What Does Viagra Do to You: How It Works and Side Effects

Viagra (sildenafil) increases blood flow to the penis by blocking an enzyme that normally breaks down one of the body’s key relaxation signals, making it easier to get and maintain an erection when you’re sexually aroused. It does not create arousal on its own, does not act as an aphrodisiac, and its effects reach well beyond the genitals because the enzyme it targets exists in blood vessels throughout the body. That broader reach explains both why it causes side effects like headaches and flushing, and why doctors now prescribe it for conditions that have nothing to do with sex.

How Viagra Actually Works

When you become sexually aroused, nerve endings in the penis release nitric oxide, a small signaling molecule. Nitric oxide triggers the production of a substance called cGMP, which tells the smooth muscle lining the blood vessels in the penis to relax. As those muscles relax, blood rushes in and an erection develops. The process has a built-in off switch: an enzyme called PDE5 breaks down cGMP, which is how the erection eventually subsides.

Sildenafil works by blocking PDE5. With the enzyme held back, cGMP accumulates for longer and at higher levels, so the relaxation signal stays strong. The key point is that sildenafil amplifies a process that sexual arousal has already started; without that initial release of nitric oxide, there is no cGMP to protect and the drug has little to do.¹ In blood vessel smooth muscle, the higher cGMP levels reduce intracellular calcium, which is the direct trigger for muscle contraction. Lower calcium means more relaxation, wider vessels, and more blood flow.²

How Quickly It Kicks In and What Affects Absorption

Most people notice the effects within 30 to 60 minutes after swallowing a tablet on an empty stomach. If you take it with or shortly after a meal, absorption slows down: one pharmacokinetic study in healthy men found that food delayed peak blood levels by about an hour and reduced the peak concentration by roughly 29%.³ Total drug exposure dropped by about 11% with food, so a heavy meal does not just delay the effect but slightly weakens it. The standard advice to take it on an empty or light stomach comes directly from this kind of data. Alcohol can compound the issue, both because it often accompanies a meal and because it has its own blood-pressure-lowering effect.

Sildenafil’s absolute bioavailability is around 41%, meaning less than half of what you swallow reaches the bloodstream as active drug. The rest is broken down during first-pass metabolism in the liver and gut. Typical doses range from 25 mg to 100 mg, and the drug’s effects generally last four to six hours, though that varies with age, liver function, and other medications.

Common Side Effects

Because PDE5 exists in blood vessels throughout the body, sildenafil does not limit its relaxation effects to the penis. The most frequently reported side effects are headache, facial flushing, indigestion, and nasal congestion. All of these trace back to the same mechanism: wider blood vessels where you do not necessarily want them wider. Flushing happens when blood vessels in the face and neck dilate. Headaches are a downstream consequence of vasodilation in the head. Indigestion occurs because PDE5 is present in the smooth muscle of the gastrointestinal tract.

Nasal congestion gets less attention but is worth knowing about. The drug widens blood vessels in the nasal lining, causing the tissue to swell and restricting airflow. A placebo-controlled study in young men found that sildenafil combined with sexual arousal significantly reduced the minimum cross-sectional area of the nasal passages and shrank endonasal volume, with visible swelling of the inferior turbinates on nasal examination.⁴ If you already have sinus issues or a deviated septum, the stuffiness can be more noticeable. These side effects are generally mild and resolve as the drug wears off.

What It Does to Your Blood Pressure

Sildenafil is, at its core, a vasodilator. It widens arteries modestly throughout the body, which means it lowers blood pressure. In one controlled study of both normotensive and hypertensive men, a single dose dropped systolic blood pressure by about 6 mm Hg and diastolic pressure by about 4.5 mm Hg on average.⁵ That is a small shift for most people, roughly equivalent to standing up quickly. For men already on blood pressure medications, the effect was similar in magnitude.

A separate study in men already being treated for high blood pressure found a larger peak drop: at 75 minutes after the dose, brachial blood pressure was about 17/11 mm Hg lower than on the placebo day. The study also found that sildenafil reduced arterial wave reflection, a measure of vascular stiffness, suggesting the drug reduces tone in arteries beyond the pelvis.⁶ For most men, these drops are well tolerated. But if you already run on the low side or take multiple antihypertensive drugs, combining them with sildenafil could leave you lightheaded or dizzy, especially when standing up.

The Nitrate Interaction That Can Kill

The single most dangerous thing about sildenafil is mixing it with nitrate medications. Nitrates, which include nitroglycerin (used for chest pain), isosorbide mononitrate, and isosorbide dinitrate, work by flooding the same nitric oxide pathway that sildenafil amplifies. Both drugs push cGMP levels higher, and the combination can produce a massive, life-threatening drop in blood pressure.

