01When can the next tablet be taken?
After the calendar day of the last swallow. The half-life being short does not pull that wait forward. Ten milligrams tomorrow is still one tablet, about 60 minutes before the next attempt. Ritonavir is the exception that stretches a 2.5 mg ceiling to 72 hours. M1 does not add a second evening either; it accounts for about 7% of activity.
If last night felt short, the next conversation is dose or timing, not a same-day redose. A higher tablet still carries the same 4-to-5-hour half-life line.
02Is there a labelled 4-hour intercourse study I should quote?
No. This page will not invent a timed n/N. Duration here is the half-life window plus the once-a-day rule plus stimulation. Sites that print a neat 4-hour success rate are not quoting a LEVITRA duration trial. Tmax after 20 mg already spans 30 minutes to 2 hours; that is a rise window, not intercourse time.
The vardenafil shelf keeps 10 mg as the start. It does not print a bedroom stopwatch. Do not quote a 4-hour intercourse count that the label never wrote.
03Does the usable time start only when you try?
The usable window still needs a trigger. The half-life clock starts at the swallow. The usable window starts when you actually try, if enough drug is still there. Fasted peaks after 20 mg already span 30 minutes to 2 hours, so a late try can still sit inside the rise.
Sitting through the first hour without a trigger wastes the rise, not the half-life. Nitric oxide still has to be released locally later in the evening if that is when you try. A drink that blunts that trigger is an alcohol problem, not a short half-life.
04Why does once a day still sit next to that window?
The maximum recommended frequency is once per day even though the half-life is a handful of hours. Residual drug and the next evening are why the cap exists. A short half-life is not permission for a second tablet at midnight. Ritonavir turns that gap into 72 hours after 2.5 mg. Indinavir keeps 2.5 mg inside 24 hours.
The everyday question is about that cap, not about stretching one swallow. Ten milligrams tomorrow is still one tablet.
05When does an erection become the wrong kind of long?
An erection lasting more than 4 hours needs urgent care. Priapism in this class is a painful erection past 6 hours. Untreated, the tissue can scar. Sickle cell disease, multiple myeloma, and leukemia raise that risk; so does a bent anatomy such as Peyronie's. The patient leaflet uses the same 4-hour call.
That 4-hour emergency is a safety clock. It is not the same 4-to-5-hour half-life. Do not wait at home to see if it fades after hour four.
06What does a 4-to-5-hour half-life mean at the bedside?
Terminal half-life of vardenafil is about 4 to 5 hours. Clearance is listed at 56 L/h. After an oral dose, about 91% to 95% leaves as metabolites in feces and about 2% to 6% in urine. Those numbers describe washout, not a guaranteed erection length. Absolute bioavailability is only about 15%, so the swallow that starts the clock is already a small fraction in plasma.
The major circulating metabolite M1 sits at about 26% of parent levels and accounts for about 7% of total pharmacologic activity. People who want a stopwatch number are asking a different question than the one the PK section answered.