the classes, and the one thing they share
Drug addicts tend to fall into different classes: those who like downs and Mr. Hope rarely enjoy stimulants, while coke- and ’drine-fiends as a rule abhor marijuana.
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.