drug users have successfully argued that drug use doesn't create any harm, and they haven't been effectively countered or challenged. the argument is false - drug use creates a myriad of harms.
addressing the issue in canadian law means addressing the harms that drug use creates, not dwelling on the defects of character, which has not been successful. canadian law tends not to concern itself much with moral considerations or normative values. you can basically do what you want in canada and get away with it, without consideration to moral degradation, just so long as you do not harm anybody.
the public nuisance angle addresses that by focusing on the harm that drug users create and not on normative value judgments. it should be effective but it's up to the crown to make the case.
otherwise, the police need to focus on the more obvious criminal behaviour like theft and vandalism, which creates clear harm, and can be addressed.