This is a potentially risky practice, for a few reasons. First is the risk of serotonin syndrome, which can happen with amphetamine combos, although rare. Secondly, this combination is very harsh on your striatum (possible neurotoxicity), and without striatal dopamine agonism you will just be stimulated without the desired level of euphoria. You are basically going to burn the candle at both ends and won't notice the long-term effects until loss of pleasure and potential psychosis have already occurred. There are abstracts that show neuroprotection, but they involve methamphetamine and dull the euphoria. Thirdly, DXM will slow liver metabolism of amphetamine, so beyond burdening an important organ, you may have more insomnia and all sorts of metabolites hanging around longer than you want. If you are using gels or syrup, you also have to process all the thickeners/fillers.
Honestly, NMDA antagonists may be the answer, but I don't think *mixing* them with stimulants is. If one were to detox themselves from speed for a few days so there is none present in their system, and went on a ketamine binge, they would regrow striatal dopamine receptors faster and reduce glial activity, recovering from the crash sooner. This doesn't mean they will have a lower tolerance, they will simply go back to baseline as though they took a couple months off. Tolerance is "learned" by the brain, and successfully blocking this system from further exposure dulls some of the the cognitive benefits and euphoria of stims, and will burn up dopamine even faster (in this case).
I honestly tried to find the abstracts for these claims, but I forgot the wording for some. I've spent enough time on this post already. If you care about your long-term baseline state, don't chase an upper high with a magic bullet. I have personally tried this combo before I knew all of this and had severe anhedonia when I crashed.
