I know my friend does drugs. What do I do about it? She won't admit it.?

[Lindsey]

I have gotten super close with my friend abbie the past year. She has a different group of friends at school than I do but all her friends do drugs. I recently found out that she drinks and does weed. I asked her about it and she says she has no clue what I am talking about. She keeps making up stories and tells me again about her getting taken away from her parents from drugs and that she wouldn't be that stupid to do something like that. I know that she is lying. Especially in her tone. She is more like yelling at me than convincing me that she doesn't do drugs. She isn't that good of a friend, I know this, but I don't want a friend like that around in my life, even though we have become really close, but this past few months have been terrible with her. She keeps messaging me on facebook telling me that she doesn't want to loose me and that I mean a lot to her, but yet today she ditched me at lunch to go with her "other friend" without texting me that she can't make it. I just can't deal with this anymore. I want to get out of this relationship!
what do i say when i talk to her if she just keeps making up stories and saying that she doesn't do those things?