That is separate from saying that changing the legality of it impacts the usage though. There is not reason to take the result beyond what it indicates.
Taking Sweden as control, for example, they spend loads on health services and treatment; but without decriminalisation, their numbers are still terrible.
You would probably not want this kind of prescription to be signable by just any MD though, but that's another story.
You can not before treatment and for keeping it illegal.
These are mutually exclusive, as treatment for drug addiction has to be voluntary on the part of the user, and often time criminal convictions, and stress of legal issues are what drive people to use drugs.