Viagra (sildenafil) is a prescription medication primarily used to treat erectile dysfunction in men. It was the first oral drug approved for ED and remains one of the most widely prescribed, with a systematic review of randomized trials showing that roughly half to two-thirds of sexual intercourse attempts succeed with sildenafil versus about a fifth to a third with placebo, depending on the underlying cause. Sildenafil also has a second, lesser-known approved use for a serious lung condition called pulmonary arterial hypertension, sold under the brand name Revatio. The story behind both uses, and the practical details of dosing, side effects, and interactions, is more nuanced than most people realize.
How Viagra Works
An erection depends on blood flowing into the spongy tissue of the penis and staying there long enough for intercourse. A molecule called nitric oxide kicks off the process during sexual arousal, triggering the production of a chemical messenger (cGMP) that relaxes smooth muscle cells in penile blood vessels. In men with ED, an enzyme called PDE5 breaks down cGMP too quickly, so the blood vessel relaxation fizzles out before or during sex. Sildenafil blocks PDE5, letting cGMP build up and the blood vessels stay relaxed longer.1PubMed. Effects of sildenafil on cAMP and cGMP levels in isolated human cavernous and cardiac tissue The drug does not create arousal on its own. Sexual stimulation is still needed to start the nitric oxide cascade; sildenafil simply amplifies the signal once it begins.2PubMed. Effects of sildenafil, a type-5 cGMP phosphodiesterase inhibitor, and papaverine on cyclic GMP and cyclic AMP levels in the rabbit corpus cavernosum in vitro
How Well It Works for Erectile Dysfunction
Effectiveness depends partly on the cause of a man’s ED. A systematic review and meta-analysis published in JAMA Internal Medicine pooled data from multiple randomized trials and broke results down by cause. Men whose ED was primarily psychological reported successful intercourse during about two-thirds of attempts on sildenafil, compared with roughly a third on placebo. Around nine in ten of those men achieved at least one successful attempt during treatment. For men with an organic (physical) cause, about half of attempts succeeded on sildenafil, versus about one in six on placebo, and roughly three-quarters experienced at least one successful attempt.3JAMA Internal Medicine. Sildenafil for Male Erectile Dysfunction: A Systematic Review and Meta-analysis
Those numbers mean sildenafil is effective for most men, but it is not a guarantee. The organic-cause group had lower success rates, which makes sense because conditions like diabetes, nerve damage after prostate surgery, or severe vascular disease can limit how much blood reaches the penis regardless of PDE5 inhibition.
Dosage and Timing
Sildenafil for ED is available in 25 mg, 50 mg, and 100 mg tablets. Most men start at 50 mg, taken roughly an hour before sexual activity. In clinical testing, the median time to the onset of an erection after a 50 mg dose was about 27 minutes, with roughly seven out of ten men responding within half an hour and over eight out of ten within 45 minutes.4PubMed Central. Onset and duration of action of sildenafil for the treatment of erectile dysfunction The effect tapers over the next several hours. When sexual stimulation was started two hours after dosing, men still achieved erections lasting a median of about 20 minutes; by four hours post-dose, the median dropped to about five minutes.4PubMed Central. Onset and duration of action of sildenafil for the treatment of erectile dysfunction The practical window for most men is roughly 30 minutes to four hours after taking the pill.
A doctor may adjust the dose up to 100 mg or down to 25 mg based on how well it works and whether side effects are bothersome. Higher doses are linked to a greater chance of side effects, so the goal is the lowest dose that works reliably.
