Imagine if all the other kids at school used drugs on a daily basis, that there was advertising plastered everywhere telling you that drugs are cool, that successful people use drugs and that your worth in this world can be directly tied to successful use of drugs.
I don't mean to sound mean here, but are you a parent? If not I suspect you're not really aware of the realities of parenting, particularly once a child becomes teenage. You can only do so much. And you certainly can't win against a multi-billion dollar enterprise determined to make your rebellion-inclined teenager do something.