yeah, from what i've seen, it's fucking aweful even if your addiction is only moderate (is there such a thing?).
i had one first-hand experience with it, an acquaintance of mine ODd in my living room, mainly because he was a huge retard, i had snorted a small line of the gear which i always do before i even consider shooting it, and it was enough to really buzz me, usually this does next to nothing to as the smack around here isn't that good, but this had been cut with god knows what, probably fentanyl, and even though i warned him he cooked up a 'smaller' shot, but since he'd been using forever and i am very on and off with opiods he didn't think it was sensible to listen to me, or try a line beforehand, as i urged him to do. well, he fired home and instantly went into full OD mode, stopping to breathe. i was getting cigarettes, a five minute walk from where i was living back then when that happened. my then-girlfriend instantly phoned an ambulance and started doing cpr, and i came back maybe two or three minutes after the injection. i didn't have any pure naloxone, but a few tilidine pills, which contain 100mg tilidine and 5mg naloxone. i figured that that should work until the EMTs arrive, so i cooked it up, spilling parts and barely filtering it because i was in such a hurry, and then shot him up with it. to my surprise it wasn't instantaneous, as i thought it would be, but he came out of it pretty fast and he was in a terrible way. i mean i've seen opiate withdrawal before, but that was something else entirely. i - probably foolishly - gave him 10mg diazepam and when the EMTs arrived they said that the cpr plus the the shot i gave him definitely helped a lot to save his life and prevent brain damage. i'm still kinda proud of that, although to be honest he was a huge scumbag.
edit: yeah, the half-life of naloxone is something like 63 minutes, which is why they took him into the hospital for a 24-hour observation, and probably to give him more, as the tilidine on top of what he already shot couldn't have been good, as it's much longer lasting