Just because she made some bad choices in the past doesn't mean that is who she is in this moment.
Example- I was taking ephedra pills, overdosing on meds, and drinking cough syrup in high school for around a year. I finally wisened up, and I am a completely happy person who only occasionally gets drunk and smokes a little.
Give her a little credit. I'm sure there are things in your past that you aren't proud of either. If she seems adorable, why not let her prove it to you? You always have the power to break things off if she seems to be getting back into drugs...