Seriously, holy shit it has a terrible reputation it really does not deserve. Unfortunately it's the OTC high of choice for desperate teenagers, and nigh universally found in cough syrups and cold remedy gel caps where it is accompanied by acetaminophen and guaifenesin.
Acetaminophen overdose is a horrid way to go. It will absolutely wreck your liver if you try to use an associated DXM product in tripping quantity.
Guaifenesin is a lot less horrifying, but at high doses will give you intense nausea and make you vomit. It's not hard to see why this would ruin the headspace of any trip.
And last but not least, cough syrups and gel caps are not meant to be taken in large quantities needed to get an effective dose of DXM. They are full of artificial sweeteners that will give you bad diarrhea and a painful, upset stomach.
All of this is to say, most people who try DXM end up with a horrible, shitty experience. In the worst case, the acetaminophen will literally kill you. Most prospective drug users are savvy enough to avoid formulations with APAP, but this is where DXM get its reputation as a particularly hard drug on your liver and kidneys. It's not, it's actually almost as physically benign as opiates are, it's metabolized in the exact same way and has a similar chemical structure despite the fact that it doesn't activate opioid receptors. There is a rare possibility of bladder damage, but this is a property it shares with Ketamine and other dissociatives.
Recently the FDA approved a new prescription antidepressant called Auvelity that features a combination of DXM and Bupropion. It's approved for daily use at a much larger dose than recommended by cough medicine, so I'm not talking out of my ass here when it comes to the safety profile.
Finding OTC formulations without guaifenesin is hard, though. Even harder still is finding non-gel cap/non-cough syrup formulations. Taking any of those is very likely to make you feel as if you poisoned yourself. When DXM is the only active ingredient, people are going to blame that before they blame the inactive ingredients.
And that was how I thought of DXM for a long time because my first experiences with it came in the form of gel caps. I recently had the opportunity to try a more pure formulation of Dextromethorphan HBr, and it is amazing. It doesn't feel like a dirty high at all, and the comedown comes with this peaceful, dreamy, almost opiate-like afterglow feeling.
In smaller doses you get an intoxicating alcohol-like high without any of the downsides like hangovers, liver damage, or obvious loss of motor control.
In larger doses you get what many compare to a more psychedelic, more introspective Ketamine-like trip. But unlike psychedelics, there is a far lower chance of having a bad trip because of how dissociative it is. In my experience, it totally blunts feelings of paranoia and anxiety.
Anyway, I'm on a DXM afterglow right now and thought I'd use this vibe to put my thoughts to paper on it.
I looked into it and couldn't find anything specific. While some of the people I know who ODed on the combo were using clean opiates, I'd assume that my generation's love of fentanyl probably throws a wrench in it. If you're on a maintenance dose that's fine, but if you ever take more than your daily dose, I'd suggest avoiding dextro for safety.
Speaking of maintenance, this is good info for you with DXM. My girlfriend is on methadone, and there have been a few times where her doses were fucked up (pressure boiled over on flights) so she had 20mg a day when she normally takes 65mg a day. Low dose dextro was enough to keep her out of withdrawal for weeks. If you ever get fucked on your Suboxone, like you're out of town and lose a few doses, DXM is an essential in your toolkit
That'd honestly be my guess as well regarding street fentanyl where most end users don't actually know what the potency is, what other downers it might be cut with, or even which analogue of fentanyl it is. In a lot of cases it's not fent at all but some newer, weirder opioid RC. It's hard to say how much of a role having DXM in your system would have on an overdose here because it's just another wildly confounding variable among way too many.
Given the very wide array of receptors DXM is known to bind with, I'd say it's probably not worth the risk to use DXM in these situations where you don't actually know exactly what drugs you're taking in what quantity. Maybe there are directly lethal interactions happening with some of the nastier analogues, but it might also be the case that DXM is indirectly mediating opioid effects and pushing what may have been a near lethal overdose over the edge.
With less deadly legit pharma opioids like Hydrocodone, Oxycodone, or Morphine (not IV), I wouldn't worry too much about it. Especially if you have a large tolerance to begin with and you're not being excessive with the dosing. The tolerance reduction and potentiation may even be an easy means of stopping your dose of full agonists from spiraling out of control like it typically does in the development of opioid addiction. And with partial agonists like Kratom and Buprenorphine, I'm not sure there is any danger at all. My maintenance dose of Buprenorphine is already near the ceiling dose and I was breathing just fine on my last 3rd plateau DXM trip.
That said, taking more extreme caution can't hurt.
But also yeah, DXM is a very good tool when you run out of opioids and have to go through withdrawal. It's funny you mention it because I did actually get kind of fucked by the pharmacy last time I picked up my prescription. For some reason they were still out of stock for a whole three days after I ran out despite taking my dose as prescribed and calling to ask for the refill a whole week ahead of time. Buprenorphine has a long ass half-life, so I wasn't feeling too bad by the end, but I was definitely getting very uncomfortable at an increasing rate. It was that point I just threw down a bunch of DXM tablets on a whim to take my mind off it, kind of expecting to just sorta distract myself from the withdrawal, and two hours into a second plateau dose I realized the symptoms had become miraculously unnoticeable.
Then the next day I got my Buprenorphine, dosed it during the DXM afterglow, and once again experienced a miracle when it felt almost like my first experiences with full agonists all those years ago. The only thing missing was the sleepiness and nodding.
Yeah, absolutely no danger with kratom. Love kratom and dextro. But I think you hit the nail on the head. Dextro takes opiate cases that were right on the edge and pushes the over with the tolerance rollback. It's different for people who are physically dependent on opiates, but if you're not then your tolerance isn't consistent enough to even know what a safe dose of opiates is, even with clean product.
DXM+Kratom took me through full fent withdrawal almost comfortably, it's amazing. Seriously, if you start tapering, dextro that shit.