Right. My understanding is that it's very common to have fake oxycodone or heroine that's fentanyl or carfentanyl, and the short half-life of fentanyl and carfentanyl means that addicts will get withdrawl symptoms in just a few hours. My understanding is that heroine and oxycodone have long enough half-lives that many people can hold down jobs and generally be pretty functional by taking small doses before work (and maybe mid-shift, not sure). My understanding is that it's nearly impossible to do something similar with fentanyl/carfentanyl.

Though, I have little exposure to hard drug culture, and get most of my information from headlines.