Imagine a farmer chooses to spend little time planting crops or tending his field one year, then has a famine the next (when he decides again to tend to his field), then everything turns out okay afterwards. So was it okay to not tend to his crops? Sure he had a famine, put his family at risk, etc, but everything turned out okay in the end!
It may be more effective to de-emphasize the results, and emphasize on what is lost, what is put at risk, what kind of person do the actions create, etc, in the experience and process.
Independence, maturity, freedom to choose, etc, are maybe important values. It's easy to see that there are many things kids can do that would be more effective in helping them attain those values. Kids can do drugs, and there are also many other things they could do that would be more effective in helping them become independent, mature adults with the freedom to choose what they want to do, etc. They might not be able to discern which are more effective, which is why there are adults, parents, teachers, and others, around to help them and set them straight. But even this line of thinking is results oriented.
Back to the famine example. In that case, what makes the "things turned out okay" not persuasive to me is that the experience of going through a famine is bad and painful. Maybe the drugs case is different in that the experience isn't like that. Maybe when the drugs become an addiction then, and only then, does the experience turn bad. So what's wrong with doing drugs without forming an addiction? Maybe there isn't anything wrong, so long as you can remain in control and not let it form an addiction. I guess for adults it's for themselves to decide how in control they can be, but how much can we trust kids to be in control? Maybe kids simply haven't developed enough control to be allowed to choose in regards to drugs.
As for pain killers. Again, maybe it's better to de-emphasize the morality around particular things and, instead, emphasize the morality around what people do, and the process or experience people put themselves through, and what kind of person that makes them. How could anyone be fully independent and free if they were under constant and unbearable pain? From that point of view, it's easy for me to see the difference between regular pain killers (or any medically necessary drugs) and the "drugs" in the so-called drug war.
Of course there's always the different kinds of pleasure argument that I rather like.
Better to be a human being dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied. And if the fool, or the pig, are of a different opinion, it is because they only know their own side of the question (John Stuart Mill).