Aplunk pretty much got it right.
It not only affects focus and concentration, but it can make you not even realize you are supposed to be focused and concentrating. You're thinking about doing something cool, or being someone cool, and completely forget you were supposed to be thinking in the present because it was kind of important. I don't know if that's just escapism because I didn't like who I was, or the drug, though.
I have felt clearer in the head though since I started taking the drug at night, though.
I've also not been working and sleeping a lot, so I'm pretty refreshed mentally.
It's funny before coming to this site I didn't think I had any side effects, and even for a while here when I heard people talking about the sides I didn't think I had sides. But you sort of clue in..yeah, I had/have that.