A study measuring the combined effect in coronary arteries found that sildenafil on its own caused modest vasodilation, but together with a nitrate, the result was large and prolonged decreases in both systemic blood pressure and blood flow through narrowed coronary arteries.⁷ When healthy volunteers took sildenafil and then received sublingual nitroglycerin, the drop in systolic blood pressure was roughly four times larger than the drop seen with nitroglycerin alone.⁸ In real-world cases, this combination has caused fatal hypotension.⁹

This is not a theoretical concern or a fine-print warning to gloss over. If you use any form of nitrate, whether a daily pill for angina, a patch, or a spray you keep for emergencies, sildenafil is absolutely contraindicated. The same caution extends to recreational drugs called “poppers” (amyl nitrite and butyl nitrite), which are chemically nitrates and pose the same risk.

Vision Changes and Eye Safety

Sildenafil is designed to target PDE5, but it has a weak affinity for a closely related enzyme called PDE6, which is found exclusively in the light-sensing cells of the retina. At standard doses, this cross-reactivity is minor, but at higher doses or in people who are particularly sensitive, it can produce visual symptoms: a blue-tinged haze to vision, increased sensitivity to light, or slightly blurred sight. These effects are temporary and resolve as the drug clears from the body.¹⁰

More rarely, sildenafil has been linked in case reports to a condition called nonarteritic anterior ischemic optic neuropathy (NAION), a form of sudden vision loss caused by reduced blood flow to the optic nerve. The association has not been definitively proven as causal, but it has been reported enough times that the drug’s label carries a warning about it. Men with certain pre-existing risk factors for NAION, including diabetes, heart disease, a small optic disc, or a history of sudden vision loss in one eye, should discuss the risk with their doctor before using sildenafil.

Can It Affect Your Hearing?

Hearing-related side effects are far less common but have gotten attention after a handful of reports of sudden sensorineural hearing loss in men taking PDE5 inhibitors. A review of clinical reports and the published literature concluded that these drugs should be considered a risk factor for sudden deafness, though the evidence is limited and better-controlled studies are still needed.¹¹ Animal studies paint a more reassuring picture: chronic administration of sildenafil in mice over 30 days did not affect hearing sensitivity at all in two different strains.¹² The disconnect between human case reports and controlled animal experiments suggests the risk, if real, is either very rare or involves specific predisposing conditions. Still, if you notice sudden hearing loss or ringing in your ears after taking the drug, stop using it and get checked out.

Priapism Is Rarer Than You Think

Priapism, a painful erection lasting more than four hours, is the side effect people worry about the most and the one they are least likely to experience. A study that reviewed all cases of drug-induced priapism reported to the FDA since 1998 found 411 cases linked to Viagra, Cialis, or Levitra combined. To put that in context, second-generation antipsychotic medications were responsible for about 2.6 times as many cases, and trazodone, an antidepressant commonly used as a sleep aid, accounted for about twice as many. In the published medical literature, PDE5 inhibitors accounted for only about 3% of all drug-induced priapism cases.¹³ Priapism is a medical emergency requiring treatment, but the risk from sildenafil alone, at prescribed doses, is genuinely low.

How Viagra Compares to Cialis and Levitra

Sildenafil was the first PDE5 inhibitor to market, but two others followed: vardenafil (Levitra) and tadalafil (Cialis). All three block the same enzyme and work through the same pathway. The practical differences are in potency, selectivity, and how long they last. Vardenafil is more potent and more selective for PDE5, meaning it takes a lower dose and has somewhat less cross-reactivity with other PDE enzymes. Tadalafil has a much longer duration of action, often called “the weekend pill” because its effects can last up to 36 hours compared to sildenafil’s four to six.¹⁴ In clinical trials, all three drugs showed broadly similar effectiveness for erectile dysfunction. The choice often comes down to lifestyle preference: do you want a shorter-acting drug you take right before sex, or a longer-acting one that gives you a wider window?

Treating Lung Disease, Not Just Erectile Dysfunction

Sildenafil’s journey to market is one of the more famous stories in pharmaceutical development. It was originally designed as a heart drug to treat angina, but in early clinical trials for that purpose, it did not work particularly well for chest pain. Test subjects did, however, report erections as a side effect, and the development program pivoted.¹⁵ Then, years after its approval for erectile dysfunction, the same blood-vessel-relaxing properties that worked in the penis turned out to be valuable in the lungs.

