Dextroamphetamine
"I would suggest anyone on this, to stop or take a break once you've noticed your reasoning, memory recall abilities becoming hindered.  Even though I'm happy with the benefits of focus, it's not worth losing your short-term memory.  Overtime, I noticed my math skills declined, followed by my word recall, spelling and now I can't remember stories or study material.  It's extremely debilitating, I'm trying to figure out how to reverse the side effects."