True potentiation is a tough one with codeine. Not many things work on CYP2D6, at least not as inducers. One thing that does, when taken long term, is valerian.
https://pubmed.ncbi.nlm.nih.gov/17214607/. You'll not be able to find it, but doriden (Glutethemide) was gold for this.
Blackseed oil is a good one for opioid tolerance, I feel it makes opioid receptors more sensitive.
https://pubmed.ncbi.nlm.nih.gov/22768141/
DLPA is another great supplement for opioid potentiation
Blue lotus, (d2 antagonism, slightly antipsychotic)
Corydalis yanhouso works on opioid receptors, but another with antipsychotic traits
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8540887/
And the final "neuroleptic", glaucium flavum
Turmeric and black pepper extract
The following are not potentiation, but rather additive effects (some above also have additive effects, but actually potentiate the codeine).
Antihistamines (promethazine is the best I know)
Gabaergics (very dangerous)
Valerian and hops are both gaba, but also have effects on opioid systems when taken together.
Pain is a common undertreated worldwide complaint. The need to explore the antinociceptive potential of alternative herbal products is essential. Although used as a mild sedative, limited evidence focused on the potential antinociceptive effect of ...
www.ncbi.nlm.nih.gov
NMDA antagonists.
Clonidine and similar
Kava
Passion flower
Chamomile
Lemon balm
Skullcap
Chasteberry
Mulungu
Vervain
Betonica Officinalis
Magnolia
Klip dagga
Tilia
Kratom can fuck up your tolerance, but it might help more than codeine its self.
A small amount of nutmeg essential oil can be a good sedative (in a capsule, taste is vile)
Myrrh
E: you have gabapentin, have you tried lyrica?
