I gave the half life data of Viagra (4 hours) Cialis (17.5 hours) and Levitra (about 4-5 hours)  and people have written to ask about meaning of 1/2 life
This is the time it takes from ingestion of a medication to the point where only half the drug is left in active form. It is a reflection of the length of time during which the medication will be effective though ti doesn't reflect that exactly.  It is close enough for most clinical decisions.

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I don't know if this is just me, but....

I often take 100 mg of Viagra, which is a large dose, but, 3-4 hours in advance of when I need it to be working. Given the half life, this is equivalent to taking 50 mg right before when I want it to work. So, what's the point of the larger dose, taken hours in advance if they drug is just dissipating? I find that side effects (nasal stuffiness, facial flushing) peak 1-2 hours after the drug is taken. So, with the larger dose I get past the mild side effects but still have plenty of the viagra still active in my system when I need it.

YMMV, but that's how it is for me.

Hello my experience is...I always have 1 hour dates(I am 63 after all). I take the "V" one hour prior to my arrival. I have cut the 100's in half, but once I was disappointed in the results. So I now just bit the bullet and take the 100mg($$$$). I have not been as hard since I was a teenager. Most of the ladies make some comment of how big & hard I was...I always thought they were being kind(I am 63!), but I have to say I LOVE THE "V", never tried anything else(I my in the future). I used to get the side affects stuffy nose, seeing BLUE...has not happened in at least two years. I wonder if the side affects go away after a while?? I ordered some of the generic from somewhere(I think India...?), has not arrived as of yet.
Will let you know how it works...at a fraction of the price. generic viagra 100mg x 20 (sildenafil citrate) = USD 49.95
Old Guys Rule

Bummer   I am 78    was quite active on TER but I had a tiny R thalamic stroke in 2015.   No obvious side effects, except I lost any sign of morning wood, no feeling at all in sex, and overactive bladder.   Mayne I should just advertise myself as a "squiter"    stimks    but my ladies are loyal because I am very good to them, and apparently I give amazing oral.  The stroke didnt affect that, and watching a woman cum 6 or 7 times in a row pleases me a whole lot !!