Viagra (sildenafil) increases blood flow to the penis, making it easier to get and maintain an erection when you’re sexually aroused. It does this by blocking an enzyme that normally breaks down a chemical signal responsible for relaxing blood vessels. The drug doesn’t create arousal on its own, and it won’t increase your sex drive. It only works when you’re already sexually stimulated.
How Viagra Works in the Body
When you become sexually aroused, your body releases nitric oxide in the penis. This triggers the production of a molecule called cGMP, which relaxes the smooth muscle in blood vessel walls and lets blood flow in to produce an erection. Normally, an enzyme called PDE5 breaks down cGMP quickly, which is part of what ends an erection.
Sildenafil blocks PDE5 from doing its job. With PDE5 out of the way, cGMP builds up and sticks around longer, keeping those blood vessels relaxed and open. The result is a stronger, more sustained erection. But because the whole chain starts with sexual arousal triggering nitric oxide release, the drug does nothing if you’re not aroused. It amplifies a process that’s already happening rather than starting one from scratch.
How Long It Takes and How Long It Lasts
Viagra typically starts working within 30 to 60 minutes after you take it. Peak blood levels are reached somewhere between 30 and 120 minutes, with 60 minutes being the median. The effects can last up to four hours, though the response at the four-hour mark is noticeably weaker than at two hours.
Eating a high-fat meal before taking it slows things down. A fatty meal delays the peak by about an hour and reduces the amount of drug that reaches your bloodstream by roughly 29%. If timing matters, taking it on an empty stomach or after a light meal gives you faster, more reliable results.
The Standard Dose
The typical starting dose is 50 mg, taken about an hour before sexual activity. Depending on how well it works and how you tolerate it, the dose can be adjusted down to 25 mg or up to 100 mg. It shouldn’t be taken more than once a day.
Common Side Effects
Most side effects come from the same blood-vessel-relaxing mechanism that produces the main effect. Because PDE5 exists in blood vessels throughout your body (not just in the penis), Viagra dilates vessels elsewhere too. In clinical trials at the 100 mg dose, the most frequently reported side effects were:
- Headache: 28% of users (compared to 7% on placebo)
- Flushing: 18% of users, often a warm redness in the face, neck, or chest
- Indigestion: 17% of users
- Nasal congestion: 9% of users
At the lower 50 mg dose, these numbers drop: 21% for headache, 19% for flushing, 9% for indigestion, and 4% for nasal congestion. These effects are generally mild and fade as the drug leaves your system. Some men also notice temporary changes in color vision, particularly a blue-green tint, because a related enzyme in the retina gets mildly affected.
What Viagra Does Not Do
Viagra does not increase sexual desire. It has no effect on libido, arousal, or attraction. If the underlying issue is low desire rather than difficulty with erections, sildenafil won’t help. It also doesn’t cure erectile dysfunction. It’s a treatment that works for the occasion you take it, not a permanent fix.
It’s not an aphrodisiac, and it won’t cause a spontaneous erection. Men without erectile dysfunction sometimes take it recreationally expecting enhanced performance, but in men who already have normal erectile function, the benefits are minimal. The drug works by preventing the breakdown of a signal your body is already producing, so there needs to be something to amplify in the first place.
Dangerous Interactions With Nitrates
The most serious risk with Viagra involves combining it with nitrate medications, which are commonly prescribed for chest pain (angina). Nitrates work by releasing nitric oxide, which relaxes blood vessels and lowers blood pressure. Adding sildenafil on top of that creates a double hit: nitrates flood the system with nitric oxide, and sildenafil prevents the resulting cGMP from being broken down. The combined effect can cause a severe, potentially fatal drop in blood pressure.
Research published in the American Heart Association’s journal Circulation found that the blood pressure decrease caused by nitrates was significantly larger and longer-lasting when sildenafil was in the system. In patients with narrowed coronary arteries, this can trigger a dangerous cycle where falling blood pressure reduces blood flow to the heart, causing cardiac ischemia, which drops blood pressure further. This interaction is the reason Viagra carries an absolute contraindication with any form of organic nitrate, including nitroglycerin tablets, patches, and sprays.
Rare but Serious Risks
Priapism, an erection lasting four hours or longer, is a rare but well-known risk. It requires urgent medical attention because prolonged erection without blood circulation can permanently damage penile tissue.
There have also been reports of a condition called non-arteritic anterior ischemic optic neuropathy (NAION), a type of sudden vision loss caused by reduced blood flow to the optic nerve. However, large surveillance studies have not found a clear cause-and-effect link. The incidence rate among men using PDE5 inhibitors was about 2.8 cases per 100,000 patient-years, which is similar to the background rate in men over 50 who aren’t taking the drug. Sudden hearing loss has also been reported rarely.
Uses Beyond Erectile Dysfunction
The same enzyme that Viagra blocks in the penis also exists in the lungs. In people with pulmonary arterial hypertension, a condition where blood pressure in the lung arteries is dangerously high, PDE5 breaks down cGMP in lung blood vessels, keeping them constricted. By inhibiting PDE5, sildenafil relaxes those vessels and improves blood flow through the lungs. It’s sold under a different brand name (Revatio) for this use, at a lower dose taken three times daily rather than as needed.