the mechanism of action is also poorly understood, and while this article may certainly be an important mechanism, ketamine is a "dirty" drug with many potentially active components that act in ways we dont understand
ketamine is also a drug that has been fda approved for anesthesia for years. to properly study its effects in depression would require large clinical studies. however, the drug is off patent, and no company will fund this work
other companies have developed "new" drugs that are structurally similar to ketamine, but different enough to patent. in 2015 allergan bought a company called naurex that had a few phase 2 drugs that acted on the NMDA receptor (the main receptor that ketamine is thought to act on, though there are others -- hence it being a "dirty" drug) but with some chemical modifications aimed at getting the antidepressant effects of ketamine without the "bad" dissociative effects. however, several docs i spoke with said that there isnt sufficient evidence that their chemical only gets the "good" effects, and it may actually be less effective than ketamine because they removed some of the chemical structure that moderated depression. the real reason they used a new chemical was so they could patent it
J&J has another similar product, called esketamine, is an enantiomer of ketamine (imagine ketamine occurred in two forms, a left hand and a right hand one. esketamine is just the left hand version without the right hand). they also deliver it intranasally (while the approved ketamine formulation is IV). both of these tactics help extend patent life or exclusivity. they published phase 2 data last december that looked pretty good
these companies will probably aggressively market their drugs as "better" than ketamine because they have "improved" the chemistry, although there are no data to definitively state this. many anesthesiologists currently offer ketamine treatment for depression, though it is not widely reimbursed or offered. i believe some groups reimburse for it though. it can be expensive because you need to be supervised when you take it, so you have to pay for the time of a medical professional to monitor you.
as others have noted though, the effects wear off basically right after you stop taking the drug. some people are studying a protocol where you "jolt" someone into a non-depressed state with ketamine, and then stop ketamine treatment and start treating them with CBT or another form of therapy. jury is still out on whether this works though seems promising