Eddie was still on NZT-48 at the end of the movie.The whole being off of NZT-48 but still keeping most of the brain processing power with it thing, was Eddie lying to Carl to avoid his threats. After all, NZT-48 does make you feel "invincible", and it is not approved and illegal. He would also probably be lying to Lindy as well.
NZT was actually just a really small amount of meth, modified to be ingested orally.Pretty self-explanatory. Eddie was addicted to meth and only felt "limitless" when on it. He already had the ability to become an investment genius etc. It just made him extremely focused and organized. This also fits with the fact that the 90% of Your Brain theory is just a myth.
NZT is used by the Assassins of Assassin's Creed.The real life Assassins used Hashish (aka cannabis) so they already have a taste for mind-altering drugs. Add the fact that NZT makes you like jumping off high places and better at fighting multiple enemies, and it seems obvious that NZT would their drug of choice. It also meshes well with the Assassin's philosophy: that people are free to risk their own mistakes in an attempt to become better people.