Sildenafil citrate is a prescription medication that relaxes blood vessels, most widely known under the brand name Viagra for treating erectile dysfunction (ED). It is also sold as Revatio for a separate condition: pulmonary arterial hypertension, a type of high blood pressure in the lungs. Both brands contain the same active ingredient at different doses.
How Sildenafil Works
Your body naturally produces a chemical messenger called cGMP that tells smooth muscle cells in blood vessel walls to relax. When those muscles relax, the vessels widen and more blood flows through. An enzyme called PDE5 breaks down cGMP, which is a normal part of the cycle. Sildenafil blocks that enzyme, so cGMP levels stay elevated longer and blood vessels remain relaxed.
In the context of erectile dysfunction, this means more blood can flow into the penis when a person is sexually aroused. Sildenafil does not create arousal on its own; it amplifies the body’s existing response. In pulmonary arterial hypertension, the same vessel-relaxing effect reduces pressure in the arteries of the lungs, making it easier for the heart to pump blood through them.
An Accidental Discovery
Sildenafil was never intended to treat erectile dysfunction. In 1986, researchers at Pfizer identified the compound while searching for a drug to treat angina, a form of chest pain caused by reduced blood flow to the heart. During clinical trials, the drug showed limited benefit for angina but participants reported an unexpected side effect: improved erections. Pfizer pivoted, and the FDA approved Viagra for erectile dysfunction in March 1998. It later received a second approval as Revatio for pulmonary arterial hypertension.
FDA-Approved Uses
Sildenafil has two distinct approved indications. As Viagra, it treats erectile dysfunction in adult men. As Revatio, it treats pulmonary arterial hypertension in adults to improve exercise ability and delay clinical worsening. The two products are not interchangeable because the doses differ significantly, and taking both at the same time is not recommended.
Dosing and How to Take It
For erectile dysfunction, the standard starting dose is 50 mg taken roughly one hour before sexual activity, though it can be taken anywhere from 30 minutes to 4 hours beforehand. Adults 65 and older typically start at 25 mg. Either way, the maximum is one dose per day. A doctor may adjust the amount up or down based on how well it works and how well it’s tolerated.
For pulmonary arterial hypertension, the dosing schedule is completely different: 20 mg three times a day, spaced 4 to 6 hours apart. This lower, more frequent dosing keeps a steady level of the drug in the bloodstream to continuously reduce lung artery pressure.
Onset and Duration
Sildenafil is absorbed quickly. Blood levels peak within about an hour of swallowing a tablet, though some people reach peak levels in as little as 30 minutes. The drug’s half-life is 3 to 5 hours, meaning its effects gradually taper over that window. Most people find the practical effect on erections lasts roughly 4 to 6 hours, though this varies with dose, metabolism, and whether the drug was taken with a heavy meal (which can slow absorption).
Common Side Effects
Side effects are generally mild and short-lived. In one clinical practice study, about 32% of patients experienced at least one adverse event, but none dropped out because of severity. The most frequently reported effects were:
- Flushing: 30.8% of patients
- Headache: 25.4%
- Nasal congestion: 18.7%
- Heartburn: 10.5%
Higher doses are linked to a higher chance of side effects. Some people also report temporary changes in color vision, particularly a blue-green tint, because a related enzyme in the retina is mildly affected by the drug. These visual changes are uncommon at standard doses and resolve on their own.
The Nitrate Interaction
The single most important safety concern with sildenafil is its interaction with nitrate medications, which are commonly prescribed for chest pain. Nitrates work by releasing nitric oxide, which also increases cGMP, the same chemical messenger that sildenafil preserves. Combining the two creates a double effect on blood vessel relaxation that can cause a dangerous drop in blood pressure.
In clinical testing, taking sildenafil with a common nitrate drug roughly doubled the blood pressure drop compared to taking the nitrate alone, with standing systolic pressure falling by an average of 52 points versus 25 points on nitrate alone. This is large enough to cause fainting, dizziness, or in serious cases, a cardiovascular event. For this reason, sildenafil is strictly contraindicated with any form of nitrate, including nitroglycerin tablets, patches, sprays, and recreational nitrite “poppers.”
Who Should Be Cautious
Beyond the nitrate restriction, sildenafil requires caution in people with certain cardiovascular conditions, liver or kidney problems, or conditions that make them more susceptible to the effects of blood vessel dilation. People taking alpha-blockers for prostate enlargement or high blood pressure should also be aware that combining them with sildenafil can lower blood pressure further.
Sildenafil is not appropriate for everyone with erectile dysfunction. Because erection problems can be an early sign of cardiovascular disease, the underlying cause matters. A prescriber will typically assess cardiovascular fitness before writing a prescription, not just to ensure sildenafil is safe, but to make sure a more serious condition isn’t being overlooked.