Have you tried illicitly obtained ketamine?
Yes, along with a bunch of related drugs. Unfortunately I didn't have the chance yet to try medicinal ketamine, so I can't truly compare as I don't know what impact the very probably present impurities had.
I believe its dose is much higher than hospital prescribed.
Ketamine is primarily used as an anesthetic, and the recreational effects are below the anesthetic threshold. If it's dosed much higher then it's either impure or the user developed a tolerance, in the latter case medical ketamine would have to be dosed equally high. It's one of the very few drugs that are under-dosed when used recreationally.
But yes, the medically administered dose for antidepressant purposes will be relatively small to minimize side effects (those which recreational users are searching for).
Does it interact with other prescribed drugs, normal AD's?
It's used in emergency for instabile patients (correct me if I'm wrong) because of the relative lack of bad interactions, it doesn't depress respiration for example but acts more like a mild stimulant. As long as you don't have preconditions or crazy regimens, there should be little danger. You could even combine ketamine with an opioid.
And some anecdotes have raved about it's AD effect lasting months.
I assume you haven't found this to be the case...?
That it's more of a weekly thing, even shorter perhaps?
This I can't answer, because I'm not sure how to think about it for myself. a) I'm atypical (as all of us are) b) dissociative use might have changed me more than I realize c) I very probably do have Asperger's which might be related to some NMDAr abnormalism, leading to point a.
Things beside, the effect is real and unique. For some one "hit" might be enough to make things rolling, open the road for permanent / lasting benefits when used in the right moment and the right people or therapeutical setting. Others might require more than one attempt.
I think of the consciousness, the brain and the neurobiochemical system as kind of a yin and yang. Every thought or feeling has a chemical counterpart, and every neurotransmitter activity has an emotional / cognitive counterpart. Nothing is static here.