How do I tell my girlfriend that I plan to be a medical marijuana patient.?

Before anyone starts with the "pot head" claims or the "you're just doing it to smoke pot" umptions let me educate you. While serving in the Air Force I injured my back and now suffer from chronic and sometimes severe back pain which can make even sitting painful and uncomfortable. As well as having chronic to severe joint pain in my knees due to my career field in the Air Force. I want to avoid prescription meds such as percoset or vicoden due to personal experiences and being borderline addicted to them while prescribed the medication. I've told my girlfriend that if I ever needed pain medication I would become a medical marijuana patient as an alternative option for pain relief. The reason I'm concerned about discussing the topic with her is because she is very anti drug use and I don't want to lose our relationship over being an mm patient, we've been together for a year and a half and we recently moved in together. I'm just looking for some advice on how to approach the situation and maybe make her understand I'm only interested in using it as an alternative to prescription medication and the possible dependency I've experienced before and not as an excuse to smoke pot. Please serious replies only.