Methylphenidate
"Works like a charm.  It has significantly helped me with concentration- whether sitting in a lecture or retaining my train of thought, restlessness, and some impulsiveness.  One thing- for me, mixing with caffeine is a no-no, as it makes me jittery and shaky.  But I can do a half-caf coffee. Only negative things: tolerance does develop over time, unfortunately; it only lasts up to 12 hours, and as a student, I'm up early and go to bed late, so I take a 10mg Methylphenidate tablet 40 minutes before or after dinner to keep me focused for a bit longer.  Around 1 am though, I get pretty restless."