Using a dissociative during surgery is a pretty infrequent rate of use, most people only experience anasthetic like that a handful of times in their lives, and the benefit there (painlessness) far exceeds the harm (the experience of having a dentist rip out your teeth).
Using dissociatives frequently involves the harm of neurotoxicity for the benefit of recreation, or in this case, removal of DMT-fear. This does not seem to bear the same calculated cost-benefit weight as surgical use, although I will admit a DMT experience could potentially be beneficial enough to match or exceed the benefits of some dental procedures.
Nitrous oxide is a widely used analgesic agent, used also in combination with anaesthetics during surgery. Recent research has raised concerns about possible neurotoxicity of nitrous oxide, particularly in the developing brain. Nitrous oxide is an N-methyl--aspartate ...
www.ncbi.nlm.nih.gov
"Now research in rodent models has found that homocysteine can be linked to neuronal death and possibly even cognitive deficits."
Extensive research has failed to clarify the mechanism of action of nitrous oxide (N2O, laughing gas), a widely used inhalational anesthetic and drug of abuse. Other general anesthetics are thought to act by one of two mechanisms-blockade of NMDA glutamate receptors or enhancement of GABAergic...
www.ncbi.nlm.nih.gov
"The favorable safety record of N2O may be explained by the low concentrations typically used and by the fact that it is usually used in combination with GABAergic anesthetics that counteract its neurotoxic potential."