Sildenafil opens up veins for more blood. It elevates Blood pressure in most people, funny enough it was originally meant as a heart medication and they "accidentally" noticed this "side effect." I guess the only medications that propably would "block it" are beta blockers and such.
I have to admit that i don't know too much about it and since i'm a healthy (relatively

) young man, i'll stay away from it as long as i can (hopefully 4ever).
Many young guys do it just to get better sex experiences, but from my understanding if you use it and don't actually "need it" it can have pretty nasty sides. If you are interested about Viagra i'd go to
www.drugdigest.org or to some similar web-page.