What Is Viagra? Uses, Side Effects, and Dosage

Viagra is the brand name for sildenafil citrate, a prescription medication originally developed for heart-related chest pain that became the first oral pill approved to treat erectile dysfunction. It works by blocking an enzyme called PDE5, which allows blood vessels in the penis to relax and fill with blood during sexual arousal. Since its approval in 1998, sildenafil has expanded well beyond that original use, and the science behind it, its risks, and its limits are worth understanding clearly.

How Viagra Works in the Body

During sexual arousal, the body releases nitric oxide in the tissue of the penis. Nitric oxide triggers production of a chemical messenger called cGMP, which relaxes the smooth muscle lining the blood vessels there. That relaxation allows blood to rush in and produce an erection. Under normal circumstances, an enzyme called PDE5 breaks down cGMP fairly quickly, which is part of why erections subside. Sildenafil blocks PDE5, so cGMP sticks around longer and the blood vessels stay relaxed. The drug does not create arousal on its own; it amplifies what happens when arousal is already present.

Sildenafil is quite targeted at PDE5, but it also has a minor effect on a related enzyme called PDE6, which exists only in the light-sensing cells of the retina.¹ That crossover explains why some people notice a temporary blue tinge to their vision or increased sensitivity to light after taking the drug. This side effect is dose-related and typically harmless, but it is a direct consequence of the chemistry.

Approved Medical Uses

Sildenafil has two distinct FDA-approved uses, marketed under two different brand names.

For erectile dysfunction, it is sold as Viagra. In clinical trials, men taking sildenafil reported significantly more erections firm enough for intercourse compared to placebo, along with improvements in erection hardness, duration, and overall satisfaction with their sex lives.² These results hold whether the erectile dysfunction has a psychological cause, a physical one, or a mix of both.

For pulmonary arterial hypertension, the same drug is sold under the brand name Revatio at a much lower dose. Pulmonary arterial hypertension is a condition where blood pressure in the arteries of the lungs is dangerously high, forcing the right side of the heart to work harder and eventually fail. Because PDE5 is also present in lung blood vessels, sildenafil relaxes those vessels too, lowering pulmonary artery pressure and improving the heart’s ability to pump blood through the lungs.³ Trials have shown improvements in exercise tolerance and several measures of heart and lung function.⁴ The recommended dose for this use is 20 mg three times a day, far smaller and more frequent than the typical erectile dysfunction dose.⁵

The journey from angina candidate to erectile dysfunction treatment to pulmonary hypertension therapy happened in less than two decades, making sildenafil one of the more notable examples of a drug finding its real purpose after its original intended use fell through.⁶

Dosage and Timing for Erectile Dysfunction

Eating a large or fatty meal before taking Viagra can slow things down. Food delays peak blood levels by about an hour and lowers the peak concentration by roughly 29%, though the total amount of drug absorbed drops only modestly. Researchers have noted these food effects are not large enough to be clinically important for most people, but if the drug seems to take longer to kick in after a big dinner, that is the reason.⁸ Taking it on an emptier stomach generally produces faster, more predictable results.

A lower starting dose of 25 mg is recommended for older adults and for people with significant kidney or liver problems. Both aging and organ impairment slow the body’s ability to clear the drug, which means more of it stays in the bloodstream longer and side effects become more likely at standard doses.⁹

Common Side Effects

About a third of men in clinical practice report at least one side effect, though the effects are generally mild and short-lived. In one study tracking real-world use, the most frequent complaints were flushing (about 31% of those with side effects), headache (about 25%), nasal congestion (about 19%), and heartburn (about 11%). Nobody in that study dropped out because the side effects were too severe.¹⁰ A systematic review confirmed that men taking sildenafil had a higher risk of headache, flushing, indigestion, and visual disturbances compared to placebo, and that the risk climbed with higher doses.¹¹ Other less common effects reported at lower rates include dizziness, diarrhea, rash, and urinary tract infection.¹²

