I would say "девочку-наркоманку" as well. "Зависимый от наркотиков" sounds a bit unnatural. There is a medical term "наркотическая зависимость", and if you want to make accent on the medical aspect of a person's condition, you could say "девочка, страдающая наркотической зависимостью" - this would be OK in a medical text or something like that.