Sildenafil

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Also seen as: Viagra

The Dose Guy’s verdict

Sep 1, 2026
3 sources

The first PDE5 inhibitor, still the strongest single-dose hit in the class, and backed by more human data than almost any compound on this site. The trade-off versus tadalafil is simple: sildenafil hits harder and faster, but it wants an empty stomach to work right and it is gone in a few hours, so you are always timing it. When you want the sharpest possible peak for a specific window, sildenafil. When you want to stop thinking about it, tadalafil.

Would I take it?Yes, the sharper tool.

What it is

Sildenafil is a PDE5 inhibitor. During arousal, nitric oxide signals the penile tissue to produce cGMP, which relaxes smooth muscle and lets blood flow in. PDE5 is the enzyme that clears cGMP out. Block it, and the erection response to stimulation is stronger and lasts longer. It still needs arousal to start the process; it amplifies the signal, it does not create one.

The mechanism is the same as tadalafil, but the pharmacokinetics split them. Sildenafil peaks fast, hits hard, and clears in a few hours. That makes it the better acute tool, but it also means every dose is an event you plan around: take it on an empty stomach, wait half an hour, and work within the window. Tadalafil’s long duration lets you skip that ritual with a daily dose.

What the research shows

The most-studied erection drug in history. The original trial data is from the late 1990s and it has only gotten deeper since:

In the flexible-dose trial, 69% of intercourse attempts succeeded with sildenafil versus 22% with placebo, and men averaged 5.9 successful attempts per month versus 1.5.NEJM 1998 · pivotal RCTs · n=861

Across 83 double-blind trials, sildenafil 100 mg had an odds ratio of 9.06 versus placebo for satisfactory erectile function.J Sex Med 2026 · network meta-analysis · 83 RCTs · n=6029

Food delayed peak concentration by about one hour and cut it by 29%. That is why the empty-stomach rule matters: a full meal does not just slow sildenafil, it meaningfully blunts the peak.Br J Clin Pharmacol 2002 · PK crossover

Realistic protocols

Take it on an empty stomach, 30 to 60 minutes before you need it. That is the core of the protocol and the part that matters most. Food, especially anything fatty, delays and weakens the peak noticeably, which is the single biggest practical difference from tadalafil. If dinner is part of the evening, either take it well before the meal or switch to tadalafil for that night.

Most people start at 50 mg and adjust from there. If 50 does the job, stay at 50. If it is not enough, 100 mg is the clinical ceiling and where the network meta-analysis numbers peak. Going above that does not add much function and adds more flushing and headache. Going below, 25 mg works for plenty of people, especially younger ones or anyone stacking it with something else. The window is roughly four to five hours from the time it kicks in, with the strongest effect in the first two.

The gym-pump use is real, same PDE5 vasodilation in different vascular beds. People run 25 to 50 mg pre-workout for the fullness. It works, and the short duration means it is in and out without lingering effects.

Same absolute rule as every PDE5: never with nitrates. Nitroglycerin, isosorbide, poppers. The combined vasodilation drops blood pressure to dangerous levels. The interaction window is shorter than tadalafil’s because sildenafil clears faster, but the danger during it is identical.

Standard50 mgEmpty stomach, 30–60 min before, the starting point for most people.

Full dose100 mgClinical ceiling. More sides, only if 50 is not enough.

Low dose25 mgEnough for many, especially younger users or when stacking.

Gym pump25–50 mgPre-workout, same vasodilation in different tissue.

Side effects

Well-known, dose-related, and generally mild at standard doses:

commonHeadache, flushing, and dyspepsia. The PDE5 basics. Dose-related and usually tolerable, but headache can be sharp at 100 mg.

commonA blue-tinged tint to vision at higher doses, from weak PDE6 cross-reactivity. Harmless and temporary, but alarming the first time.

monitorNitrate interaction. The blood-pressure crash with nitroglycerin, isosorbide, or poppers is the same kill risk as with tadalafil. Non-negotiable, every time.

monitorNasal congestion and occasionally a drop in standing blood pressure, especially if you are on an antihypertensive. Get up slowly after lying down.