Also one dead giveaway in some, is if for your H tolerance, a batch comes round so potent, that even were it uncut, medical grade diamorphine, it COULDN'T exert the effects it does at a dose the street stuff did. I've had that happen, could taste H, but the effects, it was so potent there was no way for the H to take effect at any dose the fent or derivative of it wouldn't have killed me. It reliably knocked me out cold, with a couple of tiny tokes from a TINY amount in a glass pipe. Pure H, or dipropionylmorphine even, significantly more potent than H, with no cut, prepared from pharm morphine would not cause effects worth noticing at the doses that street stuff was taken, even by IV.
So that rules out it being pure.
As for duration, fent is a short acting drug, few hours. And a lot of people say it feels cold, clinical, lacking in euphoria or warmth. I've had it, both street (unfortunately) as a H cut, and medical supplied. I have to admit, the presence of H at least altered it that little bit to make it SOMEWHAT inclined to make one come back for a second hit.
Otherwise, it's cold, dry as old bones, and meh. I lasted one day, on a fentanyl script. For some reason the doc wanted to try me on it, I agreed to give it a trial.
Patches. But the damn things were slow as fuck to get going and produce any plasma level sufficient to bring me out of WD from the oxy I had been taking previously. Had to resort to extracting and either shooting, or plugging doses to get some opioid agonism going on, due to the slow speed of transdermal uptake.
And what I did get from it, honestly, was crap. Euphoria? none to speak of.