I take 50-100 mg of Hydroxyzine with opium especially if I want to use less. It kills the nose tip itchiness, which I like actually, that comes with the opium starting to work and it makes the intoxication more noddy. It also seems to relax the muscles much more as well. Especially in the morning , when opium is taken with Hydroxyzine, it definitely kills my lower back pain much more rapidly.Unfortunately, Hydroxyzine doesn't really add to the opiate euphoria.
Phenprobamate, a muscle relaxant, was causing nodding with smaller amounts of Opium and making it more effective for pain (muscle, back, neck pain) but no change in euphoria.
Another medication I take for potentiating the opium is Methylphenidate. I didn't know that it potentiated opiates but I felt this effect by accident. When I first started using opium, I just was after pure opium intoxication but a couple months later, I started reminiscing about how I used to feel shooting H&C together. I decided to take 10-20 mg of Methylphenidate with opium. The result was astonishing. Within 30 minutes of dosing, I was getting nearly double the buzz that I got from opium or Methylphenidate by themselves. There was less nodding but the euphoria was much better. It also seemed much better as a painkiller probably because of the high physical pleasure. After that I read about the potentiating effects of Ritalin and I was sure that it wasn't just a wishful placebo.
I used all of the above with dried poppy pods and as expected, got the same results.
Now, I'm really interested about the potentiating effects of various antidepressants and St. John's Wort but it scares me a bit because of the risk of serotonin syndrome. Another one I came across was Propranalol which also is a little risky as well.
Ondansetron (Zofran) is said to have opiate potentiating effects but again there's a risk of serotonin syndrome when combined with opiates.
I've read about nigella sativa oil too but from what I understood, it's more helpful in not building a dependency rather than potentiating opiates, if I recall correctly. Never tried it myself.