Isn't wood supposed to last only as long as there's stimulation? Without stimulation, Ol Duke just goes back to sleep. I suppose some guys may react more aggressively but the idea is that the drugs make it easier to achieve and maintain wood but that it still requires stimulation.
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Going along with Big T's story, I have an acquaintance who was recently divorced (lack of sex among other reasons). Three hours before a rendezvous, he pops a Viagra. Afraid that it might wear off before the rendezvous, he takes another one right before. Well, there was nothing the poor girl could do to get it down. Apparantly Viagra works all too well!