Viagra pills help men get and maintain erections by increasing blood flow to the penis. The active ingredient, sildenafil, works by blocking an enzyme that normally limits blood flow to erectile tissue. It doesn’t create arousal on its own, and it won’t cause a spontaneous erection. Sexual stimulation is required for the drug to have any effect.
How Viagra Works in the Body
During sexual arousal, nerve endings in the penis release a chemical called nitric oxide. This triggers a chain reaction that produces a molecule called cGMP, which relaxes the smooth muscle tissue inside the penis and allows blood to flow in. That blood flow is what creates an erection.
Normally, an enzyme called PDE5 breaks down cGMP almost as fast as the body makes it. In men with erectile dysfunction, this breakdown happens too quickly or the signaling is too weak, making it difficult to get or keep an erection. Sildenafil blocks PDE5, so cGMP builds up to higher levels and sticks around longer. The result is stronger, more sustained blood flow to the penis when you’re aroused.
This is why Viagra has no effect without sexual stimulation. The drug doesn’t start the process. It amplifies what your body is already doing. At recommended doses, sildenafil has zero effect in the absence of arousal.
How Well It Works
In clinical trials, 67% to 86% of men taking sildenafil reported improved erections, compared to just 24% on a placebo. Men on the standard or higher dose went from being able to achieve adequate erections “a few times” to “most times,” a statistically significant improvement. The drug works across a range of causes of erectile dysfunction, including diabetes-related, vascular, and psychological.
Timing, Onset, and Duration
The standard recommendation is to take Viagra about one hour before sexual activity. Effects can begin in as little as 30 minutes and last for up to four hours, though the response tends to be strongest around the two-hour mark and diminishes after that.
Eating a high-fat meal around the same time you take the pill delays absorption by about an hour and reduces peak levels of the drug in your blood by roughly 29%. An 11% drop in overall drug exposure also occurs. So if you want the fastest, strongest effect, take it on a relatively empty stomach or after a light meal.
Available Doses
Viagra comes in 25 mg, 50 mg, and 100 mg tablets. The typical starting dose is 50 mg. From there, the dose can be adjusted up or down based on how well it works and whether side effects are a problem. It’s taken as needed, not daily, and you can take it anywhere from 30 minutes to 4 hours before sex.
Common Side Effects
The most frequently reported side effects are headache, facial flushing, nasal congestion, and indigestion. These are all related to the same blood vessel relaxation that makes the drug work for erections. Because sildenafil affects PDE5 throughout the body (not just in the penis), blood vessels elsewhere can dilate slightly too. Most side effects are mild and temporary, resolving within a few hours as the drug wears off. Some men also notice temporary changes in color vision, particularly a blue tint, because a related enzyme in the retina is mildly affected.
The Nitrate Interaction
The single most important safety concern with Viagra is its interaction with nitrate medications. Nitrates, commonly prescribed for chest pain (nitroglycerin, isosorbide), work by relaxing blood vessels. When combined with sildenafil, this vessel-relaxing effect is dramatically amplified, potentially causing a sudden, dangerous drop in blood pressure.
This interaction isn’t a mild concern. The American Heart Association describes it as “potentially fatal.” The risk applies to all forms of nitrates: long-acting pills, short-acting sprays, patches, and even recreational nitrate inhalants sometimes called “poppers.” If you’ve taken any nitrate in the past 24 hours, sildenafil is strictly off-limits. The reverse is also true: if you’ve taken Viagra, nitrates should not be used for at least 24 hours afterward.
Beyond Erectile Dysfunction
Sildenafil is also FDA-approved under the brand name Revatio to treat pulmonary arterial hypertension, a condition where blood pressure in the arteries of the lungs is dangerously high. The mechanism is identical: by blocking PDE5 in the lung’s blood vessels, the drug relaxes those vessels and lets blood flow more easily. The dosing is very different, though. For pulmonary hypertension, patients take 20 mg three times daily at regular intervals rather than a single larger dose before sex.
This dual use reflects the fact that PDE5 exists in smooth muscle tissue throughout the body, not just in the penis. The drug’s effect on blood flow is systemic, but the erectile tissue of the penis happens to have an especially high concentration of PDE5, which is why the effect is most pronounced there.