^^^^I also believe anti-histamines arent great potentiators of opiates, but phenergan is not a typical anti-histamine (like benadryl), it is also a dopamine antagonist, which means it has additive sedative effects when mixed with an opiate. Nevertheless, i have taken phenergan orally in attempts to potentiate opiates, and never found it that effective. And yes, I believe doctors administer phenergan with demerol (yes, a shitty drug) to make it less enjoyable, and essentially ruin whatever high you might get from meperidine on its own.
Like i've always said, the best way to potentiate opiates, is to take more. But real potentiators that i've found effective (much more than anti-histamines) are drugs that effect the NDMA receptors, like ketamine or even proglumide. DXM is in this catagory, but i dont like it as a potentiator, its not very potent and it affects the nature of the high. Ketamine also changes the nature of the high, but is far more effective than DXM. I used to shoot oxycodone and ketamine IV in the same syringe, and there was some powerfull synergy. Usually oxycodone and ketamine on their own arent too sedative (unless ketamine in anaesthetic doses), but when combined, it leads to a strong, euphoric nod. Quinine and quinidine, also work pretty well (500mg-750mg of quinine works well), it helps opiates cross the blood brain barrier.