WMG: Limitless

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.
  • This is supported by the fact the he offhandedly states that he has more than one lab. Why bother bringing that up, if he was off the stuff anyway?
  • He also says that he "tweaked" it. He's not on NZT, he's on something similar that the labs managed to cook up for him that isn't quite as dangerous. (And NZT was never actually illegal, it just doesn't exist as far as any regulatory agency is concerned - you can't get a prescription for it, but the DEA isn't going to come busting down your door if you have a supply. In the real world, there are lots of rejected or experimental drugs that you can buy from chemical supply companies as "research chemicals" without attracting too much attention.)

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.
  • Doesn't explain his expertise on law based on glimpses of a law textbook in his college days or speaking Russian by hearing the Russian thugs.
  • Non-fantastic aspects of NZT resemble Cocaine, MDMA, and also amphetamines. Amphetamine has been used in armies and airforces to heighten awareness of soldiers and pilots.

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.
  • Eagle Sense also seems to be a way of representing NZT's heightened insight.

NZT has side effects not specifically explained in the film.
In addition to the side effects portrayed in the film, NZT also messes with your impulse control and ability to see long-term consequences for your actions. This explains Eddie's unwillingness to just wait a couple days to get his $100,000, and later unwillingness to wait a couple more to get his $40,000,000. This also accounts for his failure to pay back the loan shark. His entire life begins to revolve around getting more money and more importance, the two things he felt he lacked in the beginning of the film. Hence he gains money and runs for senate, but doesn't seem to have any sort of endgame in mind.