I'm sorry but you need to tell your parents. She will need a lot of support to get off the drugs and even then she HAS to want to.

Take her to her GP and see what they say. Also get a referral for a psychologist as well.

This is not going to be easy and you need support as well. A referral to a psychologist for you will give you some help in working out strategies to assist her and counter the manipulation she is sure to use.

Good luck!!