managing the internal weather chemically — "this circular routine, if your basic…"
this circular routine, if your basic wiring’s OK to begin with, can work surprisingly well throughout adolescence and sometimes into one’s like early twenties, before it starts to creep up on you.
Context
The campus's harder-core chemistry described as a self-regulating cycle — uppers before matches, benzos after, something to short-circuit the swing — and then the quiet fuse in the last clause: it works, surprisingly well, right up until it doesn't. "Before it starts to creep up on you" is the whole disease in a subordinate phrase, the functionality that is real and temporary at once.