Perhaps his lack of concentration when he stopped for a month wasn't because the drug helped him concentrate, but rather that his dependence on it made him not be able to concentrate without it anymore - that is, when hes on the drug, his concentration is at the same level as those not on the drug.

Now, I don't know if this was the case or not, but its worth thinking about before starting this.

Personally, in the past I've used large quantities of energy drinks to get me through long project or study sessions. While it worked in keeping me awake, and more alert than I would have been without, I did notice my performance drop. That is, if I was well rested and worked for some number of hours, I'd get MORE work done that if I worked twice as many hours, but substituted rest for energy drink.

Sometimes the trade-off is worth it (eg because the deadline is earlier than it would take to get it done with rest; quality is less important than time etc etc), but other times its not. I bet amphetamines are similar, though I've personally never tried.

In the past, I have tried a drug for concentration purposes; not quite sure what it was exactly, some sort of legal cannabis alternative which contained small amounts of valium and other similar ingredients. Large amounts would put you to sleep, but small amounts really did help concentration a lot and a lot of great ideas came from this (including a project which myself and a friend came up with, designed and built a prototype and it is now being used by a certain university for research purposes, after rebuilding our prototype with proper manufactured parts). So, it certainly is possible to use drugs to enhance performance, but I'm wondering at what cost. I'm not convinced that prolonged use is safe and would be pretty skeptical.

You'll have to decide for yourself if the risks and trade-offs are worth it for you.