"drugs" is too general of a term. you have to be more specific when having this discussion. i feel he's at his best on weed and acid, but should avoid the benzo's and opiates. not saying i suggest he goes back to that phase, but i do feel there was some great music due to it. that 2004 interview he's obviuosly benzo'd out. but this is him (in 1999) on acid (as you can see by his eyes).
http://youtu.be/lO1FZRgo4jI?t=28s
http://youtu.be/G8Qpf4kUyy8?t=12s
http://youtu.be/VwO9fFkCPcU?t=30s
i speak from personal experience when i say he's fucking gone in those free's. he's so out there that the beat/lyrics are all he can focus on.
edit: i'm also certain he's on acid during the hiphop witch interview. but of course it was a little movie thing so they could've just told him to act as if. but the way he was telling to homeboy and passing that blunt it's pretty undeniable.