I think Randy Marsh from "South Park" had the most effective anti-marijuana speech I've ever heard: Stan: I've been told a lot of things about pot, but I've come to find out a lot of those things aren't true! So I don't know what to believe! Randy: Well, Stan, the truth is marijuana probably isn't gonna make you kill people, and it most likely isn't gonna fund terrorism, but… well, son, pot makes you feel fine with b…
If you want to convince teenagers not to try (certain) drugs - have them watch Trainspotting and Requiem For A Dream. Emotionally punishing, both of them.
Edit: my brother died of a heroin overdose, so I have zero fucking tolerance for this bullshit simplistic reasoning.