I think it's contributed to a built up tolerance. I use caffeine pills esp. during a long 12 hour night shift. I started off with 2 pills (400 mg.) and was zooming fine. Now I could take 3 pills at once and not feel a thing. I'm not sure, but the lipo/ephedra has alot more zoom

than amp does. If you're used to alot of ephedra alkaloids then switch to something like amp with 200 mg or so of caffeine, you won't feel a thing IMO.