Sildenafil is taken as an oral tablet, swallowed with water, roughly one hour before sexual activity. The standard starting dose is 50 mg, and you can take it only once per day. Beyond that basic answer, timing, food, and a few safety details can make a real difference in how well it works.
When and How to Take It
Swallow the tablet whole with a glass of water. The ideal window is about one hour before you plan to have sex, but anywhere from 30 minutes to 4 hours beforehand works. The drug reaches its peak concentration in your blood at around 60 minutes on an empty stomach, though that can range from 30 to 120 minutes depending on the person.
Sildenafil does not cause an automatic erection. It works by relaxing blood vessels in the penis so that when you’re sexually stimulated, blood flow increases enough to produce and maintain an erection. Without arousal, the tablet won’t do much on its own.
How Food Affects Absorption
A heavy or high-fat meal eaten around the same time you take sildenafil slows it down noticeably. Research shows a high-fat meal delays the drug’s peak concentration by about one hour and reduces the amount that actually reaches your bloodstream by roughly 29%. That means if you take it right after a large steak dinner, it may feel weaker or take significantly longer to kick in.
A light meal or snack generally won’t cause much interference. If you know you’ll be eating a big meal, take the tablet well before you sit down, or wait until the meal has had time to digest.
How Long the Effects Last
A single dose lasts up to about 4 hours, though its effects are strongest in the first 2 hours. After that, the response gradually diminishes. The drug’s half-life is around 4 hours, meaning half of it has been cleared from your system by that point. You won’t feel an abrupt cutoff, just a tapering window of effectiveness.
Dose Adjustments
Most people start at 50 mg. Depending on how well that works and whether you experience side effects, your prescriber may move you up to 100 mg (the maximum single dose) or down to 25 mg. The ceiling is one dose per day, regardless of the strength.
A lower starting dose of 25 mg is typical in a few situations:
- Age over 65: The body clears the drug more slowly, so less is needed to achieve the same blood levels.
- Liver or severe kidney problems: These organs are responsible for processing and eliminating the drug, so impaired function means it stays in your system longer.
- Certain other medications: Some drugs, particularly antifungals and certain antibiotics, slow down the enzyme that breaks sildenafil apart, effectively making each dose stronger.
- Alpha-blockers: If you take an alpha-blocker for blood pressure or prostate symptoms, sildenafil should be started at 25 mg to reduce the risk of a sudden blood pressure drop.
The Nitrate Warning
Sildenafil must not be combined with nitrate medications. This is the single most important safety rule. Nitrates include nitroglycerin patches, nitroglycerin tablets placed under the tongue, and isosorbide pills prescribed for chest pain.
Both sildenafil and nitrates relax blood vessels, but through overlapping pathways. Sildenafil works by preventing the breakdown of a molecule called cGMP, which keeps blood vessels dilated. Nitrates boost production of the same molecule. Taken together, the combined effect can cause large, sudden drops in blood pressure, and in people with narrowed coronary arteries, it can also reduce blood flow to the heart. Studies have confirmed that these drops are not only dramatic but prolonged, lasting well beyond what either drug causes on its own. If you use any form of nitrate, sildenafil is off the table entirely.
Sildenafil for Pulmonary Hypertension
Sildenafil is also prescribed under a different brand name for pulmonary arterial hypertension, a condition involving high blood pressure in the arteries of the lungs. The dosing schedule is completely different: 5 mg or 20 mg taken three times daily, spaced 4 to 6 hours apart. Unlike the as-needed approach for erectile dysfunction, this is a daily regimen. If you’ve been prescribed sildenafil for this condition, follow that schedule consistently rather than the timing advice above.
Side Effects Worth Knowing About
Common side effects include headache, facial flushing, nasal congestion, and mild indigestion. These are related to the same blood-vessel relaxation that makes the drug work and tend to be mild and short-lived.
One side effect that requires immediate medical attention: an erection lasting longer than 4 hours. This is called priapism, and while it’s rare, it can permanently damage penile tissue if not treated. If an erection becomes painful or simply won’t subside after 4 hours, that’s an emergency room visit, not a wait-and-see situation.
Sudden changes in vision or hearing, though uncommon, also warrant prompt medical attention. Some people notice a temporary blue-green tint to their vision at higher doses; this is a known effect of the drug on a related enzyme in the retina and typically resolves on its own.