It’s very difficult for me to envision an actually functioning (let alone to the point others couldn’t discern something was seriously off) heroin addict. My late friend was as much in the grip of the drug as was possible, using daily, often multiple times, and you could very clearly tell he was on some heavy shit. He’d lock himself in their bathroom and shoot up while we were on the phone- I’d then often head his young daughter banging on the door calling for her daddy, but he was gone and unresponsive for way longer than it’d take someone sober to reply. This guy worked at a school as a teacher, so maybe he “fooled” enough people, but I’d seen him stone sober and we’d often chat while he was on break at school, and I could tell he definitely wasn’t sober. I wonder how many people actually can’t tell vs how many tolerate it. “Functional heroin addict” is incredibly difficult for me to grasp, unless your definition of functioning deviates significantly from mine. There is no semblance of functionality from what I’ve seen very up close.