Sildenafil citrate is a prescription medication used for two FDA-approved purposes: treating erectile dysfunction (ED) and treating pulmonary arterial hypertension (PAH), a serious condition involving high blood pressure in the lungs. It’s sold under the brand name Viagra for ED and Revatio for PAH. Though the active ingredient is the same, the doses and dosing schedules differ significantly between these two uses.
Erectile Dysfunction
Sildenafil’s most widely known use is for erectile dysfunction. It doesn’t cause an erection on its own. Instead, it makes the body’s natural arousal response work more effectively by relaxing the smooth muscle in blood vessel walls, allowing more blood to flow into the penis when you’re sexually stimulated.
The effects typically last several hours, giving a practical window for sexual activity. However, eating a high-fat meal around the time you take it can delay how quickly it kicks in by about an hour and reduce the peak amount of the drug in your bloodstream by roughly 29%. Taking it on an empty stomach or after a light meal produces faster, more reliable results.
Pulmonary Arterial Hypertension
The second approved use is for pulmonary arterial hypertension, a condition where the blood vessels in the lungs become narrowed and stiff, forcing the heart to work harder to pump blood through them. Over time, this can lead to heart failure. Sildenafil relaxes the smooth muscle in pulmonary blood vessels specifically, lowering resistance and easing the workload on the heart.
For PAH, sildenafil is taken three times a day (typically at 20 mg per dose, spaced 4 to 6 hours apart), rather than as needed. In clinical trials, patients taking sildenafil for PAH were able to walk 45 to 50 meters farther in a six-minute walk test compared to those on placebo after 12 weeks of treatment. That improvement in exercise capacity is meaningful for people whose daily activities are limited by breathlessness and fatigue.
How Sildenafil Works in the Body
Sildenafil belongs to a class of drugs called PDE5 inhibitors. To understand what that means in practical terms: when your body wants to relax a blood vessel, it produces a signaling molecule called cGMP. This molecule tells the smooth muscle cells lining the vessel walls to loosen up, widening the vessel and increasing blood flow. Normally, an enzyme called PDE5 breaks down cGMP fairly quickly, keeping the process in check.
Sildenafil blocks PDE5, so cGMP builds up and sticks around longer than it otherwise would. The result is stronger, more sustained relaxation of blood vessel walls. In the penis, that means better blood flow during arousal. In the lungs, it means lower pressure in the pulmonary arteries. The same basic mechanism drives both uses.
Off-Label Uses
Because sildenafil improves blood flow by relaxing blood vessels, doctors sometimes prescribe it off-label for conditions involving poor circulation. One of the more established off-label uses is for Raynaud’s phenomenon, a condition where small blood vessels in the fingers and toes spasm in response to cold or stress, cutting off blood flow and turning the digits white or blue. This is particularly common in people with systemic sclerosis, an autoimmune condition that can also cause painful digital ulcers from chronic poor circulation.
The UK’s National Institute for Health and Care Excellence (NICE) includes PDE5 inhibitors like sildenafil as a treatment option for people with Raynaud’s that hasn’t responded to first-line therapy, and for non-healing digital ulcers or critical loss of blood flow to the fingers. The evidence base is still built on small trials, but the clinical rationale is well supported by the drug’s mechanism.
Common Side Effects
Most side effects stem from the same blood vessel relaxation that makes sildenafil work. In clinical trials for ED, the three most frequently reported were headache (16% of users vs. 4% on placebo), flushing or warmth in the face and chest (10% vs. 1%), and indigestion (7% vs. 2%). These tend to be mild and temporary, resolving as the drug clears your system.
Some people notice a bluish tinge to their vision or increased sensitivity to light. This happens because sildenafil has a weak effect on a related enzyme (PDE6) found in the retina. It’s generally harmless and short-lived.
Rare but Serious Risks
There have been reports of a rare eye condition called nonarteritic anterior ischemic optic neuropathy (NAION), which involves sudden vision loss in one eye due to restricted blood flow to the optic nerve. Early concern about a possible link to sildenafil prompted a large-scale monitoring study. The observed rate among sildenafil users was about 2.8 per 100,000 patient-years, which falls within the expected background rate of 2.5 to 11.8 per 100,000 men per year aged 50 and older. No cases were reported across more than 13,000 patient-years of observation in Pfizer’s clinical trial database. Still, if you experience sudden vision loss in one eye, stop taking the medication and seek care immediately.
The Nitrate Interaction
The single most important safety rule with sildenafil is that it should never be combined with nitrate medications. Nitrates (commonly prescribed for chest pain or angina) work by increasing the same cGMP molecule that sildenafil protects from breakdown. One drug boosts cGMP production while the other prevents its destruction. Together, they can cause a severe, potentially dangerous drop in blood pressure, leading to fainting, dizziness, or in serious cases, a heart attack.
A single dose of sildenafil alone lowers blood pressure by a modest average of about 8/5 mmHg. That’s usually insignificant on its own, but layered on top of a nitrate, the combined effect can be far more pronounced. This interaction applies to all forms of nitrates, including nitroglycerin tablets, patches, sprays, and recreational use of amyl nitrite (“poppers”).
Viagra vs. Revatio: Same Drug, Different Uses
Because both brand names contain the same active ingredient, you should not take Viagra and Revatio together, or combine either with another PDE5 inhibitor. If you’re being treated for PAH with Revatio, taking Viagra on top of it for ED would effectively double your exposure to sildenafil, increasing the risk of side effects and dangerous blood pressure drops. Your prescriber needs to know about all PDE5 inhibitors you’re taking, regardless of the reason.