Hi Kash!
Thanks so much for the kind words about my game, I really appreciate it. I'm planning on having the game be content complete by the end of this year, and so far I'm on track to achieve that.
Excellent question. I've actually played with this sort of complication before in Scum & Villainy, another Blades in the Dark hack. The way I treated it there was as an ongoing complication that I could use when the character got less than a 6 on an action roll. When it felt appropriate I would give the player harm based on their addiction: dizziness, exhaustion, nausea, etc. I also started a clock where their addiction would replace their vice, which I could tick as a result of Devil's Bargains or further complications.
You could use similar methods in Neon Black. The character could also rid themselves of this addiction by completing a long-term project.
I assumed the group has already had this conversation, but I would also double-check and make sure that everyone at the table was on board with this complication being included in the story. Addiction and substance abuse can be very uncomfortable for people to experience in a fictional setting, so make sure everyone is being safe and happy!