Pulmonary arterial hypertension is a condition where the arteries carrying blood from the heart to the lungs become narrowed and stiff, forcing the right side of the heart to work dangerously hard. Sildenafil, marketed under the name Revatio for this indication, relaxes those pulmonary arteries through the same cGMP mechanism. A landmark randomized trial found that patients on sildenafil could walk significantly farther in a six-minute test (gains of 45 to 50 meters over placebo depending on dose), with improvements in pulmonary artery pressure and overall functional capacity across all dose groups.¹⁶ For patients with this progressive and often fatal condition, that represented a meaningful improvement in daily life.¹⁷

Recreational Use by Young, Healthy Men

A meaningful percentage of young men without erectile dysfunction use sildenafil recreationally. A survey of healthy men between 18 and 30 found that about one in five reported using a PDE5 inhibitor, often in combination with alcohol or other drugs and without medical oversight.¹⁸ The motivation is usually performance anxiety or a desire for a confidence boost, but the research suggests this can backfire. A study specifically examining the link between recreational use and erectile function found that men who used the drugs recreationally had worse erectile function scores, and the relationship appeared to be mediated by a loss of confidence in their own natural abilities. In other words, relying on the pill eroded their belief that they could perform without it.¹⁹

The risk profile changes for young healthy men, too, because this population is more likely to mix sildenafil with stimulants, alcohol, or poppers at social or club settings, combinations that stress the cardiovascular system in unpredictable ways. Using a prescription vasodilator without knowing your own blood pressure or drug interactions is a gamble that most doctors would discourage.

Counterfeit and Unregulated Products

Viagra is one of the most counterfeited drugs in the world, and the problem has intensified with online pharmacies. Counterfeit PDE5 inhibitors may contain no active ingredient at all, wildly inaccurate doses, or harmful contaminants. Because the pills look convincingly real, many buyers do not realize they are taking a counterfeit product.²⁰ The risks are compounded by “herbal” sexual-enhancement supplements sold online that claim to be natural but actually contain undeclared sildenafil or its analogs. Laboratory testing of products marketed as herbal supplements for sexual performance has repeatedly found hidden pharmaceutical ingredients, including sildenafil citrate, in products that carried no such disclosure on their labels.²¹ This means someone who thinks they are avoiding a pharmaceutical drug may actually be taking one, without the dosing control or safety warnings that come with a legitimate prescription.

Does It Work for Women?

PDE5 is present in clitoral, vaginal, and labial smooth muscle, so the same vasodilatory mechanism operates in female genital tissue. A double-blind crossover trial in women with sexual arousal disorder found that sildenafil significantly improved self-reported arousal, orgasm, and sexual enjoyment compared to placebo.²² Similar improvements were found in premenopausal women with type 1 diabetes, where blood flow measurements confirmed that the drug increased clitoral arterial flow alongside the subjective improvements in arousal and orgasm.²³

A broader review of the clinical trial data described sildenafil as “moderately effective” for female sexual arousal disorder, with potentially stronger benefits in women whose arousal problems are secondary to conditions like multiple sclerosis, diabetes, or antidepressant use.²⁴ Despite these findings, sildenafil has never been approved for use in women. Female sexual dysfunction involves a more complex interplay of psychological, hormonal, and vascular factors, and the large-scale trials needed for regulatory approval have not materialized.

Effects on Sperm

Men trying to conceive sometimes wonder whether sildenafil affects sperm. Lab studies show it has real effects on sperm behavior, though the clinical significance is not straightforward. When semen samples were exposed to sildenafil in the laboratory, the number and velocity of progressively motile sperm increased significantly.²⁵ Faster sperm sounds like a good thing for fertility, and a separate in-vitro study confirmed the dose-dependent boost in motility.²⁶ But the first study also found that sildenafil triggered a premature acrosome reaction, the chemical event that sperm normally undergo only when they reach the egg. Sperm that have already undergone this reaction too early may be unable to fertilize when it counts. The net impact on real-world fertility is unclear, and these were lab-dish experiments rather than studies of men who took the drug orally. Still, for couples going through fertility treatment, it is a finding worth discussing with a specialist.

Raynaud’s Phenomenon and Other Vascular Uses

Because sildenafil relaxes blood vessels throughout the body, researchers have tested it on other conditions where restricted blood flow causes problems. One of the more promising applications is Raynaud’s phenomenon, a disorder in which small arteries in the fingers and toes spasm in response to cold or stress, cutting off circulation and turning the digits white or blue. In a controlled trial of patients who had not responded to standard vasodilator therapy, sildenafil cut the frequency of Raynaud’s attacks by roughly a third, shortened total attack duration substantially, and more than quadrupled capillary blood flow velocity in the affected digits.²⁷

Early-stage research has also explored whether sildenafil’s ability to increase blood flow and reduce inflammation in blood vessels could benefit people with Alzheimer’s disease. In a small study of Alzheimer’s patients, a single dose of sildenafil increased cerebral blood flow and the rate at which the brain used oxygen.²⁸ Animal and cell studies have suggested additional neuroprotective mechanisms, but this line of research remains in early stages and no one should take sildenafil as a cognitive aid based on what is currently known.