Evidence shows these tactics being between ineffective [1] and backfiring splendidly [2]. It did in my peer group. Because you see a friend smoke a blunt or do a line and, lo and behold, their teeth don’t fall out. That crisis of credibility then corrodes the entire message.
Fact-based messaging together with testing to ensure purity makes more sense than Puritanical fear mongering.
We're closer in thinking than my quick sentence suggests. Some facts are a bit ruder than others.
We have long had perfectly functional alcohol, cannabis, MDMA, cocaine, LSD, mushroom, ketamine, and amphetamine users. There are good guidelines for how to safely use these and stay out of trouble.
Fentanyl, well, the folks lying in the gutter or folded over in their wheelchairs in SF at night are sort of a warning. And on the purity testing, well, it's a PITA to need to test so many things for the absence of added fentanyl.