I'm going to tell my ex's parents about her drug use
Are you just going to tell them because you want to get back at her for breaking up with you?
Yes, I'm going to tell them because I know it will be harmful to her
I'm sorry that your relationship didn't work out, but don't tell your ex's secrets just to spite her. Doing so will only cause more drama in your life. Move on and leave the relationship behind you.
She was cheating on me and probably said a lot of bad things about me so she would look better. I'm just trying to get even.
I can understand how you may be upset by all of this, but you shouldn't reveal her secret's to get revenge. If you truly care about her and her safety and she's hurting herself, then I would talk to her parents.