Are you inquiring about Stacker III with ephedra or without?
With ephedra- you'll have the rise in heart rate, possibly the shakes, and other associated responses. And you'll have one hell of an energy boost.
Without ephedra- someone else who has used the product needs to answer.
EFX- I tried it (at 190 lbs) and had no response. Seems too weak. I used RFA-1 in the past and it was one of my favorites metabolizers (EAS Betalean HP with ephedra being the other).