The visual changes deserve special mention. The blue-tinged vision or light sensitivity caused by PDE6 inhibition in the retina is temporary and dose-dependent. A more serious but very rare concern is a condition called nonarteritic ischemic optic neuropathy, a sudden loss of vision in one eye. In 2005, the FDA added a warning about this risk. Later research found that the association was not clear-cut for the broader population, though there may be a small increased risk specifically with sildenafil use, particularly in people who already have cardiovascular risk factors.¹³

Priapism, a prolonged erection lasting more than four hours, is a rare but medically urgent potential side effect. It requires emergency treatment because blood trapped in the penis for too long can cause permanent tissue damage. A case report involving accidental ingestion by a child documented a prolonged but painless erection that resolved on its own within about 24 hours, likely because of a different blood-flow mechanism than the dangerous low-flow type seen in adults.¹⁴ In adults, any erection persisting beyond four hours warrants a trip to the emergency room.

The Nitrate Interaction and Other Dangerous Combinations

The single most dangerous drug interaction with Viagra involves nitrates, medications commonly prescribed for chest pain (angina). Nitroglycerin tablets, nitroglycerin patches, and isosorbide mononitrate are all nitrates. Combining sildenafil with any of them can cause a large, sustained drop in blood pressure that may reduce blood flow to the heart itself. In a study of men with stable angina, combining sildenafil with a nitrate roughly doubled the blood pressure drop compared to the nitrate alone, with systolic pressure falling by an average of over 50 points in the standing position.¹⁵ Experimental work has confirmed that this combination can also reduce blood flow through narrowed coronary arteries specifically, making it especially dangerous for people with significant heart disease.¹⁶

This is not a theoretical concern. Nitrates and Viagra are absolutely contraindicated together, meaning they should never be used in combination. If you take nitroglycerin or any other nitrate for a heart condition, sildenafil is off the table. The same applies to recreational use of amyl nitrite (“poppers”), which work through the same nitric oxide pathway.

Alpha-blocker medications, used for high blood pressure or an enlarged prostate, also interact with sildenafil. Some patients who take both develop orthostatic hypotension, a drop in blood pressure upon standing that causes dizziness or fainting. The interaction appears to be less of an issue for patients who have been on alpha-blockers long term, likely because the body adjusts to the blood-pressure-lowering effect, but precautions are still advised.¹⁷ Doctors typically recommend separating the doses by several hours and starting with the lowest sildenafil dose.

Off-Label Uses Under Investigation

Because PDE5 exists in blood vessels throughout the body, researchers have explored sildenafil for conditions far removed from erectile dysfunction. Some of these are promising, and some have not panned out.

One of the more interesting applications involves digital ulcers in systemic sclerosis, an autoimmune condition that can constrict blood flow to the fingers severely enough to cause open sores. A pilot study found that sildenafil treatment reduced the number of these ulcers from 49 to 17 and improved pain and Raynaud’s symptoms.¹⁸ A placebo-controlled trial later confirmed that ulcer healing rates were significantly better with sildenafil at both 8 and 12 weeks of treatment.¹⁹

For women with sexual arousal difficulties, the picture is more complicated. A trial in postmenopausal women with arousal disorder found that sildenafil improved several arousal measures compared to placebo, but only in women who did not also have low desire. When low desire was present alongside the arousal problem, sildenafil did not help much, which makes sense because the drug works on the physical blood-flow component rather than the desire or psychological component.²⁰ More recently, a topical sildenafil cream applied directly to genital tissue showed improvements in arousal sensation and reduced sexual distress in a preliminary trial, suggesting that a localized approach may work better for some women than an oral pill.²¹

High-altitude use has also been explored. The theory is that sildenafil could lower pulmonary artery pressure at altitude and prevent altitude sickness or high-altitude pulmonary edema. The reality has been disappointing. A placebo-controlled trial at 5,200 meters found that sildenafil did not reduce pulmonary artery pressure in healthy lowlanders and actually worsened acute mountain sickness symptoms on the second day at altitude.²² A systematic review of PDE5 inhibitor use in high-altitude pulmonary edema similarly found no significant improvement in outcomes.²³ Despite occasional anecdotal claims among mountaineers, the evidence does not support packing Viagra for a high-altitude expedition.

