mrsgmhopkins that's an interesting question. Many of the people I went to school with (still leading very privileged lives), take drugs recreationally still. It's become a life-defining thing in only a few, but then again with access to money and similar resources it's easier to stop things getting desperate.
I wish I knew what parents could do - talk about things. I have three sons and my eldest (who is 9), is already aware that there are illegal drugs and about the dangers of alcohol.
I think something useful would be to accept that teenagers often do experiment - with drink (god knows I puked up enough stolen gin, can't stand the taste now), cigarettes (ditto silk cut, ergh), sex and drugs. Accepting that rather than going mental or insisting your PFB would NEVER is probably a good start.
Generally if the child has self-confidence they will be okay with saying no. I remember a girl saying she'd stopped taking cocaine for her GCSEs(!) - awful that she was taking it regularly at that age, but that decision to stop and that exams were more important, I dunno, it feels like that was a good sign.
As with most people who use substances to self-medicate or as a social crutch (and I did this with alcohol for a time), there are usually issues under the surface. But to say parents could stop their children experimenting is just not true. The most together, sorted teenager in the world is probably going to do something illicit at some point. Probably years before their parents would consider it even a vague risk.