Fifty milligrams an hour ahead, and what the hour is built on

One hour ahead: what the instruction is actually built on

The dose is 50 mg. The variable that decides the evening is the gap between swallowing and trying.

The hour is not a ritual. Sildenafil is absorbed quickly, and maximum plasma concentration on an empty stomach arrives between 30 and 120 minutes after the tablet, with a median of 60. Aim at the median and the attempt lands near the top of the curve. Aim ten minutes ahead and it lands on the way up.

Almost nobody uses the hour. The tablet goes in once the evening is already moving, which is the single most common correctable fault in this prescription and the one least often discussed in the room.

Two more numbers do most of the remaining work. Terminal half-life is about 4 hours for sildenafil and for its active N-desmethyl metabolite, which sets the far edge of the useful window. Absolute bioavailability averages 41 percent with an individual range from 25 to 63 percent, which is why the same 50 mg tablet behaves noticeably differently in two men of the same build.

That spread is worth holding on to. It means a report of nothing happening is not automatically a report about the dose.

Zero without arousal: the part of the instruction that has no clock

The commonest reason a correctly timed tablet does nothing has nothing to do with pharmacology.

Sildenafil raises cyclic GMP in tissue that is already receiving a signal. Without sexual stimulation there is no signal to protect and no erection, at any dose and at any point in the window. Every trial in the programme was run with stimulation as a condition of the design.

This produces the second big category of reported failure, and it has a recognisable shape. A man takes the tablet, then starts watching himself for evidence that it is working. Attention goes to monitoring instead of to arousal, and the thing being monitored does not happen. He concludes that 50 mg was not enough and asks for 100.

Nothing about the dose fixes that loop. What sometimes does is dosing far enough ahead that the tablet is out of mind by the time anything begins, which is a second and better argument for the full hour.

When the timing was right and it still failed

Even in the best-responding group in the trials, three attempts in ten came to nothing.

Expectations are usually set from the global improvement figures, and those are the friendliest numbers in the file. The diary data is more sobering and more useful. In the flexible-dose study in 268 men with diabetes, 48 percent of intercourse attempts succeeded on sildenafil against 12 percent on placebo. In the psychogenic subgroup, the best-responding population in the programme, the figures were 70 percent against 29.

Seventy percent is the ceiling of realistic expectation, in the easiest group, on a drug that is working. Roughly three attempts in ten still fail. A man told the tablet works 82 percent of the time hears always, and one poor evening then reads as proof that it has stopped working.

Before changing anything, four things are worth ruling out: the tablet went in too late, a heavy meal moved the curve, alcohol was doing its own work, or a new medicine has been added since the last successful attempt. Those four cover most of what walks back through the door.

What is left after that is genuine non-response, and it is a different conversation. Adequate attempts at 100 mg with arousal present and nothing on the list to explain it is the point at which the underlying cause deserves more attention than the prescription does. Where that leads is set out in why erections fail.

30 minutes to 4 hours: what each end of the window costs

Both ends of the licensed range are real. Neither is where the tablet works best.

At the early end there is drug on board but the level is still rising, and the man who takes a tablet as clothes come off is testing the front third of the curve. At the far end the effect is measurably present and clearly weaker. In the plethysmography work, sildenafil was still producing an erection in response to sexual stimulation 4 to 5 hours after the dose, which is where the four-hour figure in the label comes from rather than from any calculation.

The reliable middle of that range is roughly 45 to 90 minutes, and that is where the advice about an hour sits.

One asymmetry is worth acting on. If the evening cannot be predicted to the half hour, aim early rather than late. There is documented drug activity four hours in; fifteen minutes in, most men in the onset study had not yet reached the rigidity threshold it was measuring. Being an hour early costs a slightly lower peak at the moment of the attempt. Being twenty minutes late costs the attempt.

The window is also not an invitation to spread three attempts across an evening on one tablet and judge the drug by the last of them.

25 mg first: the situations that lower the opening dose

A lower starting dose is a decision with reasons behind it, not a gentler way to begin.

Fifty milligrams is the usual starting dose. Every group listed above starts lower for the same underlying reason: the same tablet produces higher plasma levels in them than it does in a healthy man in his forties.

The exposure differences behind those lines are not small. In healthy men aged 65 and over, plasma exposure to sildenafil was about 84 percent higher than in men aged 18 to 45, and free unbound exposure about 45 percent higher. In cirrhosis, exposure rose 85 percent and peak concentration 47 percent. Erythromycin raised peak level 160 percent and exposure 182 percent; saquinavir raised them 140 and 210 percent.

The mirror-image mistake is worth naming, because it happens more often than overdosing. Starting a healthy 45-year-old at 25 mg for no stated reason, then reading the poor result as drug failure, moves men into second-line treatment they do not need. Across four fixed-dose trials in 1797 men, improvement was reported by 63 percent on 25 mg, 74 percent on 50 mg and 82 percent on 100 mg, against 24 percent on placebo. There is a real gap between 25 and 50.

