The standard starting dose of Viagra (sildenafil) is 50 mg, taken about one hour before sexual activity. It comes in three strengths: 25 mg, 50 mg, and 100 mg. Most prescribers begin at 50 mg and adjust up or down based on how well it works and how you tolerate it. You should never take more than one dose in a 24-hour period.
Starting Dose by Age
For adults under 65, the recommended starting dose is 50 mg. For adults 65 and older, the recommended starting dose is lower, at 25 mg. The reason for the difference is straightforward: older adults clear the drug from their bodies more slowly. In clinical testing, healthy adults over 65 had roughly 84% higher blood levels of sildenafil compared to younger adults given the same dose. That means the same pill hits harder and lasts longer, which increases the chance of side effects.
People with significant liver problems or severe kidney impairment also typically start at 25 mg for similar reasons. The body can’t process the drug as efficiently, so a lower dose produces the same effective concentration.
How the Dose Gets Adjusted
If 50 mg works well without bothersome side effects, there’s no reason to change anything. If it’s not effective enough, your prescriber may increase the dose to 100 mg, which is the maximum. If side effects like headache, facial flushing, or nasal congestion are a problem, the dose can be lowered to 25 mg.
The most common side effects (headache, flushing, indigestion, nasal stuffiness, and visual changes like a blue tint) tend to be dose-related. A higher dose generally means a greater chance of experiencing them. Starting at the lowest effective dose keeps these to a minimum.
When and How to Take It
Take it roughly one hour before you expect to need it. The drug typically starts working within 30 to 60 minutes and remains active for several hours, though individual experiences vary. You don’t take it on a daily schedule for erectile dysfunction; you take it only when needed.
What you eat beforehand matters. A heavy, high-fat meal can delay absorption by about an hour. The drug still works, but the onset is slower. If timing is important, taking it on a lighter stomach gives you a more predictable window.
Who Should Not Take It
The most critical safety issue with Viagra is its interaction with nitrate medications. Nitrates are commonly prescribed for chest pain and include nitroglycerin patches, nitroglycerin tablets placed under the tongue, and isosorbide. Both Viagra and nitrates lower blood pressure through a related mechanism. When combined, they can cause a sudden, dangerous drop in blood pressure. This is not a minor interaction or a theoretical risk. Clinical studies showed large, abrupt blood pressure drops in most patients who took the two together. If you use any form of nitrate medication, sildenafil is not safe for you.
Alpha-blockers, often prescribed for an enlarged prostate, can also interact with sildenafil and cause low blood pressure. If you take one, your prescriber may start you at a lower dose and advise spacing the two medications apart.
Available Forms
Sildenafil for erectile dysfunction is available as a standard tablet and as an oral dissolving film, both in the same strength options: 25 mg, 50 mg, and 100 mg. The branded Viagra and generic sildenafil contain the same active ingredient at the same doses. The generic version became widely available after the patent expired and is significantly less expensive.
Sildenafil is also prescribed at much lower doses (20 mg, taken three times daily) for pulmonary arterial hypertension, a condition involving high blood pressure in the lungs. That’s a completely different use with a different dosing schedule, so the two should not be confused.