I think as adults if we send a blanket message that drugs are bad and you must not take them, we run the risk of our children switching off at that point. Especially if we enjoy a glass of wine ourselves in the evening. Yes I know alcohol is legal and other drugs are not, but teenagers are quick to pick up on perceived hypocrisy. If they have friends who have tried drugs and enjoyed them, or if they have already experimented themselves, then they are going to be needing a fuller picture than the simplistic "drugs are bad" message.
If we can enter into an honest dialogue about reasons for taking drugs, ways of keeping safe, peer pressure and relative safety of different drugs, then I think our teenagers will be more likely to listen to our advice, and to ask questions, or confide in us if they need support.
I personally would rather have a child who experiments with drugs, but has the information they need to do so in a more safe way, and can turn to me if they get out of their depth, than one who knows that Mum totally disapproves of all kinds of drug taking and she had better never find out about that dodgy cigarette he tried last week or she will kill him....
Of course what we hope is that knowing the facts and thinking it over, a child will decide not to get involved with drugs and will have the self-confidence to stand up to the peer pressure and say no thanks. I don't think a child whose only message from parents about drugs is that they are wrong, is as likely to have the skills to do that.