Honestly, I'm speaking from personal experience with both drugs. When you're talking about overdose on dextromethorphan I think you're mischaracterizing. The LD50 of dextromethorpan is 150mg/kg in mice. A dissociative/psychedelic dose of dextromethorpan is 300-500mg in an average adult. A typical cough medicine is 15-30mg per dose, and a bottle usually has no more than 250mg or 300mg. Nobody is "accidentally" hitting a toxic dose. Overdosing really means: getting somewhat similar effects to ketamine -- along with a nasty dose of dizziness and nausea. To have an extremely serious reaction to dextromethorphan that is drastic, you'd have to consume multiple bottles of cough syrup.

On other hand I've been in, and had friends in, so-called "k-holes" because of doses of ketamine that were too high and the experience was ... awful. Imagine thinking you're dead, but you're still conscious.

Both are NMDA antagonists. Both provide emotional and physical dissociation. They are in the a similar class of experiences.

Psychotherapeutic doses of ketamine are not one-time. At least not from the reading I've been doing. They have been found to be more effective done over several sessions. I honestly worry about the potential for bladder damage because it has not been studied well and also this kind of treatment is something we're looking into for a loved one