Hopefully we all know the reasons why you shouldn't do drugs: Fries your brain, makes you evil, can kill you - all that jazz. Well, I have found an even better deterrent, especially useful for teenage girls. I have already tried it out on a couple and found it works nicely.
Simply explain to them how smuggling drugs in humans is done. And how you retrieve those drugs. End with "Don't do drugs because they might have come out of someones butt." Either the conversation will scar them for life or they will be on the floor laughing. Either way, mission accomplished.
Surprisingly, this conversation started out very innocently. You just never know where a topic is going.