I recently tried two supplements reported to enhance cognitive ability - piracetam and sulbutiamine. They didn't work for me, but even if they had there's an issue. If they had worked well and I could focus better and I wanted to take them every day - would that be an addiction?
Stopping piracetam and sulbutiamine was a no brainer since they didn't work for me, but in general if it's not found in nature, and there are no large placebo controlled trials for effectiveness and safety I think it's best to stay away. (Here's a cool balloon race of supplements and their trial results. All in-vitro and small trials thankfully excluded.)
atm, I'm currently testing out creatine -- for my brain, not my body. There is a mechanism for creatine in generating ATP quickly in the brain when the brain is fuel limited. (My working assumption is that more ATP equals better mental performance, but is that right?)