You'll never change their mind. They've been brainwashed by years of anti drug rhetoric.
Most of these people have never tried drugs and wouldn't know the difference between MDMA, Coke, Weed, Smack, Ketamine, Shrooms, Hash, ICE or LSD.
For the record I've tried all of those and enjoyed most of them, although there are only a few out of that list that I'd want to do regularly.