It depends on the drug, obviously:
Marijuana could improve focus and motivation.
Alcohol puts the kibosh on protein (muscle) synthesis, and can invoke injury.
Uppers (coke, speed, etc.) might be the best for both CNS and focus, but once you're a crack-head, don't plan on having any money left for good food...
Serotonergic psychedelics (tryptamines: lsd, shrooms, phenethylamines: mescaline, mdma) definately boost the CNS but destroy your focus.
Coricidin is completely stupid, and can kill you. Try Max Strength Robitussin instead, the preperation with just DXM. Though it might anesthisize the lactic acid burn, you could seriously injure yourself with a weight you can't feel. Stupid.
Opiods destroy motor control as well.
Personally, I wouldn't mess with anything, except caffeine and/or perhaps low-dose marijuana.
Don't pay too much attention to the posts that follow this one - but I'm actually that former psychonaut who's tried them all (and loved many)... and here come the DARE graduates -->