I was taught in economics class that if product “X” was made illegal, then the price will rise, as will demand to enter the business (to make money), and the alure of using it will as well. If one legalizes it, then in the short run, there would be an increase in usage and then will steadily decline.
Halachically, I would think that any product abuse (alcohol, cigarretes ect) go against the mitzvah “leshmor es nafshoseichem- to keep your body safe” and wouldnt marijuana fall under that category as well?