I agree with you OP, that parents do need to be open to the idea that their DC may use drugs at some point.
I have 2 late teen DDs, and as a former heavy weed smoker (in my late teens/early 20's), a "recreational" pill taker (a few times at festivals), and a now sober alcoholic (I've been in recovery for nearly 15 years), the best way I have found to educate them has been through sharing my own experiences with them.
Our DDs are both aware that they are genetically predisposed to addiction, that in drinking and using drugs I took risks that landed me in dangerous (totally avoidable if I hadn't been drunk/high) situations and that weed basically turned me into an anxious, paranoid mess for a couple of years.
So far, DD1 has tried alcohol twice, but stopped after 2 drinks as she "didn't like how it felt" and DD2 is currently totally opposed, mainly through the fear of getting drunk and doing something "utterly mortifying".
Talking of things that are "utterly mortifying", I think the very best deterrent so far came from a friend of DD1's at a party she was at last year. He drank the best part of a bottle of vodka and while walking down the stairs, he vomited the whole way down, then slipped in it, skidding down to the bottom and landing in a pool of his own vomit, where he promptly passed out, and shat himself. Apparently the Mum of the girl throwing the party billed him for a professional clean of her once cream carpet!