond Research has also shown effectiveness in Raynaud’s phenomenon, a condition where blood vessels in the fingers and toes constrict excessively in response to cold, cutting off circulation.30PubMed. Sildenafil in the treatment of Raynaud’s phenomenon resistant to vasodilatory therapy In at least one documented case, it improved both pulmonary hypertension and peripheral blood flow in a patient with scleroderma-related lung fibrosis and Raynaud’s.31PubMed. Sildenafil improved pulmonary hypertension and peripheral blood flow in a patient with scleroderma-associated lung fibrosis and the raynaud phenomenon These off-label uses reinforce that sildenafil is fundamentally a blood-vessel drug. The erectile dysfunction application was the surprise, not the other way around.