Viagra (sildenafil) is a prescription medication that helps men with erectile dysfunction get and maintain an erection by increasing blood flow to the penis. It works within 30 to 60 minutes, lasts up to 4 hours, and only produces an erection when you’re sexually aroused.
How Viagra Works in the Body
During sexual arousal, your body naturally releases a chemical signal called nitric oxide in the tissue of the penis. This triggers a chain reaction that relaxes the smooth muscle in blood vessel walls, allowing blood to flow in and produce an erection. A specific enzyme then breaks down the signaling molecule that keeps those muscles relaxed, which is how an erection naturally subsides.
Viagra blocks that enzyme. By slowing the breakdown of the relaxation signal, the drug lets blood flow more easily into the penis and stay there longer. The result is a firmer, more sustained erection. This is also why Viagra does nothing on its own: without sexual arousal, your body never releases the initial nitric oxide signal, so there’s nothing for the drug to amplify. You won’t get an erection just from taking the pill.
How Quickly It Works and How Long It Lasts
Viagra typically reaches peak concentration in the blood about 60 minutes after you take it on an empty stomach, though the range is 30 to 120 minutes. Most men notice effects starting around the 30-minute mark. The strongest window of effectiveness is within the first two hours, and the drug can continue working for up to four hours, though the effect gradually fades during that second half.
Eating a high-fat meal before taking Viagra delays absorption by roughly an hour and reduces the peak concentration in your blood by about 29%. A heavy burger or steak dinner before taking the pill means it will kick in later and may not work as well. Taking it on an empty stomach, or after a light meal, gives you the most predictable results.
Standard Dosing
The typical starting dose is 50 mg, taken about an hour before sexual activity. For adults 65 and older, the recommended starting point is 25 mg. The pill is taken no more than once per day. Your prescriber may adjust the dose up or down depending on how well it works and how you tolerate side effects.
Common Side Effects
Side effects are dose-dependent, meaning they’re more likely at higher doses. In clinical trials at the 50 mg dose, the most frequently reported effects were:
- Headache: 21% of users (vs. 7% on placebo)
- Flushing: 19% (vs. 2% on placebo)
- Indigestion: 9% (vs. 2% on placebo)
- Nasal congestion: 4% (vs. 2% on placebo)
- Dizziness: 4% (vs. 2% on placebo)
At the 100 mg dose, headache jumped to 28% and indigestion to 17%. About 11% of people taking the highest dose reported mild, temporary visual changes, typically a slight blue or green tint to their vision, increased light sensitivity, or mild blurriness. These visual effects were rare at lower doses.
Most side effects are mild and short-lived. Headache and flushing happen because the drug relaxes blood vessels throughout the body, not just in the penis. That same blood vessel relaxation is why nasal congestion shows up on the list.
Who Should Not Take Viagra
The most important safety rule with Viagra is straightforward: do not combine it with nitrate medications. Nitrates are commonly prescribed for chest pain (angina) and come in forms like nitroglycerin patches, sublingual tablets, or isosorbide pills. Both Viagra and nitrates lower blood pressure through overlapping mechanisms, and taking them together can cause a sudden, dangerous drop in blood pressure. Studies published in the American Heart Association’s journal Circulation confirmed that large, rapid blood pressure decreases occurred in the majority of patients who combined the two drugs.
This restriction applies even if you only use nitrates occasionally. If there’s any chance you might need a nitroglycerin tablet for chest pain, Viagra is not safe for you. The same caution extends to recreational drugs called “poppers” (amyl nitrite), which are also nitrates.
What Viagra Does Not Do
Viagra is not an aphrodisiac. It doesn’t increase desire, trigger spontaneous erections, or change your level of arousal. It specifically addresses the mechanical side of erections: blood flow. If your erectile difficulty is entirely psychological, or related to low desire, the pill may not solve the underlying problem on its own.
It also doesn’t work instantly. Planning around the 30- to 60-minute onset window is part of using it effectively, and the timing shifts if you’ve recently eaten a large meal.
Priapism: A Rare but Serious Risk
In very rare cases, Viagra can cause an erection that won’t go away, a condition called priapism. If you have an erection lasting longer than four hours without sexual stimulation, this is a medical emergency. Prolonged erection cuts off fresh blood flow to penile tissue, and without treatment, it can cause permanent damage. This side effect is uncommon, but recognizing it matters because the window for effective treatment is narrow.