The fact that they're both dissociatives doesn't mean they're substitutable in any given context.

Dextromethorphan is a much riskier drug overall than ketamine, in almost every measurable way. It's very easy to overdose on dextromethorphan, to the point where people routinely do it by accident while self-medicating for a cough. By contrast, it's actually quite difficult to overdose on ketamine - the LD50 is quite high, and a person would likely incapacitate themselves halfway through and be unable to finish.

They also have dramatically different effects - there are actually very few contexts in which both could be considered an acceptable choice for a given goal.

The bladder issues with ketamine are only documented for chronic heavy users. There's no clinical evidence of it for one-time use, and given the extensive use of ketamine in controlled settings for several decades, the absence of evidence is (in this case) strong evidence of absence.

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

It allows you to take a much lower dose of dextromethorphan and so have fewer undesirable side effects. It helped me cope with some chronic physical pain I was suffering with at the time (in addition to depression).

This is insane gouging, as you could literally make it yourself by getting a prescription for generic bupoprion and some store-bought cough medicine.