This is an interesting question.  If Keith has introduced as many chemicals into his system as have been fabled in Rock and Roll mythology, there is a good chance that he may have some neurological damage.  Gene might want to give some more detail here, but I imagine it might have similar effect on speech and thought like a stroke or closed head injury.  Even if he went stone sober, he might still have lifelong changes in his neurophysiology which would effect his speech.

So, my advice to future rockstars...go easy on the drugs and booze...and get some skin so soft.

PRH