Depends on how the teenager is; are they responsible? Do they have experience with cigs/alcohol? How is our relationship? etc.

So I would first ask them why they want to smoke it. I wouldn't accept like a "just because" answer. We'd have a whole discussion on it. Good, bad, consequences if it's not legal in our area & what I will do if he/she gets caught, etc. If they're reasoning isn't based on just "being cool", I'd then ask them if there is a necessity for it. Some people need it for this or that, what, if anything, do you need it for? They may or may not have an answer for this, but I'd go on to ask how they plan on obtaining it. (So that I know before hand how to rescue them if needed.) I already expect any future children that I have to experiment, so I would end the conversation by saying, "after having this conversation with you, I know that you know the potential consequences of your actions & I expect you to be responsible enough to make the right decision".

The end goal for me would be to make them understand fully why they do the things they do. I'd probably even throw in my own experience with weed. And, tbh, I'd rather have them smoke weed, (if it's legal that is) more so than cigs.