I’m in a minority opinion around here, but when it comes to appealing to its target demographic, I think the Truth ads have the potential to be very effective. We know that long-term effects of smoking aren’t enough to deter most teenagers from it, and many are resistant to the short-term effects. But by demonstrating that many people in authority want them to smoke, and that those people are relying on their lack of intelligence or sophistication to get them to do so. When you make not smoking look subversive, teenagers are more likely to listen.
Anti-drug commercials are harder to make along this model, since the industry that wants teenagers to start using drugs lurks in the shadows. Tobacco executives could be their parents or grandparents; drug dealers and cartel kingpins could not.
I agree that demonization of marijuana does more harm than good; kids see a lot of pot use, and they can see that it doesn’t cause the rain of hellfire and destruction that the commercials say it does, so heroin must not be as bad as they say, either.
One avenue I’d explore is how much money it costs to maintain a drug habit. Since you’re aiming at kids a few years before the age where they’ll engage in the behaviors you’d like to stop, and since amounts of money seem smaller as one gets older, this could have a lasting effect on a kid. (Of course, when you point out how much cheaper it would be to smoke pot regularly than to use crack or heroin, it may drive them to smoke pot instead, which I think would be an OK goal but most would not.)
All in all, I think it is nigh impossible to make good PSAs against “drugs” as a whole, because different drugs have very different reasons why you shouldn’t use them. Dividing and conquering would be far more effective.