How Viagra Compares to Other PDE5 Inhibitors

Sildenafil is no longer the only option. Tadalafil (Cialis) and vardenafil (Levitra) work through the same mechanism but differ in practical ways that matter to users. The biggest difference is duration: sildenafil’s effects last roughly four to six hours, while tadalafil can last up to 36 hours. That longer window is why tadalafil is sometimes called the “weekend pill.” Tadalafil can also be taken as a low daily dose for continuous readiness rather than on-demand use, which suits some people better.

At the molecular level, these drugs vary in how specifically they target PDE5 versus other PDE enzymes. All three are highly selective for PDE5 over PDE11, a related enzyme whose accidental inhibition was once a theoretical safety concern. Sildenafil and vardenafil are both about 1,000-fold or more selective for PDE5 over PDE11, while tadalafil is only about 40-fold selective. In practice, none of the three appears to cause problems from PDE11 crossover at therapeutic doses.²⁴ The clinical differences between these drugs come down mostly to their pharmacokinetics: how fast they are absorbed, how long they last, and how they interact with food and other medications.²⁵

The Counterfeit Problem

Viagra is one of the most counterfeited drugs in the world, and the fakes are not just ineffective pills with less active ingredient than advertised. Analyses of counterfeit PDE5 inhibitor pills have found contents ranging from zero active ingredient to more than double the labeled dose. Beyond inconsistent dosing, counterfeit pills have been found to contain contaminants including talcum powder, commercial paint, and printer ink. In one analysis, only about 10% of counterfeit samples fell within 10% of the dose claimed on the label.²⁶

The risk extends to products marketed as “natural” or “herbal” sexual enhancement supplements. Researchers who purchased three such products from an e-commerce platform found that all of them contained sildenafil citrate despite not declaring it. One product contained about 200 mg of sildenafil, twice the maximum daily dose, hidden inside a capsule claiming to be herbal.²⁷ For someone also taking nitrates for a heart condition, unknowingly consuming a hidden double dose of sildenafil could be life-threatening.

The practical takeaway is that purchasing erectile dysfunction medication from unregulated online vendors, gas stations, or supplement shops carries real risks beyond just wasting money. Illicit products may contain harmful contaminants and wildly unpredictable amounts of active drug, without the safety warnings that come with a legitimate prescription.²⁸ Generic sildenafil is now available at a fraction of the original Viagra price through licensed pharmacies, which makes the risk-to-reward calculation of buying unregulated products even harder to justify.

Sildenafil in Women and the Broader Research Horizon

The question of whether Viagra “works for women” comes up frequently, and the honest answer is that it depends entirely on the nature of the problem. Female sexual dysfunction is a broad category that includes low desire, difficulty with arousal, pain during sex, and trouble reaching orgasm. Because sildenafil acts specifically on blood flow, it only addresses one piece of that puzzle: the genital arousal response. The trial in postmenopausal women described earlier found benefits specifically in women whose primary complaint was physical arousal difficulty without a simultaneous drop in desire.²⁰ When low desire was also present, the drug did not outperform placebo. This makes physiological sense: increasing blood flow does nothing if the brain is not sending the “interested” signal in the first place.

The topical cream approach may eventually change the landscape. By applying sildenafil directly to the tissue rather than taking it orally, the cream concentrates the effect where it is needed while reducing systemic exposure and the side effects that come with it. The early trial data showed improvements in arousal sensation and reductions in sexual distress, but these are still preliminary findings from one trial.²¹ Larger studies are needed before this could become a standard treatment. Still, the idea of repurposing a well-understood molecule in a new delivery format for an underserved patient population is the kind of practical innovation that could genuinely matter. Female sexual dysfunction has far fewer evidence-based pharmacological options than its male counterpart, and most of the drugs that do exist target desire through central nervous system pathways rather than peripheral blood flow. Sildenafil cream could fill a specific and currently empty niche.