Common Side Effects
Most side effects are mild and short-lived. In one clinical practice study, about a third of patients experienced at least one side effect. The most frequently reported were facial flushing (roughly 31%), headache (about 25%), nasal congestion (around 19%), and heartburn (about 11%). No patients withdrew from the study because the side effects were too severe.5PubMed. Side-effect profile of sildenafil citrate (Viagra) in clinical practice A separate meta-analysis of harms confirmed the same pattern, finding that men on sildenafil had a higher risk of headache, flushing, indigestion, and visual disturbances compared with placebo, and that higher doses corresponded to more side effects.6PubMed. Oral sildenafil citrate (viagra) for erectile dysfunction: a systematic review and meta-analysis of harms
The visual disturbances deserve a closer look. Sildenafil’s main target is PDE5, but it also weakly inhibits a related enzyme called PDE6 found in the retina. This can cause a temporary blue tint to vision, increased sensitivity to light, or blurry vision in some users. These effects are generally mild and go away on their own. More rarely, case reports have linked sildenafil to a condition called nonarteritic ischemic optic neuropathy (NAION), which involves sudden vision loss in one eye due to reduced blood flow to the optic nerve.7PubMed Central. Visual Side Effects Linked to Sildenafil Consumption: An Update NAION is rare and a definitive causal link has not been established, but men who have already experienced it in one eye are generally advised against using PDE5 inhibitors.
The Nitrate Interaction
If there is one safety rule that stands above all others with Viagra, it is this: never combine it with nitrate medications. Nitrates, commonly prescribed for chest pain (angina), work through the same nitric oxide pathway that sildenafil amplifies. Together they can cause a dangerous, sometimes fatal drop in blood pressure. In one study, the combination of sildenafil with a nitrate called isosorbide mononitrate roughly doubled the drop in blood pressure compared with the nitrate alone, with standing systolic pressure falling by an average of about 52 points versus 25 on nitrate plus placebo.8PubMed. Sildenafil citrate potentiates the hypotensive effects of nitric oxide donor drugs in male patients with stable angina Animal research showed that when sildenafil was combined with a nitrate in the presence of coronary artery narrowing, both systemic blood pressure and coronary blood flow dropped substantially and stayed down for a prolonged period.9PubMed. Effects of sildenafil citrate (Viagra) combined with nitrate on the heart
Nitrates include nitroglycerin (sublingual tablets, patches, sprays), isosorbide mononitrate, and isosorbide dinitrate. Recreational drugs like amyl nitrite (“poppers”) also fall into this category. If you use any nitrate, sildenafil is off limits. Men who take alpha-blockers for blood pressure or an enlarged prostate also need caution, because the combination can cause a drop in blood pressure upon standing, though the risk appears lower if alpha-blocker therapy has been stable for a while.10PubMed. Pharmacology and drug interaction effects of the phosphodiesterase 5 inhibitors: focus on alpha-blocker interactions
Pulmonary Arterial Hypertension
Sildenafil’s second approved indication has nothing to do with the bedroom. Pulmonary arterial hypertension (PAH) is a condition in which the arteries in the lungs become abnormally narrowed, forcing the right side of the heart to work harder. PDE5 is found not only in penile tissue but also in lung blood vessels. By blocking PDE5 in the lungs, sildenafil relaxes those vessels, lowers pulmonary artery pressure, and improves the heart’s ability to pump blood through the lungs.11PubMed Central. Sildenafil in the treatment of pulmonary hypertension
A landmark randomized controlled trial published in the New England Journal of Medicine tested three doses of sildenafil in patients with PAH. All three improved pulmonary artery pressure, exercise capacity, and functional status. Side effects at the doses used included flushing, indigestion, and diarrhea.12PubMed. Sildenafil citrate therapy for pulmonary arterial hypertension For PAH, the drug is dosed differently from ED treatment: typically 20 mg three times daily rather than a single larger dose before sex. The branding is also different (Revatio rather than Viagra), which sometimes surprises patients.
