Sildenafil comes in three strengths, 25mg, 50mg and sildenafil 100mg; most men start at 50mg taken about an hour before sex, and move up to 100mg or down to 25mg only after reviewing the response with a prescriber. The maximum is one dose in any 24 hours, and 100mg is the ceiling.
The internet is full of confident nonsense about choosing between the strengths. The truthful version is more useful: dosing follows a simple clinical logic, the differences between strengths are smaller than people assume, and the questions that actually determine your dose, your health, your other medicines, your response, belong to a prescriber rather than a forum. This guide explains the logic so the conversation with your clinician makes sense, not so you can improvise around it.
What is sildenafil?
Sildenafil is a PDE5 inhibitor, a prescription medicine that improves blood flow to the penis in response to sexual arousal; it does not create an erection on its own or raise desire. It is the most prescribed erectile dysfunction tablet in the UK, and the NHS medicines guidance covers who can and cannot take it. For the mechanism in detail, see our guide to how sildenafil works.
Why sildenafil 50mg is the standard start
For most men, prescribing starts at 50mg, taken as needed around an hour before sex, at most once in any 24-hour period. That starting point is the balance the trial evidence in the official prescribing information supports: at 50mg a clear majority of men with erectile dysfunction get erections sufficient for sex, with side effects, headache, flushing, congestion, indigestion, that most find tolerable. From there the dose adjusts on results. Men who respond well but experience bothersome side effects may drop to 25mg, which retains much of the effect with a gentler profile. Men who tolerate 50mg well but get an insufficient response may step up to 100mg. The 25mg strength is also the routine starting choice for older men and for anyone whose kidneys, liver or other medicines slow sildenafil's clearance, because their blood levels run higher on any given dose.
Is it OK to take 100mg of sildenafil?
Yes, if your prescriber has moved you to it: the step from 50mg to sildenafil 100mg helps a meaningful minority of men whose response was partial, and it is a legitimate, licensed adjustment. What it is not is twice as good. Trial data shows the effectiveness gain from doubling the dose is modest, while side-effect rates climb more steeply: more headaches, more flushing, more visual disturbance, more indigestion. This asymmetry is the core fact of sildenafil dosage, and it is why the sensible sequence is always to optimise how you use the current dose before escalating it. It is also why taking more than 100mg, or taking a second tablet in the same day because the first disappointed, is genuinely dangerous rather than merely unofficial: blood pressure effects, priapism risk and visual side effects all scale with dose, and 200mg is not a licensed dose in any circumstances. If 100mg used correctly is not working, the answer is a different treatment conversation, not a bigger number.
Before blaming the dose, check the usage
A large share of apparent dose failures are usage failures, and they respond to information rather than escalation. Sildenafil taken immediately after a large or fatty meal absorbs slowly and can appear weak; on an empty or light stomach it works noticeably faster. It needs sexual stimulation to do anything at all, a fact that surprises more men than prescribers expect: the tablet enables the response, it does not create it. Timing matters: an hour before is the standard guidance, and effectiveness persists for around four hours, as we explain in how long sildenafil lasts, so both impatience and excessive advance planning can sabotage it. Alcohol beyond a drink or two works directly against the erection you are trying to have. And crucially, the trials judged response over several attempts: clinicians generally advise trying a dose properly on multiple occasions before concluding it has failed, because first-attempt anxiety alone sinks plenty of them.
Who needs extra caution, whatever the dose
Some situations change the dosing conversation entirely. Nitrate medicines for angina and chest pain make sildenafil unsafe at any strength, full stop, and the same applies to recreational nitrites. Alpha-blockers for blood pressure or prostate symptoms need careful timing and usually a low sildenafil start. Significant heart disease needs a clinician's judgement about whether sexual activity itself is safe before any tablet enters the picture. Certain antifungals, antibiotics and HIV medicines raise sildenafil levels enough that lower doses are needed. And a handful of conditions, severe liver problems, recent stroke or heart attack, certain rare eye diseases, take sildenafil off the table altogether. None of this is small print; it is the actual reason the medicine is prescription-only and why honest medical questionnaires at a regulated pharmacy protect you rather than inconvenience you. The common and rare reactions are covered in our guide to sildenafil and tadalafil side effects.
Getting the dose changed properly
If your current dose disappoints or its side effects grate, the route is a review, and it is usually quick: describe what happened over several attempts, what you took, when, with what food and alcohol, and how the side effects felt. Prescribers adjust on exactly that information, upwards for insufficient response with good tolerance, downwards for good response with rough edges, or sideways to a different medicine such as tadalafil when sildenafil's timing profile does not fit your life; our sildenafil versus tadalafil comparison covers that choice. What never helps is the improvised route: doubling tablets, stacking doses, or topping up prescribed sildenafil with unregulated online product whose actual content is anyone's guess. The three strengths exist precisely so adjustment can be done safely; use the system, it is fast and it works.
The sildenafil dosage story in one paragraph: start where the evidence starts, usually 50mg; fix usage before strength; move to 100mg or down to 25mg only through your prescriber; respect the one-dose, 24-hour, no-nitrates rules absolutely; and treat a failed 100mg as a signal to reassess, not exceed. Erectile dysfunction has more treatment routes than most men realise, and the dose ladder is only the first of them.
