someone addicted to a certain amount of heroin per day who stops using (money, access, etc) will come back to heroin with a much lower tolerance than before.
if they are sold fentanyl unknowingly, using a familiar amount can be a death sentence. if they are sold heroin cut with fentanyl, it could result in death as well.
these aren't entirely new concepts, nor are they explicitly attributable to fentanyl. addicts across the country are keeling over every day from traditional overdoses, fentanyl's presence means everyone using heroin intravenously, even the 60 year old junkie, needs to have a test shot first.
in my experience, fentanyl is good for withdrawal only. it is a cold, unfriendly high that leaves the user irritable.
it's nothing like the warm embrace of morphine.