Why It Does Not Work for Everyone
Studies put the overall non-response rate for sildenafil somewhere in the range of one-quarter to one-third of men who try it. But a striking finding from the literature is that many supposed “non-responders” are actually using the drug incorrectly. In a study of 100 men who had reported sildenafil failure, over half were found to be using it improperly. Among the most common mistakes: 45 had never tried the highest recommended 100 mg dose, 32 had taken the pill right after a heavy meal (which delays absorption), 22 took it immediately before trying to have sex without waiting for it to kick in, and 12 did not know that sexual stimulation was still required. After receiving proper instructions and dose adjustment, about a third of those previously unsuccessful men responded.13PubMed. Sildenafil failures may be due to inadequate patient instructions and follow-up: a study on 100 non-responders
A separate study of 60 prior non-responders found that roughly 77% had at least one area of major suboptimal use: 30% did not know sexual stimulation was necessary, 60% had tried fewer than four times before giving up, and 45% had never tried the full 100 mg dose.14International Journal of Impotence Research. Rechallenge prior sildenafil nonresponders Patient education is, frankly, an undervalued part of prescribing this drug. In many cases a doctor writes a prescription and sends the patient home without a follow-up visit, leading to preventable failures.15PubMed Central. Treatment Strategy for Non-Responders to PDE5 Inhibitors
True non-responders, men for whom sildenafil genuinely does not work even with proper use, tend to have more advanced vascular disease, significant nerve damage (such as after radical prostatectomy), or severe psychological factors like relationship distress or deep performance anxiety. For these men, other treatment options exist, including different PDE5 inhibitors, penile injections, vacuum devices, and surgical implants.
How Food Affects Absorption
Taking sildenafil on a full stomach is one of the most common mistakes. A pharmacokinetic study found that food delayed the time to peak blood levels by about an hour and reduced the peak concentration by roughly 29%, though the total amount absorbed changed only modestly.16PubMed Central. Pharmacokinetics of sildenafil after single oral doses in healthy male subjects: absolute bioavailability, food effects and dose proportionality A study of an oral-disintegrating formulation found even more dramatic effects with a high-fat meal, reducing peak concentration by 59% and pushing the time to peak from well under an hour in the fasting state to four hours.17PubMed. Pharmacokinetics of a novel orodispersible tablet of sildenafil in healthy subjects The practical advice: take sildenafil on an empty stomach or after a light meal, especially if timing matters. A heavy or fatty meal can substantially delay when the drug kicks in.
Older Adults and People with Organ Impairment
The body clears sildenafil more slowly in older men. In a pharmacokinetic study comparing young and elderly healthy volunteers, older men had roughly double the overall drug exposure and 60 to 70% higher peak blood levels, with the drug staying in the system about an hour longer.18PubMed Central. The effects of age and renal and hepatic impairment on the pharmacokinetics of sildenafil Severe kidney impairment produced a similar increase, and liver cirrhosis reduced the body’s ability to metabolize the drug, raising peak levels by about 47%. For all three groups, the standard recommendation is to start at 25 mg rather than 50 mg and adjust from there. Men taking certain other medications that slow sildenafil’s metabolism (like some antifungal drugs or HIV protease inhibitors) may also need a lower starting dose.
The Psychological Side of Things
ED frequently comes with a heavy psychological burden. Performance anxiety is common, and it can create a vicious cycle: a man experiences difficulty once, worries about it happening again, and the worry itself makes the next attempt harder. Research on sildenafil-treated men has found that improved erection hardness leads to increased confidence, and that increased confidence in turn reduces performance anxiety.19PubMed. Interrelationship of sildenafil treatment effects on the physiological and psychosocial aspects of erectile dysfunction of mixed or organic etiology In other words, for many men, sildenafil’s biggest benefit may be breaking the anxiety cycle so that natural function can reassert itself. Some clinicians use short-term sildenafil specifically for this purpose in men whose ED is largely psychological.
