How Do I Tell ?

O.k i have this friend that means the world to me she`s my best friend were so close were like sisters so she means alot.
Well she`s getting in to stuff that i dont like like snorting pills or worse stuff then that she says my thought about the s*it she does means alot to her but i dont no what i should say i deff dont want to loose her over something like that or if she does something worse (killes her ) what should i say to her? I dont want it to be mean cuz i dont want to loose her as a friend but im very VERY worried about her. Thanx