I'll stick around.
Drug abuse is pretty much Economics 101. The diminishing returns to scale are already covered. Supply and demand is no brainer. However, there are some things add.
1) Perfect complements vs perfect substitutes. Good X is useless without good Y vs Good X is interchangeble with Good Y.
2) Trade-offs (
https://en.wikipedia.org/wiki/Trade-off)
3) Utility (
https://en.wikipedia.org/wiki/Utility)
Different drugs satisfy different needs. For instance; I don't like speed whatsoever and having some amounts of it would yield me very little to no utility. I used opiates because I fucking hated life and being awake fucking sucked, I had no intentions of staying up for days. I would (and have in the past) trade it for less than market price for some opiates, even if there was an efficient market for the garbage.
Same thing applied to things like freedom, health and family. As those things flew further down the priority list, getting wasted (sort of) as long as possible (kind of) took more scarce resources like effort, time and money. I was not oblivious, I fucking knew what the opiates would do to me, and yet assigned a higher values to it. If there were no opiates, my choice would land on sedatives like benzos and alcohol, both preferably, or some combination of those.
Would I become addicted to meth? No. Does not work that way. Satisfies no needs, hence, useless. I don't know how to explain it, really. Maybe our Sniffie can explain.
Think of it in food terms. Some you love, some you would never eat.