The only drug I've heard about that's been proven (depending on one's definition of "prove", of course) with respect to Hubbard is Vistaril, a psychiatric tranquilizer, which was found in his blood by the coroner who examined his body upon death. You can Google "Vistaril Hubbard" to get lots of entries specific to this. I haven't found any other proof of the use of drugs, psychiatric or otherwise, by Hubbard. [Then again, it's not like Hubbard ever offered himself up for testing on the subject...]
In any case, what Scientology teachings does Kevin promote?