-Edit, Sorry it's so long, Didn't realize how much I was rambling on, this is just my experience in the past, take it for what its worth, be safe, message me if you have any questions, Ill do my best to help/answer. -
I believe we already have a thread for this.
I used opiates for about 6 years. So, I've tried everything.
I realized overtime, that most of it is nonsense and really doesn't make much of a difference when thinking of euphoria.
When mixing opiates, I found that most of the things I felt were a 'synergy' with the opiates, and didn't help in terms of potentiation. (not a real word I don't think)
Diphenhydramine made me drowsy, easily irritable, foggy, and the dry mouth drove me crazy. Helped with the nod a tiny bit though
Promethazine made me foggy,tired. Helped with nausea a great deal, and slightly increased the nod.
As for grapefruit juice, well, it depends on what opiate you're doing. (maybe, from my experience anyways.)
Morphine ,codeine, hydrocodone, all seemed to have a tiny bit of a boost when drinking with grapefruit juice. (On Hydrocodone, I'd say when taking 60mg with drinking grapefruit juice, it would feel like 65mg, nothing to rave about)
Alcohol, Well, I don't drink, I hate the taste, love the effect, but the taste will literally make me puke before I even attempt to swallow, same with smell of some alcohols.
Benzodiazapines just increase the effects opiates have on the central nervous system.
Benzos leave me feeling a little blah, I usually love the euphoria of opiates, but when I take benzos, I would say I lose some of that euphoria, and end up in a nod.
Dramamine was the same as diphenhydramine for me, not a whole lot going on other that being drowsy.
DXM, I've had some pretty mixed results with. (I have to say I don't remember very clearly, as most of my time experimenting with DXM and opiates, I was at the worst point in my addiction, and don't remember things very clearly, as I was under the influence of benzos,dxm,weed,opiates,and ambien on a daily basis. Needless to say, between the Ambien,Benzos, and Weed, I don't remember much)
DXM does seem to do something with Opiates, though I've never been able to point out exactly what that is.
I would notice that when I took DXM with Opiates, things seemed to kick in a bit faster (I would dose dxm 45-90 minutes before I would take my opiates)
Seemed to make my body warmer than normal. It might have some effect, but Im doubting its anything worth doing every time you take opiates.
To be completely honest, I've found the most effecient way of increasing the effects opiates have on my by eating a meal or food loaded fats soon after taking your opiate. That has always worked well for me.
I'd try natural alternatives before taking other drugs, to increase one drugs effects.
If you can get a little boost out of drinking a tiny bit of grapefruit juice, taking opiates, eating a meal loaded with fat, more power to you.
But, I think , you will find, as most other people who have tried to increase the effect of opiates, you can't really make it stronger with 'potentiators' , sure, you might be able to get a feeling of 5-10mg+, but, it really doesn't seem to make enough of a difference to really be of any significance. Sorry I couldn't help more.