It depends on whether you are looking for a drug which mimics an opiOID or an opiATE. Opiates are naturally occurring opioids like heroin and morphine while the definition of an opioid is any substance which attaches to mu opioid receptors, so stuff like Kratom or buprenorphine are included.
I think it's interesting that people find dissociatives like DXM to be opioid-like. That's never been my personal experience...
DXM is an atypical one, it is somewhat dirty/promiscuous and even a real opioid with affinity to mu (first it was called an opioid because of sigma affinity. Then after sigma was dropped as an opioid receptor because it doesn't induce any relevant effects, so DXM was dismissed as an opioid. Nowadays we know that it does dock to mu but with a very weak affinity and its main metabolite dextrorphan is an antagonist at mu).
But I found methoxetamine indeed to be much like what I'd imagined an opiate high must be like. Dissociatives were my first drugs, long before I got my hands wet with real opioids/opiates. The highs are indeed different and I guess an experienced user could blindly discriminate between a disso and an opioid but when you know only one group of substances the attempt to describe one will quite fit the other. Words are limited when it comes to emotions and drug effects. Even DXM can feel quite opiatey as in cozy warmth, hazy dreaming while awake states etc. DXM won't get you nodding as it's a strong norepinephrine reuptake inhibitor though. MXE was stimulating as well but more 'opiatey' than DXM if you ask me. Nodding isn't possible on dissociatives but will almost always happen on opioids when dosed high enough.
Weird thing is that we have some dissos with relevant mu affinity, e.g. the 3-HO ones like 3-HO-PCE/P but I found them to be
less similar to an opioid than e.g. MXE.
Ketamine - for this I would first have to sample some pharm grade ket straight outta a vial because so much stuff is going around labelled as ketamine but might be god knows what research chemical. Basically there are two variants of the various K I've biosampled. The nice one has hints of psychedelis, is warm, stimulating and smooth. The bad one is cold and clinical, feels edgey and not really euphoric. One guess would be that this were the two R-/S-isomers and everything in-between a racemic mixture but I'm not really sure as the descriptions of R-ketamine differ wildly from almost inactive to a pure DRI and whatnot.
But overall I loved MXE exactly for its similarities to an opioid high. The acute morphine high (acute: dosage taken without tolerance or seriously upped - chronic: same dosage taken daily) is quite dissociative-ish. The nodding marks the opioid but otherwise it would fit. This subsides with every same dosage taken and isn't as sustainable as dissociatives are. Also they don't come with a physical addiction/withdrawal at least unless seriously abused.
