Here are some pieces of advice that might help:1) First of all, try to learn as much as possible about different drugs and how they affect someones behavior.
2) Talk to your child, let them know that you know. Express your worry, care and love for the child.
3) Try to remain calm and avoid blaming. Try to find out what the child is feeling and experiencing, and how they see the situation.
4) Make it clear that you love her/him, but not the drug and what the drug does to your child.
5) Explain the consequences of alcohol/drug addiction.
6) Tell them that you are there to help and support them. Try to find out what the young one is thinking in terms of quitting and what he/she needs to help with that.
7) Set limits on what is OK and what isn't and make the consequences of crossig those limits clear.