Viagra is a prescription medication used to treat erectile dysfunction (ED). Its active ingredient, sildenafil citrate, belongs to a class of drugs called PDE5 inhibitors. First approved by the FDA in March 1998, it was the first medication of its kind and remains one of the most widely recognized prescription drugs in the world. It comes in tablet form in three strengths: 25 mg, 50 mg, and 100 mg.
How Viagra Works
An erection depends on blood flow. During sexual arousal, nerve endings and blood vessel walls in the penis release a signaling molecule called nitric oxide. This triggers a chain reaction inside smooth muscle cells that ultimately produces a substance called cGMP, which acts as the body’s internal switch for relaxing those muscles. When the muscles relax, blood vessels widen and blood flows into the spongy tissue of the penis, producing an erection.
The body also has a built-in off switch: an enzyme called PDE5 that breaks down cGMP. Sildenafil blocks that enzyme, allowing cGMP to build up for longer and in greater amounts. The result is stronger, more sustained blood flow during arousal. This is a critical point: Viagra doesn’t create arousal on its own. It amplifies the body’s natural response to sexual stimulation. Without that stimulation, the drug has no erection-producing effect.
Timing, Onset, and Duration
The standard recommendation is to take Viagra about one hour before sexual activity. It can start working in as little as 30 minutes, with peak blood levels typically reached between 30 and 120 minutes (the median is 60 minutes). The effects can last up to four hours, though some research suggests the window may extend well beyond that. One clinical study found no significant drop in effectiveness for up to 10 hours after taking it, with the highest success rates occurring between 1.5 and 2 hours.
Eating a heavy, high-fat meal before taking Viagra can delay how quickly the drug reaches peak levels in your bloodstream. The prescribing information notes reduced absorption after a fatty meal. That said, clinical trial data showed no meaningful loss of effectiveness when sildenafil was taken shortly before or with a meal, so the practical impact on most people is modest. Taking it on an empty stomach simply gets it working faster.
Dosage
For most people, the recommended starting dose is 50 mg. Based on how well it works and how well you tolerate it, a doctor may adjust the dose up to 100 mg or down to 25 mg. It’s taken as needed, not on a daily schedule, and the maximum is one dose per day.
Common Side Effects
About a third of users experience at least one side effect. In clinical practice, the most frequently reported are:
- Flushing (warmth or redness in the face and neck): roughly 31% of users
- Headache: about 25%
- Nasal congestion: around 19%
- Heartburn or indigestion: about 11%
These numbers, from an independent study (not funded by the manufacturer), are somewhat higher than what appeared in the original approval trials. The difference likely comes down to how side effects were recorded. Regardless, all of these effects tend to be mild and short-lived. Higher doses are associated with more frequent side effects.
Serious Risks
The most dangerous interaction is with nitrate medications, which are commonly prescribed for chest pain (angina). Both Viagra and nitrates work by increasing cGMP levels, so combining them can cause a dramatic, potentially life-threatening drop in blood pressure. In one controlled study, adding sildenafil to a nitrate produced a four-fold greater decrease in systolic blood pressure compared to taking the nitrate alone. This combination is strictly contraindicated, whether the nitrates are taken daily or only occasionally.
Rare but serious complications have also been reported after the drug reached the market. These include priapism (a painful erection lasting more than four hours that requires emergency treatment) and a type of sudden vision loss called NAION, which affects the optic nerve. Most people who experienced NAION already had underlying risk factors like diabetes, high blood pressure, heart disease, or a particular optic nerve anatomy. A direct causal link to sildenafil hasn’t been established, but anyone who experiences sudden vision or hearing loss while using the drug should stop taking it immediately.
Beyond Erectile Dysfunction
Sildenafil isn’t only used for ED. Under the brand name Revatio, it’s prescribed at a lower dose for pulmonary arterial hypertension, a condition where blood pressure in the arteries of the lungs is dangerously high. The same mechanism that relaxes blood vessels in the penis also relaxes blood vessels in the lungs, improving exercise capacity and slowing disease progression. It has been approved for this use in the United States since 2005.
Generic Availability and Cost
Generic sildenafil is widely available and significantly cheaper than brand-name Viagra. The FDA has approved generic versions that are bioequivalent to the original, meaning they contain the same active ingredient at the same strength and work the same way in the body. Prices for generic sildenafil tablets start around $4 to $7 depending on the strength and quantity.
One important caution: counterfeit versions of both Viagra and generic sildenafil are common through unregulated online pharmacies. These pills may contain incorrect doses, wrong ingredients, or harmful contaminants. Purchasing from a licensed, verified pharmacy is the only way to ensure you’re getting a legitimate product.