Viagra (sildenafil) increases blood flow to the penis by relaxing the smooth muscle in penile blood vessels, making it easier to get and maintain an erection during sexual arousal. It typically starts working within 30 to 60 minutes, lasts up to four hours, and requires sexual stimulation to have any effect. It does not boost sex drive or cause spontaneous erections on its own.
How Viagra Works in the Body
During sexual arousal, nerve endings and blood vessel cells in the penis release a chemical called nitric oxide. This triggers the production of a signaling molecule (cGMP) that relaxes the smooth muscle lining the penile arteries and spongy tissue. As that muscle relaxes, blood flows in and fills the tissue, producing an erection.
The body also produces an enzyme called PDE5, whose job is to break down that signaling molecule and wind the process back down. Viagra blocks PDE5, which lets the signaling molecule accumulate at higher levels for longer. The result is stronger, more sustained smooth muscle relaxation, more blood flow, and a firmer erection. This is why Viagra only works when you’re sexually aroused: without that initial release of nitric oxide, there’s nothing for the drug to amplify.
What It Feels Like and How Long It Lasts
Most people reach peak blood levels of the drug about 60 minutes after taking it on an empty stomach, though the window ranges from 30 minutes to two hours. The strongest effects are around the two-hour mark, with a noticeable but diminishing response out to about four hours. Eating a heavy or high-fat meal beforehand can delay onset.
Viagra doesn’t change sensation, arousal, or orgasm. What it changes is the mechanical part of the process: blood getting in and staying in. If you lose your erection after orgasm, that’s normal. The drug doesn’t override the body’s natural refractory period.
Common Side Effects
About one in three people experience at least one side effect. In clinical practice, the most frequently reported are:
- Flushing (about 31% of users), a warm redness in the face or chest
- Headache (about 25%)
- Nasal congestion (about 19%)
- Heartburn or indigestion (about 11%)
These side effects are driven by the same blood vessel relaxation that produces the erection. PDE5 exists throughout the body, not just in the penis, so blood vessels in the face, sinuses, and digestive tract can also dilate. Most side effects are mild and fade as the drug wears off. Some people also notice temporary changes in color vision, particularly a bluish tint, because a related enzyme in the retina is mildly affected.
The Nitrate Interaction
The most dangerous interaction is with nitrate medications, commonly prescribed for chest pain (angina). Nitrates work by flooding the same signaling pathway that Viagra amplifies. Combining the two can cause severe, sudden drops in blood pressure, reduced blood flow to the heart, and potentially fatal outcomes. This includes nitroglycerin in any form (patches, tablets, sprays) and recreational drugs known as “poppers” (amyl nitrate or butyl nitrate).
Research published by the American Heart Association showed that in patients with narrowed coronary arteries, the combination created a dangerous feedback loop: falling blood pressure reduced coronary blood flow, which weakened the heart further, which dropped blood pressure even more. This interaction isn’t dose-dependent or predictable, so the two should never be combined. If you use any form of nitrate, Viagra is not an option.
People with recent heart attacks, strokes, serious heart failure, or very low blood pressure are also generally advised against using the drug, since the blood pressure effects can be unpredictable in those situations.
Beyond Erectile Dysfunction
Sildenafil is also FDA-approved under a different brand name (Revatio) for pulmonary arterial hypertension, a condition where blood pressure in the arteries connecting the heart to the lungs is dangerously high. The mechanism is the same: by blocking PDE5 in the lung vasculature, the drug relaxes those blood vessels, lowers pulmonary pressure, and improves the heart’s ability to pump blood through the lungs. For people with this condition, the result is better exercise tolerance and slower disease progression.
What Viagra Does Not Do
Viagra does not increase sexual desire. It does not work as an aphrodisiac, and it will not produce an erection without arousal. In people who don’t have erectile dysfunction, it may slightly improve erection firmness, but it won’t fundamentally change sexual performance. It also does not treat the underlying causes of erectile dysfunction, whether those are cardiovascular disease, diabetes, hormonal imbalances, or psychological factors. It manages the symptom (insufficient blood flow) while the erection is needed, then its effects wear off.
For people whose erectile dysfunction stems from nerve damage (after prostate surgery, for instance) or severely reduced nitric oxide production, the drug may be less effective, since it depends on that initial chemical signal to have something to work with.