Viagra (sildenafil) works by blocking an enzyme in the blood vessels of the penis, allowing them to relax and fill with blood during sexual arousal. It doesn’t create arousal on its own. Instead, it amplifies the body’s natural response to sexual stimulation, making it easier to get and maintain an erection.
The Chain Reaction Behind an Erection
When you become sexually aroused, nerve endings and blood vessel linings in the penis release a chemical signal called nitric oxide. This signal triggers the production of a second messenger molecule (cGMP) inside smooth muscle cells. As cGMP builds up, it causes those muscle cells to relax, widening the small arteries in the penis and letting blood flow in. The increased blood flow is what produces an erection.
Normally, another enzyme, PDE5, breaks down cGMP almost as fast as it’s produced. Think of it as a cleanup crew that keeps the relaxation signal short-lived. In men with erectile dysfunction, this balance tips too far toward cleanup: not enough cGMP accumulates, the smooth muscle doesn’t relax fully, and blood flow stays restricted.
Where Viagra Steps In
Sildenafil binds directly to PDE5 and blocks it from breaking down cGMP. With the cleanup crew sidelined, cGMP levels rise higher and last longer inside the smooth muscle cells of the penis. The result is stronger muscle relaxation, wider arteries, and improved blood flow. This is why Viagra only works when you’re already aroused: sexual stimulation is still needed to trigger the initial release of nitric oxide that starts the whole process.
How Quickly It Works and How Long It Lasts
Viagra reaches its peak concentration in your blood within 30 to 120 minutes after you take it, with a median of about 60 minutes. That’s why the standard recommendation is to take it roughly an hour before sexual activity, though it can be taken as early as 30 minutes or as far out as four hours beforehand.
The drug’s half-life is about 3 to 5 hours, meaning it stays active in your system for a meaningful window. Studies show the effect is strongest around the two-hour mark and still present at four hours, though it starts to diminish after that. One practical detail: eating a high-fat meal around the time you take it can delay absorption by about an hour. It won’t reduce how well the drug works overall, but your timeline shifts.
Typical Doses and Effectiveness
The standard starting dose is 50 mg, taken as needed, no more than once per day. Based on how well it works and how you tolerate it, the dose can be adjusted down to 25 mg or up to a maximum of 100 mg. Older adults and people with significant liver or kidney issues typically start at the lower 25 mg dose, since the drug clears from their systems more slowly.
In clinical trials involving nearly 1,800 men with erectile dysfunction, 63% of those on the 25 mg dose reported improved erections, compared to 74% on 50 mg and 82% on 100 mg. Only 24% of men taking a placebo reported improvement. These trials included men whose erectile dysfunction had physical causes (including diabetes), psychological causes, or a mix of both.
Common Side Effects
Because Viagra relaxes blood vessels, its most common side effects are related to that same mechanism acting elsewhere in the body. In clinical trials at the 100 mg dose, 28% of men reported headaches, 18% experienced facial flushing, and 17% had indigestion. At the 50 mg dose, those numbers dropped to 21%, 19%, and 9% respectively. Some men also reported nasal congestion, dizziness, temporary changes in color vision (a slight blue tint), back pain, or nausea. Most of these effects are mild and resolve within a few hours as the drug wears off.
Why Nitrate Medications Are Dangerous to Combine
This is the most important safety concern with Viagra. Nitrate medications, commonly prescribed for chest pain (angina), work through the exact same pathway: they boost nitric oxide signaling, which raises cGMP levels, which relaxes blood vessels and lowers blood pressure. When you add Viagra on top of that, you’re amplifying the same signal twice. The result can be a severe, potentially life-threatening drop in blood pressure.
In controlled studies, men taking sildenafil who were then given a nitrate experienced a four-fold greater drop in systolic blood pressure compared to those on placebo. They were also significantly more likely to develop symptomatic low blood pressure, meaning dizziness, fainting, or worse. This interaction applies to all forms of nitrates, whether taken regularly, occasionally, or as a sublingual tablet for acute chest pain. The combination is strictly contraindicated.
What Viagra Does Not Do
Viagra does not increase sexual desire. It does not cause spontaneous erections without arousal. And it does not treat the underlying cause of erectile dysfunction, whether that’s cardiovascular disease, diabetes, nerve damage, or psychological factors. It works downstream of all those causes by making the most of whatever nitric oxide signal your body does produce. For men whose erectile dysfunction stems from very low nitric oxide production or severe nerve damage, the drug may be less effective, which partly explains why not every man in clinical trials responded.