Sildenafil is a medication with two FDA-approved uses: treating erectile dysfunction (sold as Viagra) and treating pulmonary arterial hypertension, a serious lung condition (sold as Revatio). Both uses rely on the same basic mechanism: relaxing blood vessels to improve blood flow. Doctors also prescribe it off-label for a handful of other circulatory problems.
How Sildenafil Works in the Body
Your blood vessels relax and widen when levels of a signaling molecule called cGMP rise inside smooth muscle cells. Sildenafil blocks the enzyme that breaks cGMP down, so more of it stays active. The result is wider blood vessels and better blood flow to whichever tissue has the highest concentration of that enzyme. Two areas stand out: the blood vessels in the lungs and the tissue of the penis. That’s why the same drug treats two very different conditions.
Erectile Dysfunction
This is the use most people know. An erection depends on blood flowing into the penis and staying there, and sildenafil makes that process more reliable when sexual arousal is already present. It doesn’t create arousal on its own.
In a large meta-analysis pooling 14 clinical trials, 57% of sexual intercourse attempts were successful with sildenafil compared to 21% with a placebo. Looking at it another way, 83% of men taking the drug had at least one successful attempt during the study period, versus 45% on placebo. Available doses are 25 mg, 50 mg, and 100 mg, and most prescribers start in the middle of that range.
Sildenafil reaches its peak concentration in about an hour on an empty stomach, though the window ranges from 30 minutes to two hours. A high-fat meal slows things down considerably, delaying the peak by roughly an hour and reducing the amount your body absorbs by about 29%. The effect lasts up to four hours, though it’s strongest around the two-hour mark.
Pulmonary Arterial Hypertension
Pulmonary arterial hypertension (PAH) is a condition where the blood vessels in the lungs become abnormally narrow, forcing the heart to work much harder to push blood through them. Left untreated, it leads to heart failure. Sildenafil, marketed as Revatio for this purpose, is FDA-approved to improve exercise ability and delay clinical worsening in adults with PAH.
The dosing schedule looks completely different from the erectile dysfunction version. Instead of taking a single tablet before sex, people with PAH take a smaller dose three times a day, spaced four to six hours apart. It’s a daily, ongoing medication rather than an as-needed one.
Off-Label Uses
Because sildenafil improves blood flow by relaxing blood vessels, doctors sometimes prescribe it for circulatory problems beyond its two approved indications.
One well-studied example is Raynaud’s phenomenon, a condition where small blood vessels in the fingers and toes spasm in response to cold or stress, cutting off circulation and causing painful, discolored digits. In a controlled crossover trial of patients whose Raynaud’s hadn’t responded to other treatments, sildenafil cut the frequency of attacks by about a third and nearly halved the total time spent in attacks. Capillary blood flow in the fingers more than quadrupled. Among six patients with chronic digital ulcers (open sores caused by poor circulation), all showed visible healing, and two saw their ulcers disappear entirely.
There is also some evidence that sildenafil can help with exercise capacity at high altitude by counteracting the blood vessel constriction that occurs in low-oxygen environments, though this use has less robust clinical data behind it.
Common Side Effects
Most side effects stem from the same blood-vessel-widening action that makes the drug work. Headache and facial flushing are the most frequently reported. Other common effects include nasal congestion or a runny nose, indigestion, muscle aches, and skin redness or warmth. These tend to be mild and resolve within a few hours as the drug wears off.
Serious Risks and Drug Interactions
The most critical safety concern involves nitrate medications, which are commonly prescribed for chest pain (angina). Nitroglycerin tablets, sprays, and patches all fall into this category. Combining sildenafil with any form of nitrate can cause a sudden, dangerous drop in blood pressure that may lead to fainting, heart attack, or stroke. This combination is strictly contraindicated. Even after taking sildenafil, it’s unclear exactly how long you need to wait before a nitrate can be used safely, since the drug’s blood-pressure-lowering effects can persist beyond the point where it’s otherwise noticeable.
People with already low blood pressure (below 90/50), significant dehydration, or certain structural heart conditions face additional risk from sildenafil’s blood-pressure-lowering properties.
Two rare but serious effects deserve mention. Cases of sudden vision loss from a condition called non-arteritic anterior ischemic optic neuropathy have been reported in people taking sildenafil, though a large study found the incidence rate was no higher than in the general population of men over 50. Sudden hearing loss, sometimes accompanied by ringing in the ears or dizziness, has also been reported. Either of these warrants immediate medical attention.