Answer: I would talk with them and tell them that alcohol in moderation is completely fine. So long as they are the legal drinking age. And that their first time to drink it I'd like for them to do it with me so we can tell what negative effects it will have on them. That and we'd know their tolerance for it.
How to stop all together. But you should also see a counselor or get professional help, because you can't just up and decide I'm not gonna drink anymore, it is hard. The professional help will guide you, and give you substitutes, and help you through your problems.
