Sildenafil works by blocking an enzyme that breaks down a chemical signal responsible for relaxing blood vessels. When that signal stays active longer, blood flow increases to specific areas of the body, most notably the penis during sexual arousal. The drug doesn’t create arousal on its own. It amplifies the body’s existing response to sexual stimulation.
The Role of Nitric Oxide
To understand sildenafil, you first need to understand what happens during a normal erection. When you become sexually aroused, nerve endings and blood vessel walls in the penis release a molecule called nitric oxide (NO). This molecule triggers a chain reaction inside the smooth muscle cells that line penile blood vessels: it activates an enzyme that produces a second messenger molecule called cGMP.
cGMP is the key player. It lowers calcium levels inside smooth muscle cells, which causes those cells to relax. When the smooth muscle in the walls of penile arteries relaxes, the arteries widen and blood rushes into the spongy tissue of the penis, producing an erection. The entire process depends on nitric oxide showing up first, which is why sexual stimulation is required for the drug to work at all.
What Sildenafil Actually Blocks
Your body doesn’t let cGMP accumulate indefinitely. An enzyme called PDE5 constantly breaks it down, which is how an erection naturally subsides. In men with erectile dysfunction, the balance tips too far toward breakdown. Either less nitric oxide gets released, less cGMP gets produced, or PDE5 clears it too quickly for blood flow to build up enough.
Sildenafil selectively inhibits PDE5. By slowing the destruction of cGMP, the drug lets even modest amounts of nitric oxide produce a stronger, longer-lasting relaxation of penile blood vessels. Research published in The Journal of Urology found that sildenafil has very little relaxing effect on penile tissue when nitric oxide is absent, but significantly enhances relaxation when even small amounts of nitric oxide are present. It’s a booster, not an initiator.
Timing, Onset, and Duration
Sildenafil reaches peak levels in your blood within 30 to 120 minutes after you take it on an empty stomach, with a median of about 60 minutes. Most people notice effects beginning around the 30-minute mark. The drug can remain effective for up to 4 hours, though the response at the 4-hour point is noticeably weaker than at 2 hours.
Eating a high-fat meal before taking sildenafil delays absorption by roughly an hour and reduces the peak concentration in your blood by about 29%. Overall drug exposure drops by around 11%. This doesn’t mean it won’t work after a heavy dinner, but the effect will be slower to start and potentially weaker. Taking it on an empty stomach or after a light meal gives you the most predictable results.
Typical Dosing for Erectile Dysfunction
The standard starting dose is 50 mg, taken as needed before sexual activity. Depending on how well it works and how you tolerate it, the dose can be adjusted down to 25 mg or up to 100 mg. You should not take it more than once per day. The tablets come in 25 mg, 50 mg, and 100 mg strengths.
Why It Also Treats Pulmonary Hypertension
PDE5 isn’t only found in the penis. It’s also abundant in the smooth muscle lining the blood vessels of the lungs. In people with pulmonary arterial hypertension, those vessels are abnormally constricted, forcing the heart to work harder to push blood through the lungs. By blocking PDE5 in the pulmonary vasculature, sildenafil relaxes those vessels and lowers the resistance the heart pumps against.
For this use, the drug is sold under a different brand name (Revatio) at a much lower dose: 20 mg taken three times daily, spaced 4 to 6 hours apart. Clinical trials found no additional benefit from higher doses. The mechanism is identical to what happens in the penis: more cGMP, more smooth muscle relaxation, better blood flow. The difference is simply which blood vessels benefit most.
Why Nitrate Medications Are Dangerous to Combine
Nitrate drugs, like nitroglycerin patches or isosorbide tablets prescribed for chest pain, work by releasing nitric oxide directly into blood vessel walls throughout the body. Sildenafil prevents the breakdown of the cGMP that nitric oxide produces. Combine the two and you get a massive, system-wide surge of cGMP that relaxes blood vessels far beyond what either drug would do alone.
The result is a severe, potentially fatal drop in blood pressure. Research published in Circulation described a dangerous feedback loop: blood pressure falls so low that the heart itself loses adequate blood supply, which weakens its pumping ability, which drops blood pressure even further. This interaction is not dose-dependent in a predictable way, meaning even small overlaps between the two drugs can be hazardous. This is the single most important safety concern with sildenafil and the reason you’ll always be asked about nitrate use before getting a prescription.
Common Side Effects
Because PDE5 exists in blood vessels beyond the penis and lungs, sildenafil can cause mild vascular side effects. The most frequently reported include headache, facial flushing, nasal congestion, and indigestion. These all stem from the same mechanism: blood vessel dilation in areas you didn’t necessarily want dilated. Some people also experience temporary visual changes, like a mild blue tint to vision, because a related enzyme (PDE6) in the retina gets partially affected.
These side effects are generally mild and short-lived, resolving as the drug clears your system over a few hours. They tend to be more noticeable at higher doses, which is one reason starting at 50 mg and adjusting from there is the standard approach.