Ah well, this is indeed a problem here. Many of my friends smoke weed or do stuff like cocaine or LSD. Some even eat shrooms. Whenever I'm invited to a "session", I just say no. They're always trying to convince me that drugs are harmless and society is just brainwashed to hate them, but I can't just start to believe that after I've been told that drugs are bad my whole life before.
So yeah, I only smoke weed sometimes... and alcohol of course. But I wouldn't go for more, ever. Not even out of curiosity. I don't want to mess with my health, taking drugs ain't cool.
There's not really something you can do about it, those people will just continue no matter what, it's their own decision, let them do whatever they want.