@Halif2 - 4-Fluorobutyrfentanyl?
I may be wrong but fentanyl homologues in which the key amide is shorter or longer than the most active (propanamide) have a longer duration of action. I don't know where that fluorine is as it's possible to place it in the
para position of either of the benzene rings and it produce an active. I suppose Cayman only offer reference samples of things that HAVE turned up on the street so the one that reached the street had a
para fluorine on the A-aromatic, but equally, misdirection is hardly novel and if the law controls the wrong one, that's all to the good for producers.
Oddly, such a substitution was apparently tested by Janssen who one can only conclude saw no clinical advantage. I don't think it confirs significantly different activity.
But I have said elsewhere that I have a hypothesis that opiate 'euphoria' is mediated by the binding-unbinding cycle so if less potent analogues have shorter receptor occupancy times, they may be subjectively better.
But as far as I know, fentanyl only produces a sort of 'plastic euphoria' at quite large doses i.e. although it's analgesic TI is larger than morphine, it's 'euphoric' TI is probably a lot lower than morphine.
But as with all street drugs - users are always playing the 'mystery powder game' i.e. a seller can claim almost ANYTHING because the end-user is in no position to CHECK.