Viagra (sildenafil) is primarily used to treat erectile dysfunction (ED) in adult men. It works by increasing blood flow to the penis, helping achieve and maintain an erection when sexually aroused. The same active ingredient, sold under a different brand name, is also FDA-approved to treat a serious lung condition called pulmonary arterial hypertension.
How Viagra Treats Erectile Dysfunction
When you’re sexually aroused, your body releases a chemical signal that triggers the production of a molecule called cGMP in the blood vessels of the penis. This molecule relaxes the smooth muscle in those vessels, allowing them to widen and fill with blood, producing an erection. Normally, an enzyme breaks down cGMP relatively quickly. Sildenafil blocks that enzyme, letting cGMP build up and keeping blood vessels relaxed longer. The result is stronger, more sustained blood flow.
This is why Viagra only works when you’re already sexually excited. It doesn’t create arousal on its own. It amplifies the body’s existing process.
Timing, Dosing, and What to Expect
The standard starting dose is 50 mg, taken roughly one hour before sexual activity. Depending on how well it works and how you tolerate it, your dose may be adjusted down to 25 mg or up to a maximum of 100 mg. You should not take it more than once per day.
Viagra can start working in as little as 30 minutes. Peak blood levels typically hit around the one-hour mark, and the effect lasts up to four hours, though it tends to diminish after the first two. You can take it with or without food, but a high-fat meal (think cheeseburger and fries) can slow absorption and delay when it kicks in.
Treating Pulmonary Arterial Hypertension
The same drug is sold under the brand name Revatio for pulmonary arterial hypertension (PAH), a condition where blood pressure in the arteries leading to the lungs becomes dangerously high. The heart has to work harder to pump blood through narrowed lung vessels, which over time can lead to heart failure.
Because sildenafil relaxes smooth muscle in blood vessels throughout the body, not just in the penis, it can widen those lung arteries and reduce the strain on the heart. The FDA approved it specifically for WHO Group I pulmonary arterial hypertension in adults, with the goals of improving exercise ability and delaying clinical worsening. The dosing schedule for PAH is completely different from the ED dosing, involving smaller, more frequent doses throughout the day.
Other Medical Uses
Doctors sometimes prescribe sildenafil off-label for conditions that also involve restricted blood flow. One notable example is Raynaud’s phenomenon, a condition where blood vessels in the fingers and toes spasm in response to cold or stress, cutting off circulation and sometimes causing painful ulcers. Guidelines from the UK’s National Institute for Health and Care Excellence list sildenafil as a treatment option for people with severe Raynaud’s that hasn’t responded to first-line therapy, particularly when it’s linked to an autoimmune condition called systemic sclerosis.
Common Side Effects
The most frequently reported side effects are headache, facial flushing, heartburn, and nasal congestion. These are all related to the same blood-vessel-widening effect that makes the drug work, and they’re usually mild.
Vision changes deserve special attention. Some users experience a temporary blue tint to their vision, increased light sensitivity, or altered color perception. A case series published in Frontiers in Neurology examined 17 patients whose visual side effects lasted beyond 24 hours. Among them, about 53% had some degree of light sensitivity, roughly 76% showed disrupted color perception, and 47% had abnormally dilated pupils. While these effects were temporary, they highlight that vision-related side effects can occasionally be more than a brief inconvenience.
Prolonged erections lasting more than four hours (priapism) are rare but require immediate medical attention, as restricted blood flow for that long can cause permanent tissue damage.
The Nitrate Interaction
The single most important safety concern with Viagra is its interaction with nitrate medications, commonly prescribed for chest pain (angina). Drugs like nitroglycerin patches, sublingual tablets, and isosorbide work by boosting the same cGMP pathway that sildenafil enhances. Combining the two causes a compounding effect: blood vessels throughout the body relax far more than intended, producing large, sudden drops in blood pressure.
Research published in the American Heart Association journal Circulation found that when sildenafil was combined with nitrates, the decreases in blood pressure were not only severe but prolonged. In patients with narrowed coronary arteries, blood flow to the heart itself dropped significantly. This combination is strictly contraindicated, meaning you should never take Viagra if you use any form of nitrate medication.
Generic and Newer Formulations
Viagra’s patent exclusivity expired years ago, and generic sildenafil is now widely available at a fraction of the original brand-name cost. The generic contains the same active ingredient in the same doses (25 mg, 50 mg, and 100 mg tablets) and is held to the same FDA manufacturing standards.
A newer formulation, an oral dissolving film, received FDA approval and is expected to become commercially available in the U.S. in early 2026. This thin film dissolves in the mouth rather than requiring a traditional tablet and water. Fast-dissolving versions have been available through compounding pharmacies for years, but the new product is the first to go through the full FDA approval process, giving it a level of standardization and quality assurance that compounded versions lack. Direct home delivery will be available in all states, reflecting a broader industry shift toward discreet, convenient access to ED treatment.