so i've never used viagra, but according to most opinions on the internet, viagra doesn't help with permanent gains. at least no one really sees that happening. no really studies, just what popular opinion seems like. It seems counter-intuitive, but i can't really speak from experience.
Seems like it'd be better to take it after the pump, since both pumping and viagra can induce stress on the body, and consistent stress can build up into something harmful. that's the only's a guess. Also, being able to get fully erect after a pump is important, and can help bring out a lot of size from the pumping.