Important: this page is educational. Sildenafil is a prescription drug and requires a clinician evaluation, especially to screen for nitrate use and cardiovascular risk.
What is sildenafil?
Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor sold as Viagra. It was the first oral medication approved for erectile dysfunction in 1998 and remains the most prescribed PDE5 inhibitor worldwide. Generic sildenafil is widely available at low cost.
How does sildenafil work?
Sexual arousal triggers nitric oxide release in the penis, which raises cyclic GMP and relaxes smooth muscle in the corpus cavernosum, allowing the blood inflow that produces an erection. PDE5 breaks down cyclic GMP. Sildenafil inhibits PDE5, prolongs the signal, and supports a firmer and more reliable erection in response to arousal. Sexual stimulation is still required; the drug does not produce spontaneous erections.
How is sildenafil dosed?
Standard dosing ranges from 25 mg to 100 mg taken 30 to 60 minutes before sexual activity. Most men start at 50 mg and titrate based on response and tolerability. Maximum recommended dose is 100 mg once in 24 hours.
The half-life is roughly four hours, so the practical window of effect is four to six hours. Plan dosing around when intimacy is likely.
Food interaction
Sildenafil absorption is significantly slowed and reduced by fatty meals. A heavy steakhouse dinner can delay onset by an hour and lower peak effect. Take sildenafil on an empty stomach or after a light meal for the most predictable result.
How long until sildenafil works?
Onset is typically 30 to 60 minutes on an empty stomach, with peak effect around one hour. For most men the window of effect closes around four to six hours after the dose.
Side effects of sildenafil
Common side effects include headache, flushing, nasal congestion, indigestion, and visual changes (often a mild blue tint or increased light sensitivity). Most are mild and resolve as the dose clears.
Rare but serious effects include sudden vision loss (non-arteritic anterior ischemic optic neuropathy), sudden hearing loss, and priapism (erection lasting more than four hours). Any of these is a reason to seek emergency care.
Who should not take sildenafil?
Absolute contraindications include current or recent nitrate use, certain pulmonary hypertension regimens, and known severe hypersensitivity. Use caution with alpha-blockers, significant hepatic or renal impairment, recent stroke or myocardial infarction, unstable angina, severe heart failure, and uncontrolled blood pressure. Strong CYP3A4 inhibitors raise sildenafil exposure and may require dose adjustment.
If you are in your 30s and seeing new changes in erectile function, sildenafil is a tool but not a diagnosis. Read ED in your 30s for context on root causes worth addressing in parallel.
Sildenafil vs tadalafil
Tadalafil has a 17.5-hour half-life and a 36-hour effective window, which supports spontaneity and daily low-dose use. Sildenafil has a four-hour half-life and is taken before specific activity. Tadalafil is less affected by food. Side-effect profiles overlap; visual changes are slightly more common with sildenafil while back pain is more common with tadalafil. Either works for most men, and many cycle between them based on schedule.