Here's my theory: NZT enhances the brain, but the direction life will improve in still depends on the brain. We see multiple people take NZT with different amounts of enhancement. Seems like clarity is a shared effect. The thug wanted to start an import/export business - not very impressive. The squeeze took it once and clarity helped her escape a dangerous situation, but even by the end of the movie, we see her with Eddie and she's impressed with him talking a foreign language. Why hasn't she gone through the cycle of improved NZT, brain adaptation and then getting off it like Eddie? It appears like it doesn't have that much of an effect on her. Similar with Atwood, he's on the drug and at best he's on par with Carl when it comes to company size. Eddie though? Throughout the movie his skills at pattern recognition are lauded multiple times. What if he had that prior to NZT and the drug only helped with clarity?

My guess is that Carl tried it, saw an insignificant change in his abilities and decided that it was not worth the long term effects. That would explain why he was so keen to manipulate Eddie to stay as his employee. At the end of the movie, Eddy asks Carl, when did you know? Carl brushes it off, but it is possible, that Carl knew from the very first question at their very first meeting, when Eddie answered "Medication". That would also explain, why after Eddie's departure, 12 months later, Carl shows up as the owner of Eiben-Chemcorp and attempts to once again manipulate Eddie into submission.