addiction defined before the drugs
They'd sit there and look into Gately's eyes and lie and believe their own lies, and Gately would have to call in the debtors' lies and sob-stories and get Sorkin's explicit decision on if to believe them and what to do. These types were Gately's first exposure to the concept of real addiction and what it can turn someone into; he hadn't yet connected the concept to drugs really, except coke-heads and hardcore needle-jockeys, who at that point all seemed to him just as furtive and pathetic as the gambling-addicts, in their own way.
Context
The book's cleanest statement of what addiction is, and it's delivered through gamblers, before Gately ever thinks to apply it to himself: the compulsive who lies and believes his own lie, who bets his way deeper to escape the hole the betting dug. Gately watches the disease's whole psychology — the furtive begging, the self-deception, the person hollowed into "what it can turn someone into" — years before he recognizes the same thing operating in his own pocket's Tylenol bottle. Addiction located in the structure, not the substance.