48 hours: the interaction that changes the interval rather than the dose

Most interactions here move the milligrams. One moves the calendar.

Ritonavir raises sildenafil exposure eleven-fold and peak concentration four-fold. The scale of it is easier to grasp from the tail: 24 hours after a 100 mg dose, plasma sildenafil was still around 200 ng/mL with ritonavir on board, against roughly 5 ng/mL when the same dose was given alone. Hence the instruction, which is a single 25 mg dose at most in any 48-hour period.

That converts an on-demand drug into a planned one, and it is the only place in this label where the clock stops being about tonight.

Induction runs the other way and is easier to miss because nothing dramatic happens. Bosentan cut sildenafil exposure by 63 percent and peak level by 55 percent; rifampin is expected to do more. A man on an enzyme inducer who reports no effect may be reporting a drug level rather than a diagnosis, and the label offers no upward adjustment to compensate.

Nitrates sit outside all of this as an absolute bar rather than a dosing problem, along with riociguat and recreational nitrites. There is no interval and no dose that makes that combination workable.

Minus 29 percent: what a high-fat meal takes off the peak

Food does not stop this drug working. It moves the whole curve to the right and flattens it.

Taken after a high-fat meal, the rate of absorption falls: mean time to peak is delayed by 60 minutes and mean peak concentration drops 29 percent. The label documents the delay and the lower peak. It puts no figure on total exposure, so the honest reading is that the shape of the curve changes rather than that the drug is lost.

The patient section of the label is unusually concrete about what counts, giving a cheeseburger and fries as the example. That is the useful test. A steak with a cream sauce qualifies. A salad does not.

The failure mode is easy to picture and common. Heavy restaurant dinner, tablet swallowed with the dessert, attempt forty minutes later. The man has combined a delayed peak with an early attempt and taken both penalties at once, and he reports that the drug does not work on him.

Two ways out, and only one of them involves eating differently. Take the tablet before the meal rather than after it, or accept the delay and push the attempt later. Antacids are not part of the problem: a single dose of magnesium and aluminium hydroxide did not affect bioavailability at all.

25 minutes: the fastest honest onset figure, and its spread

Where the onset claims come from, and what the study actually measured.

Two studies were built specifically to find the time window in which sildenafil could produce an erection in response to stimulation. In a penile plethysmography study of fasted men, the median time to an erection of 60 percent rigidity, which is enough for intercourse, was 25 minutes, with a range of 12 to 37 minutes.

That 25 minutes is the fastest defensible claim about this drug, and it carries two conditions people drop. The men were fasted, and 25 is a median rather than a promise. Half of them were slower.

The range matters more than the median for anyone deciding when to dose. Twelve to 37 minutes is a threefold spread inside one controlled study, in men selected for it. Add the bioavailability spread of 25 to 63 percent and the honest position is that a man has to find his own number over several attempts, which is also why one disappointing evening tells you very little.

Advertising that promises fifteen minutes is quoting the lucky end of that range as though it were the middle.

Four hours of half-life: what is left at the end of the evening

Four hours is short. That is the trade this molecule makes for its speed.

Sildenafil and its active metabolite both have terminal half-lives of about 4 hours. The drug distributes widely, with a steady-state volume of distribution of 105 litres, and both parent and metabolite are around 96 percent bound to plasma protein.

The practical consequence is that this is an evening drug and not a night drug. A second attempt four hours after the first is happening at a substantially lower level, and an attempt the following morning is happening on very little. Men who want the tablet still present the next day are describing a different molecule, and that one is covered in the tadalafil briefing.

One dose per 24 hours is the ceiling, which is what makes the timing question expensive. A mistimed tablet cannot be corrected with a second one. Get the gap wrong and the evening is spent, whereas on a long-acting drug the same mistake is barely a mistake.

The note to make after an attempt that did not work

Bring numbers to the review, not adjectives.

Six pieces of information turn a vague complaint into something a prescriber can act on, and none of them can be reconstructed a fortnight later. Minutes between swallowing the tablet and the attempt. Strength taken. What had been eaten and how long before. Units of alcohol. Whether arousal was present and stimulation continued. And whether there was a partial response, because some rigidity that faded is a different problem from nothing at all.

A partial response points at dose or timing. Nothing at all, repeatedly, points at cause, adherence, or something on the medicine list. Those two findings lead to opposite next steps, which is why the distinction is worth recording at the time rather than remembering.

Three or four such notes are also what makes a dose change defensible instead of a guess. Men who arrive with them tend to leave with the right change, and often the change is the hour rather than the milligrams. Anyone weighing this molecule against the other two before that point will find the label figures set side by side in the class comparison, and the third drug, its speed and its rhythm caution, in where vardenafil fits.