I'd go so far as to propose that Dez would reject any diagnosis that put him in a corner where he might be hearing imaginary voices, Dez couldn't stand that denigration of his powers of perspicacity, his firm belief in his own super shrewdness. If he was scripted medication they would have to lie to him, and tell him its for his bones or something. Dez believes only he is able to see round those corners, you see.
It's his illness burdened with his delusions of grandeur that have got him where he is today!!!