There are no short cuts with opioids. If you're going to stop using them you have to just stop and wait it out.
At which age did you first use opioids and how was your abuse pattern?
Granted, I probably wasn't really abusing them but in common maintenance doses (bupe 4mg, metha 40-60mg, morphine 200-600mg) and altogether more than 2 years. Just that I started late, after the age of 25, like smoking, which also is very easy to quit for me. Stims were at the age of 20, they are medium yet still what I really struggle with is serotonin, these SSRI fuckers entered my life with 17 and daily.
For me the NMDA vs opioid thing is among the only drug related concepts that really reliably works. I can switch from 200mg morphine to memantine or DXM without withdrawal and after a few days my tolerance is back to baseline. I suspect this works better the less intense cold turkeys somebody went through.
Other thing that works is, again NMDA and stim but here the wrecked serotonin system lets me struggle and it is my primary concern.
Wonder whether something like microdosing in psydelics could heal it a bit over time, at least they come with loads of nice growth factors.
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Never tried to withdraw with K but guess its half life is too short. This effect works different than the rapid AD one and needs you to be on a disso while the body re-adjusts to skip w/d. I agree that DXM and memantine might be too weak for hefty habits. Combination is worth a try, its more lucid. I've tried deschloroketamine and O-PCE, very different but hints of psychosis specially with PCE. The ketamines can be combined with DXM or memantine, don't take all three - psychosis risk.
ULN sounds really promising, it's the only thing which actually lowers NMDA density but I have a brief experience with naloxone (above UL though) which waa scary.