Off-Label Uses and Ongoing Research
Because PDE5 is present in blood vessels throughout the body, sildenafil’s ability to relax those vessels has attracted interest well beyond ED and PAH. One area where evidence is building is Raynaud’s phenomenon, a condition where blood vessels in the fingers and toes spasm in response to cold or stress, turning them white and painful. A double-blind crossover trial in patients whose Raynaud’s had not responded to standard vasodilator therapy found that sildenafil cut the frequency of attacks by about a third and the total duration of attacks by nearly half. Capillary blood flow velocity in the fingers more than quadrupled on the drug.20PubMed. Sildenafil in the treatment of Raynaud’s phenomenon resistant to vasodilatory therapy
Sildenafil’s effects on blood vessel health more broadly are also under study. In patients with heart failure, six months of sildenafil treatment improved the ability of blood vessels to dilate, reduced pulmonary artery pressure, and increased exercise capacity.21PubMed. Long-term use of sildenafil in the therapeutic management of heart failure In patients with cystic fibrosis, four weeks of sildenafil improved vascular endothelial function, likely by increasing the activity of an enzyme involved in nitric oxide production.22PubMed Central. Sildenafil improves vascular endothelial function in patients with cystic fibrosis These are still research findings rather than standard treatments, but they illustrate how a drug originally tested for chest pain found its way to ED, then to lung disease, and may eventually find use in vascular conditions that nobody anticipated in the 1990s.
Preliminary research has also explored whether sildenafil could help with female sexual arousal disorder. A randomized controlled trial testing a topical sildenafil cream in women diagnosed with arousal difficulties found that those using the cream reported improvements in arousal sensation, desire, and orgasm scores compared with placebo, along with reduced sexual distress.23PubMed Central. Preliminary Efficacy of Topical Sildenafil Cream for the Treatment of Female Sexual Arousal Disorder: A Randomized Controlled Trial The findings are early-stage, and oral sildenafil has not been approved for women, but the research underscores how the drug’s blood-flow-enhancing mechanism could have applications beyond its current labels.
From Angina Drug to Blockbuster
Sildenafil’s origin story is one of the most famous accidental discoveries in pharmaceutical history. Pfizer researchers in the late 1980s developed the compound as a treatment for angina, hoping it would relax coronary arteries and relieve chest pain. Early clinical trials were disappointing for that purpose, but male volunteers kept reporting an unexpected side effect: improved erections. Rather than shelving the drug, the company pivoted, and Viagra was approved for ED in 1998.24PubMed Central. Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond The PAH indication followed in 2005 as researchers recognized that the same PDE5 mechanism relevant to penile blood vessels also applied to pulmonary arteries. It is a textbook case of drug repurposing, where understanding a molecule’s basic biology opens doors nobody planned for.
Priapism
One rare but serious risk with any ED medication is priapism, a prolonged erection that does not go away on its own. It results from a failure of the normal mechanisms that drain blood back out of the penis after arousal subsides.25The Journal of Sexual Medicine. Priapism: Pathogenesis, Epidemiology, and Management While priapism is uncommon with sildenafil at recommended doses, it is a medical emergency. An erection lasting more than four hours can damage penile tissue and potentially cause permanent difficulty with erections. Men with sickle cell disease, leukemia, or certain other blood conditions are at higher risk. If an erection becomes painful or lasts well beyond the point of any sexual stimulation, seek emergency care immediately.
Counterfeit Viagra
Because Viagra is one of the most recognizable drug brands in the world and ED carries social stigma, a massive counterfeit market exists. Men who purchase pills online without a prescription often have no way of knowing what they are actually taking. Analyses of seized counterfeit pills have found wildly inconsistent amounts of the active ingredient, ranging from zero to more than double the labeled dose, along with contaminants including commercial paint and printer ink.26PubMed Central. Counterfeit phosphodiesterase type 5 inhibitors pose significant safety risks Beyond the direct danger of unknown ingredients, more than 30% of men who purchase ED drugs through unofficial channels report no interaction with a healthcare provider at all. That means potential drug interactions go unchecked and underlying conditions like cardiovascular disease, diabetes, or hormonal problems that are causing the ED never get diagnosed.27PubMed Central. The dangers of sexual enhancement supplements and counterfeit drugs to “treat” erectile dysfunction Getting sildenafil through a legitimate prescription is not just a legal formality. It is the only reliable way to know you are getting the right drug, at the right dose, with medical screening for the conditions that make it unsafe.