Most of us already take a cognitive enhancer to help us with school or work, so that ship sailed a long time ago. The most commonly used cognitive enhancer is caffeine, which seems to be pretty benign, although it is at least mildly addictive. Some people also take nicotine, which is severely addictive and (at least as commonly administered) quite harmful.
Then of course there are people who are diagnosed with ADHD. Although there is almost certainly some biological, and probably genetic, basis for ADHD, there is currently no objective diagnostic test that will distinguish people with ADHD from those without--diagnosis is by an essentially arbitrary set of behavioral criteria. Where the line is drawn between "normal" variation and disease is pretty arbitrary. Sometimes, the line is drawn based on the efficacy of a treatment, and at one time it was thought that ADHD drugs like methylphenidate (Ritalin) and amphetamine (Adderall) only benefitted people with ADHD, but this turns out to be untrue, so this also does not qualify as a diagnostic criterion.
So far, the ADHD medications seem to be fairly benign, although they too can be habit forming (except perhaps for atomoxetine [Strattera] which is probably less effective as a cognitive enhancer). I say so far, because it has been just a bit over a couple of decades since we began chronically treating children with these ADHD drugs on a large scale, so if they were going to come down with, say, early Parkinson's Disease in middle age, we probably would have seen at least a hint of it by now, but it's still a bit early to be certain.