OP how to you want the conversation to go? I mean, are you trying to put them off ever doing drugs or giving advice on how to stay safe if they do?
I took a shit load of pretty much everything when I was younger, and less regularly since then. When I was a teen my parents tried to come down really heavily and ‘ban’ me from doing it, but it had the opposite effect. Teenagers can be quite determined like that.
Do you have reason to think your teenager is taking drugs? If so, I would advise telling them to be moderate, always let friends know what they’ve taken, sip water, don’t drink booze and make sure they are in a safe place when they do it. And that they can always call you if things have gone a bit sideways for them. A lot of clubs/festivals have places where you can take drugs to have them tested, without any risk of getting handcuffed, so that’s a way of staying safe and finding out if you’re about to take anything dodgy.
Do they have friends who seem like druggies? If not, you probably don’t even need to have a conversation about drugs - it’s very unlikely your kid will do them if their mates aren’t. And a lot of young people today really aren’t interested at all. So could be a non issue.
Anecdotally, over 20 years on from our ‘hay day’, me and the majority of friends I have who took a lot of drugs back then turned out just fine - careers, families etc. For some that’s all completely in the past, others still dabble occasionally. The odds are very good that it won’t wreck their lives, so try not to freak out if they have been experimenting. Of course, it would be better if they didn’t do drugs but it’s not necessarily a huge deal. If you can, be non judgemental with them, and let them know they can always be honest with you about their drug use. That way you’ll know if things are getting out of control.