First - you can take the focus away from this child. YOU CAN DETACH. This doesn't mean that you stop loving them, it means that you give them the opportunity to make their own choices. They need to do this in order to experience the consequences of their actions.
Click here to see some tips about detachment.

When you detach, you'll have the time and the energy to look at your own life - your own problems. You'll learn that blaming others for your own unhappiness is unrealistic and useless.
Remember you are responsible only for your own behaviour. You will help your child if you put your own life back in order.

Then - you can SET BOUNDARIES - these represent what is acceptable to you and what isn't.
So you can set rules for your children - such as no using alcohol or drugs in your home - based on your own boundaries. And you STICK TO THEM - FIRMLY. Breaking these rules will have CONSEQUENCES - perhaps legal consequences, loss of pocket money or free time etc.
Both the rules and the consequences must be clearly set out. Writing them down is a great idea.
Click here to see some tips about setting boundaries.