Viagra (sildenafil) is a prescription medication that helps men get and maintain an erection by increasing blood flow to the penis. It does not cause arousal on its own. The drug works only when you’re already sexually stimulated, amplifying your body’s natural response to make erections firmer and easier to sustain.
How Viagra Works in the Body
When you become sexually aroused, your body releases a chemical signal that produces a molecule called cGMP in the blood vessels of the penis. This molecule relaxes the smooth muscle lining those vessels, allowing them to widen and fill with blood, which creates an erection. Normally, an enzyme called PDE5 breaks down cGMP fairly quickly, which is part of why erections naturally subside.
Viagra blocks PDE5. With that enzyme out of the way, cGMP accumulates and the blood vessels stay relaxed and open for longer. The result is a stronger, more sustained erection. This is why Viagra doesn’t work without sexual stimulation: your body needs to produce that initial chemical signal first. The drug simply prevents it from being broken down too fast.
How Well It Works
In a large pooled analysis of 48 placebo-controlled trials involving more than 11,000 men, about 80% of men under 65 reported improved erections with sildenafil compared to placebo. Effectiveness does decline somewhat with age: roughly 69% of men between 65 and 74 reported improvement, and about 59% of men 75 and older. Those are still substantial numbers, which is why the American Urological Association lists drugs in this class as a first-line treatment for erectile dysfunction.
Men with low testosterone may find Viagra less effective on its own. Clinical guidelines note that combining it with testosterone therapy can improve results in those cases.
Timing, Onset, and Duration
The standard approach is to take Viagra about one hour before sexual activity, though it can be taken anywhere from 30 minutes to 4 hours beforehand. Most men notice its effects within 30 to 60 minutes. The effects generally last several hours, giving a reasonably wide window.
A high-fat meal can delay absorption by about an hour. The drug still works, but it takes longer to kick in. If you’re planning to eat something heavy, consider taking Viagra an hour before the meal or about two hours after. Alcohol is also worth watching. Drinking around the same time you take Viagra can worsen side effects like flushing and low blood pressure, and heavy drinking can make erectile dysfunction worse on its own.
Common Side Effects
Side effects are generally mild and occur in more than 1 in 100 users. The most frequently reported ones include:
- Headache
- Facial flushing or hot flushes
- Indigestion
- Stuffy nose
- Dizziness
- Nausea
These side effects happen because Viagra doesn’t act exclusively on blood vessels in the penis. It has a mild relaxing effect on blood vessels throughout the body, which is why you might notice flushing or a drop in blood pressure. For most people, these effects are short-lived and manageable.
Dosage
The typical starting dose is 50 mg, taken as needed. Based on how well it works and how you tolerate it, your dose might be adjusted up to 100 mg or down to 25 mg. Adults over 65, people with liver problems like cirrhosis, or those with severe kidney impairment usually start at 25 mg because the drug clears from their bodies more slowly, leading to higher blood levels.
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 patches, sprays, and tablets you dissolve under your tongue. Both Viagra and nitrates relax blood vessels through overlapping pathways, and combining them can cause a sudden, dangerous drop in blood pressure. Studies have shown large, rapid blood pressure decreases in the majority of patients who took both together.
If you carry nitroglycerin tablets for occasional angina, you should not use them within 24 hours of taking Viagra. People with severe liver or kidney disease are generally advised against using the drug as well.
Rare but Serious Risks
Priapism, a painful erection lasting more than four hours, is a rare but well-known risk. It happens when blood becomes trapped in the penis and can’t drain. Left untreated, it can permanently damage erectile tissue. While the incidence is low, any erection that persists well beyond sexual activity and becomes painful needs urgent medical attention.
There have also been rare reports of sudden vision changes linked to a condition affecting blood flow to the optic nerve. This is uncommon, but any sudden loss of vision or significant change in eyesight after taking Viagra warrants immediate evaluation.
Viagra Beyond Erectile Dysfunction
Sildenafil is also approved under a different brand name, Revatio, to treat pulmonary arterial hypertension, a condition where blood pressure in the arteries of the lungs is dangerously high. The mechanism is the same: by relaxing blood vessel walls, the drug reduces the resistance the heart has to pump against. The dose for this condition is much lower (20 mg, taken three times daily) compared to the 50 to 100 mg used for erectile dysfunction, and the pill itself looks different: a small, round, white tablet rather than the familiar blue diamond.