Viagra is a prescription medication used to treat erectile dysfunction (ED) in men. Its active ingredient, sildenafil citrate, works by increasing blood flow to the penis, making it easier to get and maintain an erection during sexual activity. It does not cause spontaneous erections on its own; sexual arousal is still required for it to work.
How Viagra Works
When a man becomes sexually aroused, his body releases a chemical signal (nitric oxide) that triggers a chain reaction in the blood vessels of the penis. This chain reaction produces a molecule called cGMP, which relaxes the smooth muscle in penile blood vessels and allows them to widen, letting blood flow in and producing an erection. Normally, an enzyme called PDE5 breaks down cGMP fairly quickly, which is part of what ends an erection.
Viagra blocks that PDE5 enzyme. With the enzyme out of the way, cGMP stays active longer, blood vessels stay relaxed and open, and blood flow to the penis is sustained. The result is a firmer, longer-lasting erection. But because the whole process starts with arousal and that initial nitric oxide signal, Viagra won’t do anything without sexual stimulation.
How It Was Discovered
Viagra was never designed to treat erectile dysfunction. Researchers discovered sildenafil in 1989 while looking for a drug to treat coronary heart disease and angina (chest pain). Early clinical trials in the 1990s showed disappointing results for heart conditions, but male participants kept reporting an unexpected side effect: improved erections. That incidental finding redirected the entire course of the drug’s development, and the FDA approved it for erectile dysfunction in 1998.
How Effective It Is
In clinical trials, Viagra significantly outperformed placebo. A meta-analysis of trial data found that with dose optimization, 49% of men had successful sexual intercourse on at least 60% of attempts, compared to just 11% with placebo. For the broader measure of overall improvement in erections, the drug worked so reliably that only about 1.7 men needed to be treated for one to report meaningful improvement. That’s a strong result by pharmaceutical standards.
Viagra is not a cure for the underlying causes of ED. It treats the symptom (difficulty achieving or maintaining erections) while the medication is active, but once it wears off, the underlying condition remains. Many men use it on an ongoing, as-needed basis.
Timing, Food, and How Long It Lasts
The standard recommended dose is 50 mg, taken roughly one hour before sexual activity. It can be taken as early as four hours beforehand or as late as 30 minutes before. The drug is absorbed quickly after swallowing, and it reaches peak levels in the bloodstream within about an hour on an empty stomach.
Food matters. Eating a high-fat meal around the same time you take Viagra delays its peak concentration by about an hour and reduces the amount of drug your body absorbs by roughly 29%. This means it may take noticeably longer to kick in and may feel less effective. Taking it on an empty stomach, or after a light meal, gives the best results.
Once active, Viagra’s effects typically last around four hours, though the response at the four-hour mark is weaker than at two hours. The drug and its active breakdown product both have half-lives of about four hours, meaning half the medication has been cleared from your system by that point.
Common Side Effects
Side effects are dose-dependent, meaning they become more likely at higher doses. In clinical trials comparing the 25 mg, 50 mg, and 100 mg doses:
- Headache was the most common side effect, affecting 16% of men at the lowest dose and 28% at the highest (compared to 7% on placebo).
- Facial flushing affected 10% to 19% of men, depending on dose.
- Indigestion ranged from 3% at 25 mg to 17% at 100 mg.
- Vision changes were rare at lower doses (1–2%) but reached 11% at the 100 mg dose. These are typically mild and temporary, involving a slight blue or green color tinge, light sensitivity, or blurred vision.
- Nasal congestion affected up to 9% of men at the highest dose.
Most of these side effects are mild and resolve on their own as the drug leaves your system. Back pain, muscle aches, nausea, dizziness, and rash each occurred in a small percentage of men across all doses.
The Nitrate Danger
The most important safety concern with Viagra is its interaction with nitrate medications, which are commonly prescribed for chest pain and heart conditions (nitroglycerin patches, nitroglycerin tablets, isosorbide). Both Viagra and nitrates lower blood pressure through related pathways. Combining them can cause a sudden, severe drop in blood pressure that reduces blood flow to the heart. In studies where the two were combined, large and rapid blood pressure drops occurred in the majority of participants. In men with narrowed coronary arteries, this combination can potentially trigger a fatal cardiac event. Viagra is strictly contraindicated for anyone taking nitrates in any form.
Priapism: A Rare but Serious Risk
An erection lasting longer than four hours, called priapism, is a medical emergency. This is rare with Viagra, but it can happen. Prolonged ischemic priapism cuts off fresh blood flow to penile tissue, leading to oxygen deprivation, tissue damage, and potentially permanent erectile dysfunction if not treated promptly. Any erection that persists beyond four hours requires immediate emergency care, regardless of whether it’s painful.