to use as a minor you have to lie, and lie well
This is one reason why his juvenile recreational-chemical phase passed so quickly compared to some of our colleagues at E.T.A. If you are going to do serious drugs while you are still a minor and under your parents' roof, you are going to have to lie often and lie well.
Context
The concealment mechanic that governs every underage user in the book, stated flat: serious drugs under a parent's roof require constant, skillful lying, and the ones who can't sustain the deception either quit or get caught. It's the structural counterpart to Hal's pump-room secrecy-attachment — the using and the hiding fused, the habit only as durable as the lie that covers it — and Bain offers it to explain why Orin, a "strangely stupid liar," dropped out of substances early: the concealment was more than his talent for it could carry. The lie is load-bearing infrastructure of the adolescent habit.