So because he was well manicured, charming and sharply dressed, that is suppose to indicate that the drug didn't inhibit him from thinking over other things in his life? The guy said himself that his brain was forcing itself on solving problems and couldn't. ever. relax. His mind couldn't dwell on things that he never registered as a problem to begin with. He had to jump off cliffs and drive 200mph just to keep his mind occupied.
It has been a while since I have seen the movie, but the only problem I remember him putting his mind to was getting more of it. I don't think he even focused on making his own until he saw that dealer guys sister looking like a meth head. No matter what, he doesn't think at all about any of the issues I mentioned in my previous post. (And when I said source of supply, I was referring not to getting more, but figuring out who is behind making them, where they come from, and why). Getting more is the basic instinct of any addict. To answer your question as to why his was pushing that drug, I assume this drug was never intended to just make money on the street. I figured it was a military experiment or stolen/smuggled drug from a pharmacy of some sort.
I think the movie does mention quite a bit how the drug is flawed....I think a few things they mentioned, and only directly from the dudes observations, were loss of appetite, inability to control thoughts, blackouts, sleep deprivation, personality shifts, killing chicks, being a smug SOB. In the end he talked to deniro about needing to change the forumula to fix it up as well. At that point alone, he seemed to be far more even and normal in thought, and yet just as intelligent.