Promethazine definitely potentiates opioids, like oxy for instance! Its not very safe though.. My friend did this once, and over did it big time! He found a script of promethazine when I was helping him move in his parents drug drawer, and already had done well over 200mg of oxy sniffed and smoked, and well he ate like over 100mg of promethazine and was pretty much comatose for a few hours, I was scared he had od'ed and was going to die, but his heart kept beating, and I kept giving him slaps in the face like "wake up man!" and he would pop up just about every time saying a bunch of gibberish nonsense.. he was too sedated for that to have been any fun for him.. he woke up 3 hours later not knowing shit about what had happened, and never did that combo again as far as I know. I have had good results mixing diphenhydramine with all different opioids, including subutex(partial agonist opioid) which was incredibly nice, surprisingly! There was a thread on here a while back about mixing IV bupe with IV antihistamines, so I went ahead being the needle addict, sub addict, opiate addict I am, was wanting to get some kind of nice rush from my subs, so I mixed sleepinal (diphenhydramine) with bupe in one shot, prepped the diphen first then after it was turned into a clear solution, put that on top of the bupe in another spoon, mixed them up, and shot it.. I got a pretty nice feeling actually! Even got a rush from the diphen! but I started to become addicted to mixing those two together IV, and figured it was time to stop... so now I am just back to plain bupe, no antihistamines for me anymore, at least not IV...