Ok slow down.. breathe...
You can only take care of yourself right now. As you know, you are the one that takes the drink.. that snorts the coke. Your friends will do what they do.. Your best friend made choices on his own too.
If you were to overdose on coke, or die an alcoholic death, would you want anyone feeling blame? I doubt it.
This is the time for you to love YOU, and worry about YOU, and be responsible for you... Who knows, maybe you'll turn into the "good influence" guy, or maybe your friends will find another 'bad' influence to depend on.
We are all here for you!