What Is Sildenafil? Dosage, Half-Life & How to Track It
Also known as: Viagra · Revatio
Sildenafil — sold as Viagra for erectile dysfunction and Revatio for pulmonary hypertension — is a PDE5 inhibitor that works by relaxing blood vessels, taken as needed roughly 30–60 minutes before it is needed, with a half-life of about 4 hours. It is a well-established, FDA-approved drug with a strong safety record within its guardrails, and the one guardrail that matters most is absolute: it should never be combined with nitrate medications. Educational information only, not medical advice.
What sildenafil is and how it works
Sildenafil inhibits an enzyme called phosphodiesterase type 5 (PDE5), which normally breaks down a signaling molecule (cyclic GMP) responsible for relaxing smooth muscle in blood vessel walls. By blocking that breakdown, sildenafil lets blood vessels — including those in the penis — relax and fill more readily in response to sexual stimulation, which is the entire mechanism behind its effect on erectile function.
It was FDA-approved in 1998 as Viagra for erectile dysfunction and separately as Revatio for pulmonary arterial hypertension, where the same vessel-relaxing mechanism lowers pressure in the lungs’ blood vessels rather than acting on erectile tissue specifically.
It requires sexual stimulation to produce an effect — it is not a spontaneous trigger — and it does not increase libido or testosterone; it addresses the vascular mechanics of an erection, not desire itself.
Typical dosing & half-life
Standard doses run 25–100 mg, usually taken 30–60 minutes before anticipated activity, with 50 mg being the typical starting point for most people. A high-fat meal can slow absorption and delay onset, which is worth knowing if timing matters.
At a roughly 4-hour half-life, its effect window is generally described as lasting 4–6 hours, though that varies by individual and dose.
| Dose | Typical onset | Effective window |
|---|---|---|
| 25 mg | ~30–60 min | ~4 hours |
| 50 mg (standard starting dose) | ~30–60 min | ~4–6 hours |
| 100 mg (max typical dose) | ~30–60 min | ~4–6 hours |
Individual response varies meaningfully — a prescriber typically starts at 50 mg and adjusts based on effect and tolerability, not the other way around.
The nitrate interaction — the one absolute rule
PDE5 inhibitors and nitrate medications (nitroglycerin and related drugs used for chest pain/angina, and some recreational nitrite "poppers") both lower blood pressure through overlapping pathways, and combining them can cause a sudden, severe drop in blood pressure that has been linked to fatal outcomes. This is not a minor interaction footnote — it is the single most important safety rule attached to this entire drug class.
It is also relevant to anyone using other vasodilating substances or with cardiovascular risk factors from AAS or TRT use — a resting blood pressure check and an honest conversation with a prescriber about anything else you are taking is the standard due diligence before using a PDE5 inhibitor at all.
What to monitor & the real risks
Common effects are generally mild — headache, flushing, nasal congestion, and mild indigestion are the most frequently reported. Sildenafil specifically is also associated with a temporary blue-tinged visual disturbance in a minority of users, a quirk of its slight cross-reactivity with a related enzyme in the retina.
Rare but serious risks include priapism (an erection lasting more than 4 hours, a medical emergency) and sudden vision or hearing loss, both of which warrant immediate medical attention rather than waiting them out. Blood pressure is worth knowing at baseline, particularly if it is being used alongside other cardiovascular-active medications or supplements.
How to track Sildenafil in Stackeddd
Log use against timing and dose so you know what actually worked, and keep a note of anything else cardiovascular-active you are taking on the same timeline — the nitrate interaction is the one thing worth never losing track of.
Model it yourself · free, no account
Blood-Level Simulator
Log your sildenafil dose and timing, and keep it on the same timeline as any other cardiovascular-active medication or supplement so a nitrate conflict is never a guessing game.
Related compounds
Related reading: TRT Math: what mg/mL actually means · Injection frequency: daily vs EOD vs weekly
Frequently asked questions
How long does sildenafil (Viagra) last?
Its effective window is typically 4–6 hours, matching its roughly 4-hour half-life, with onset around 30–60 minutes after taking it — longer if taken with a high-fat meal, which can delay absorption.
Can you take sildenafil with nitrate medications?
No. Combining PDE5 inhibitors like sildenafil with nitrates (used for angina/chest pain) or nitrite-based recreational drugs can cause a dangerous, potentially fatal drop in blood pressure. This is an absolute contraindication, not a dose-dependent caution.
Does sildenafil increase libido or testosterone?
No. It works on blood vessel mechanics to support an erection in response to stimulation — it does not affect desire or hormone levels. It requires sexual stimulation to have an effect.
What are the serious side effects of sildenafil?
Priapism (an erection lasting more than 4 hours) is a medical emergency requiring immediate care. Sudden vision or hearing loss are rare but reported and also warrant immediate medical attention. Common, non-emergency effects include headache, flushing, and nasal congestion.
This is educational information, not medical advice. Doses and protocols above reflect published references and community practice — protocol decisions belong with your prescribing physician. Generic names used throughout. How this reference is built & how the AI is tested →
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