Viagra helps men get and maintain erections by increasing blood flow to the penis. It does this by blocking an enzyme that normally limits blood flow, making it easier for the natural arousal process to produce a firm erection. The recommended starting dose is 50 mg, taken about one hour before sexual activity, and it can be used no more than once per day.
How Viagra Works in the Body
When a man becomes sexually aroused, nerve endings in the penis release a chemical called nitric oxide. That triggers a chain reaction: nitric oxide activates an enzyme that produces a molecule called cGMP, which relaxes the smooth muscle tissue inside the penis. As that tissue relaxes, blood flows in and an erection forms.
The body also produces an enzyme called PDE5 that breaks down cGMP, which is part of why erections naturally subside. In men with erectile dysfunction, this breakdown happens too quickly or the process doesn’t generate enough cGMP to begin with. Viagra (sildenafil) blocks PDE5, allowing cGMP to build up and do its job more effectively. The result is stronger blood flow into the penis and a firmer, longer-lasting erection.
It Only Works With Arousal
One of the most common misconceptions is that Viagra automatically produces an erection. It doesn’t. The entire process depends on nitric oxide being released first, and that only happens when you’re sexually stimulated. According to the FDA’s prescribing information, sildenafil “has no effect in the absence of sexual stimulation.” So taking a pill and sitting on the couch won’t do anything. Viagra amplifies your body’s existing arousal response; it doesn’t create one from scratch.
Timing, Onset, and Duration
Most men take Viagra about an hour before sexual activity. Effects typically begin within 30 to 60 minutes, and the drug remains active in the body for roughly four to six hours. That doesn’t mean you’ll have an erection for that entire window. It means that during those hours, your body will respond more easily to sexual stimulation.
What you eat matters. A high-fat meal taken around the same time as Viagra delays peak absorption by about an hour and reduces the peak concentration of the drug in your blood by 29%. A lighter meal or an empty stomach allows it to kick in faster and work more reliably.
Common Side Effects
Viagra’s side effects are well documented from large clinical trials. The most frequent ones are headache, facial flushing, and indigestion. At the 100 mg dose, about 28% of men reported headaches, 18% experienced flushing, and 17% had indigestion. At the lower 50 mg dose, those numbers drop: 21% for headaches, 19% for flushing, and 9% for indigestion. For comparison, only 7% of men taking a placebo reported headaches.
Some men also notice nasal congestion, dizziness, or temporary changes in vision, such as a blue-green tint or increased sensitivity to light. These effects are typically mild and resolve within a few hours as the drug leaves the system.
The Nitrate Warning
Viagra lowers blood pressure slightly on its own, since it relaxes blood vessel walls. This becomes dangerous when combined with nitrate medications, which are commonly prescribed for chest pain (angina). Both drugs widen blood vessels, and together they can cause a sudden, severe drop in blood pressure. Men who take any form of nitrate, including nitroglycerin tablets, patches, or sprays, should not use Viagra. This interaction is the single most important safety concern with the drug.
Beyond Erectile Dysfunction
The same active ingredient in Viagra is also sold under the brand name Revatio for a completely different condition: pulmonary arterial hypertension, a type of high blood pressure in the lungs. The mechanism is identical. By blocking PDE5 in the blood vessels of the lungs, sildenafil relaxes those vessels and reduces the workload on the heart. The doses used for this purpose are much lower than those used for erectile dysfunction.
What Viagra Does Not Do
Viagra does not increase sex drive or libido. If low desire is the underlying issue, the drug won’t help. It also doesn’t treat the root cause of erectile dysfunction, whether that’s cardiovascular disease, diabetes, nerve damage, hormonal imbalance, or psychological factors like anxiety and depression. It addresses the symptom (difficulty getting or keeping an erection) rather than the underlying problem.
It also doesn’t make erections larger than your normal size. Viagra restores the ability to achieve a full erection when arousal is present, but it won’t push things beyond your body’s natural capacity. And it won’t prevent sexually transmitted infections or serve as a form of contraception.
For many men, Viagra is an effective and straightforward treatment. But because erectile dysfunction often signals other health issues, particularly cardiovascular problems, the erection difficulty itself can be worth investigating rather than simply masking with medication.