the classes, and the one thing they share — "Note that pretty much every class…"
Note that pretty much every class of addicts drinks, though.
Context
The book's amateur addictionology: a taxonomy by preference — downs-and-heroin people versus stimulant people versus the marijuana-averse — closing on the universal solvent, the deadpan "pretty much every class of addicts drinks, though." Alcohol as the common denominator under every specialized appetite.