It is indeed true. There was a study about it done on rats, which I will link here. Note that while the majority of dissociative neurotoxicity studies have not been done on humans and people like to use this as an arguing point, I'm not really aware of any compelling reasons having been put forth to think that it
shouldn't happen in humans; the logic behind why we believe it happens seems like it could perfectly reasonably translate to humans as well based on studies that
have been done on human brains.
Anyway, here is the link:
Ketamine potentiates cerebrocortical damage induced by the common anaesthetic agent nitrous oxide in adult rats.
And here is an important excerpt from the abstract....
Ketamine and N2O were administered alone or in combination by various dosing regimens to adult female rats for a duration of 3 h and the severity of cerebrocortical neurotoxic changes was quantified histologically. Because GABA agonists are known to protect against the psychotomimetic and neurotoxic effects of NMDA antagonists, we also evaluated whether the combined neurotoxicity of ketamine+N2O can be prevented by certain commonly used GABA agonists.
When ketamine and N2O were used in combination the neurotoxic reaction was enhanced to a degree much greater than can be explained by simple additivity. The apparent synergistic interaction was particularly striking when low doses of the agents were combined, the degree of toxic synergism at higher doses being masked by a ceiling effect. GABA agonists protected against ketamine/N2O neurotoxicity.
First of all, it's significant to note that this study is in agreement with many others on one not yet directly stated fact: ketamine and nitrous oxide are completely capable of producing this kind of neurotoxicity on their own. So, if you accept their conclusion that the combination can increase this effect, then you have to consider this as well.
Second, the major takeaway here is that not only are ketamine and nitrous oxide synergistically, rather than just additively neurotoxic, but they are so much so that when higher doses of each are combined their synergism is masked by the fact that they already start to produce the maximum possible neurotoxocity that can be produced by this mechanism. That sure doesn't sound great....
However, there is more to dissociative neurotoxicity than simply damage being done and leaving the brain that way forever. Sometimes the brain is capable of repairing itself, and I recall over the years reading in several places that dissociative anesthesia lasting fewer than three hours very rarely produces any damage in the brains of animals that can still be detected after a few days. Do not quote me on this as I have not gone out to find these studies again, but I believe that they were able to produce some injury with very high doses of ketamine, but failed to do so with any dose of nitrous oxide.
Personally, what I get out of this is: as long as you don't overdo this dissociation, you'll
probably be fine, probably. In other words, doing ketamine in reasonable doses every now and then probably won't hurt you, and the same with other not too long lasting dissociatives like MXE especially in lower doses. Combining high doses of ketamine and nitrous oxide might be more risky based on this study, so for harm reduction purposes I really cannot recommend it... but can simply say that, obviously, people use dissociatives all the time and even get carried away with them and don't come out of it seemingly damaged forever, usually. The research statistics may make the problem seem more significant than it actually is when moved over to the impact on the human mind, but we honestly just simply don't know enough to say that right now. I'd say that it will probably be a little while before we truly know, but in another few decades dissociative use will have been far more widespread and rampant for quite some time, so we may have some better idea by then.
As for whether or not it will be more neurotoxic than a night of meth, that I'm afraid I don't feel qualified to speculate on. If I was considering doing this combination for myself, I suppose my mindset would just be: it's potentially dangerous, like many things. Is the reward worth the risk? Only you can decide that.
In all my readings from all the way back to its inception, this is the one constant in BL that I'm very impressed about.
