the whole movie is about drugs

the milk they drank had drugs in it, that helped the Ultra-Violence, or the violence the system thinks drug users have

he goes into treatment, and then comes out "cured"...but then he realised that he was not cured, he was just changed...and he wanted to be HIM, not what others wanted him to be, so he went back

i love Kubricks films...A Clockwork Orange, Full Metal Jacket, Paths to Glory, The Shining (original), et cetera

Kubrick was also working with Spielberg on Artificial Intelligence, but died in the middle of production