What To Say To 16 Yr Old Depressed Child Who Uses Marijuana To Feel Better…?

OK, please dont be judgmental or make any rude comments about parenting here…..My daugher is 15 almost 16 and has been in and out of counseling, on different meds for depression, and nothing makes her feel better. The meds make her not hungry, or cant sleep , or sleep too much, or????? the list goes on. Just recently, she came to me and said she smokes pot every now and then and it makes her feel better, happy, refreshed ect…She wants me to get her the medical card for it (which there is no way theyd give it to her for depression as far as I know) My personal beliefs are, it is a natural substance and can have its place for medicinal purposes. I don’t want her on chemical meds, but I don’t want her smoking pot either. Shes my 15 yr old child (ugh)….so I am not quite sure how to go about this….she will go behind my back and do it anyhow, But i don’t want her to think its ok…
I have explained to her all the negative consequenses she can encounter, and will have to face those on her own if she makes decisions to do this. Shes the kind of kid if you lecture, or threaten, or nag, she clams up and wont talk about it, so i am glad she came to me about it. But if you knew the entire situation, she has been miserable emotionally for a couple yrs and its hard to watch your child have the issues she does suffer…
so knowing just a piece of the situation, what else can i say to help her make a better decision about this??