Well, it's not really a physical thing as much a psychological thing. I know that when I am coming down off Salvia, my body and such feels so strange that I don't want to travel to the "Spiritual Realm" again, but that's just me.

Oh, by the way, the most comprehensive Salvia tripper's Guide (in my humble opinion) can be found at www.sagewisdom.org (It's the "Salvia User's Guide" or something like that. You'll know it when you see it). It's the best.