Codinex as corridor-extender: the addict's bargain
A tiny percentage of codeine gets metabolized into good old C17-morphine, affording an agonizing hint of what real relief from The Bird might feel like. I.e. the cough syrup did little more than draw the process out, extend the corridor—it slowed up time.
Context
The pharmacology of the losing bargain, stated with clinical precision: cough syrup is not relief but the memory of relief, "an agonizing hint" that keeps the corridor from ending. The substitute substance does the addict's characteristic thing — it does not solve the problem, it "extend[s] the corridor," trading a faster catastrophe for a slower, more drawn-out one, and paying for it in the currency the whole scene is about, time. This is the theme's mechanism of the substitute high in its purest, saddest form.