Thanks a lot for this! I've tried combining 200mg L-theanine with 60mg DXM, then a hit of marijuana, and found this a very enjoyable and therapeutic combination. I'd love to see a clinical trial of this combination (minus the marijuana) for major depression.
60mg seems to be something of a 'sweet spot' for DXM, I've discovered. It's the highest dose at which I feel perfectly functional, and get no negative aftereffects, but the lowest dose at which I feel some mental effects from it. I actually can study on this dose, and hold onto the material quite well. (On 90mg, forget about it!).
Adding the theanine makes the DXM feel a bit more like very low dose ketamine. I think this is not only the added NMDA antagonism, but also the fact that, just like k, theanine enhances dopamine, which DXM doesn't.
At higher (reacreational or visionary) doses of DXM, I've experienced an absolutely amazing afterglow, that feels somewhat like low-dose amphetamine, followed by a crash, that leaves me emotionally flattened an depressed, not unlike an amphetamine crash. My working theory is that high-dose DXM upregulates DA receptors so much (the afterglow), that it precipitates a rebound downregulation (the crash), leaving your CNS with lower DA tone than before taking DXM, and defeating the whole purpose of taking DXM as a DA enhancer.
This is getting a bit off topic, but I'm finding more and more that my dopamine system can't be cheated. Increasing my dopaminergic tone far above its natural set point, no matter the mechanism (amphetamines, NMDARAs, DA precursors, etc.), will inevitably lead to my body yanking those DA receptors and leaving me in an anhedonic state. Perhaps 60mg of DXM is just below the threshold of DA enhancement where this rebound downregulation happens.