I mean, you'd think that simply "if you use it a few times, you will get physically addicted and your body will demand that you obtain more of it, regardless of the fortunes it will cost you and the massive risks you will run of either falling afoul of the law or getting caught up in other criminal activities" would do the trick. It's not necessarily about health as such, it's about the health risk AND the very high probability of finding yourself compelled to waste ever more money on it AND the increasing risk of finding yourself in jail one day in the near future - or dead.

Of course, that only goes for the really hard drugs - it won't stop anyone from snorting the occasional line of coke, if they hang out in the kind of circles where that is considered acceptable.