I used to get gabapentin on to potentiate the analgesia of the MST-Continus and Hydal Retard (basically the hydromorphone analogue of MST-Continus) but had lots of problems with memory, which is ironic since when nerve pain is especially bad I have hyoscine/scopolamine, usually mixed with opioids to, inter alia, serve as an amnestic, to mix in with the immediate release SC/IM/IV for breakthrough pain (nicomorphine, dihydromorphine, hydromorphone, oxymorphone) and I thankfully remember all of it. I would be disappointed if I kept forgetting such a cocktail hitting the base of my skull and spreading into my cranium and down my spine . . . the hydroxyzine, tripelennamine, and orphenadrine does what they intended the gabapentin to do to the extended-release --actually all of the opioids I take. So gabapentin, pregabalin, and clonazepam would go with the oxycodone or dihydrocodeine, whereas as a lot of the work promethazine does is metabolic so I would either do it 2-3 hours before or steadily around the clock starting a few hours before the bender and continuing for the duration.
I would take either the chlorphenamine or the diphenhydramine or both along with the opioids. Promethazine is a phenothiazine antihistamine, chlorphenamine is an alkylamine antihistamine, and diphenhydramine is an ethanolamine antihistamine, so the additive effects are more than the synergistic effects. In my case, orphenadrine is an ethanolamine antihistamine so I do not take it with diphenhydramine. The general opioid potentiator, hydroxyzine, is a piperazine antihistamine, and the one that makes the morphine (and its derivatives) more euphoric and therefore an even stronger analgesic, tripelennamine, is an ethylenediamine antihistamine, so I take one from five of the six classes of first-generation antihistamines, and take the promethazine separately from the others because of it working on metabolism, and is good to have already working when the opioids hit. There is also a piperidine antihistamine, cyproheptadine, which really helps out opioids too, as do practically all first-generation antihistamines.
As far as benzodiazepines, I have only used tetrazepam and nitrazepam to potentiate both the extended-release, immediate release, and injected opioids (in the latter case taking it before by mouth, not injection) and I had good results from the two benzodiazepines (one at a time) as well as diazepam with dihydrocodeine and oxycodone both, amongst every other opioid.
Oxycodone and promethazine is even better not only because it is stronger ipso facto but also oxycodone is indeed made into a decent amount of oxymorphone by Cytochrome P450 II-D-6 to begin with.. It is the codeine derivative I have had the most recently as it is in the morphine range, comes in both immediate and extended release, and hydrocodone and thebacon have been gone or rare as chicken teeth for a long time here. Diphenhydramine + oxycodone is great.
