Viagra relaxes blood vessels in the penis, making it easier to get and maintain an erection during sexual arousal. It does not create arousal on its own. The drug works only when you’re already sexually stimulated, and its effects typically last up to four hours after taking it.
How Viagra Works in the Body
When you become sexually aroused, nerve endings and blood vessel walls in the penis release a chemical signal called nitric oxide. That signal triggers a chain reaction: cells produce a molecule (cGMP) that causes the smooth muscle lining penile blood vessels to relax. As those muscles relax, the vessels widen, blood flows in, and an erection forms.
Normally, an enzyme called PDE5 breaks down cGMP almost as fast as it’s produced. That’s the body’s built-in off switch, and it’s what eventually returns the penis to a soft state. Viagra (sildenafil) blocks PDE5, slowing that breakdown. With more cGMP available, the blood vessels stay relaxed longer and blood flow improves. In men with erectile dysfunction, where nerve signals or blood flow are already compromised, this boost can be the difference between a partial response and a full erection.
The key point: Viagra amplifies a process that sexual stimulation starts. Without arousal, nitric oxide isn’t released, cGMP isn’t produced, and the drug has nothing to amplify.
How Quickly It Works and How Long It Lasts
Most people take Viagra about an hour before sexual activity. Peak blood levels are reached somewhere between 30 minutes and 2 hours, with a median of 60 minutes on an empty stomach. The effect can last up to four hours, though it’s strongest around the two-hour mark and noticeably weaker by hour four.
A high-fat meal changes the math. Eating something greasy around the time you take Viagra delays peak absorption by roughly an hour and reduces the peak concentration in your blood by about 29%. That doesn’t mean it won’t work, but it will take longer to kick in and may feel less effective. Taking it on an empty stomach, or after a light meal, gives you the most predictable timing.
Dosing
Viagra comes in 25 mg, 50 mg, and 100 mg tablets. The standard starting dose is 50 mg, taken as needed. From there, a prescriber may adjust up to 100 mg or down to 25 mg depending on how well it works and whether side effects are bothersome. It’s a one-pill-per-day maximum, not a daily medication for most people.
Common Side Effects
The most frequently reported side effects are headache, facial flushing, nasal congestion, and indigestion. These are all tied to the same mechanism that produces the intended effect: blood vessel dilation. When vessels in the head, face, and nasal passages relax alongside those in the penis, you get those familiar side effects. They’re generally mild and tend to fade as the drug leaves your system.
Some people notice a temporary blue-green tint to their vision. This happens because sildenafil has a slight effect on a related enzyme (PDE6) found in the retina. It’s not harmful in most cases and resolves on its own.
The Nitrate Interaction
One interaction is genuinely dangerous: combining Viagra with nitrate medications. Nitrates are prescribed for chest pain (angina) and work through the same nitric oxide pathway that Viagra uses. When both are active at the same time, cGMP levels surge in blood vessels throughout the body, not just the penis. The result is a dramatic, potentially life-threatening drop in blood pressure.
Research on this interaction found that when subjects took sildenafil alongside a common nitrate (glyceryl trinitrate), the decrease in systolic blood pressure was four times greater than with the nitrate alone. That drop happened within seven minutes. This is why sildenafil is strictly contraindicated with any form of organic nitrate, whether taken as a daily pill, a spray, or a patch. If you use nitrates for a heart condition, Viagra is off the table entirely.
Beyond Erectile Dysfunction
The same blood-vessel-relaxing mechanism that helps with erections turns out to be useful in the lungs. Sildenafil is also FDA-approved, under the brand name Revatio, for treating pulmonary arterial hypertension (PAH). In PAH, the arteries carrying blood from the heart to the lungs become narrowed and stiff, forcing the heart to work much harder. Sildenafil widens those pulmonary arteries, lowering blood pressure in the lungs and improving the heart’s ability to pump blood through them. The result is better exercise tolerance and less strain on the heart.
The dosing for PAH is completely different from the erectile dysfunction version. Revatio is a smaller, round white 20 mg tablet taken three times daily, compared to the familiar blue diamond-shaped Viagra pill taken as needed. Same molecule, different purpose, different regimen.