The effectiveness of ketamine for depression is all about the state it puts your mind in. This is obvious because the drug itself has totally left your body shortly after administration, but the effects can last much longer. In this respect you can think of it as your mind actually healing itself, by being induced to by the ketamine, rather than the ketamine itself impacting your brain's structure. This is different from most conventional psychiatric medicine where the drugs need to build up and remain in your system over very long periods of time to be effective.

Optimizing the dose & duration to get your mind into that state, and for the right amount of time, makes a huge difference in how long the effects last. This is why I prefer using an actual doctor with ketamine depression treatment experience, because they have experience figuring out the right dose & duration for you.