Im not gonna read the other responses, but give you my immediate response.
What does it matter?
Seriously? Have you tried Coke? Its pretty damn awesome, ive had some insane nights with that stuff.
No i don't encourage you to use it, but its a part of life and culture nowadays.
Would I do it if I had the chance? Sure, every so often its fine. Why don't I? Can't afford the stuff!
Basically I guess what im trying to say is that so what, it makes no difference, you've perceived and treated this girl the same for so many years, and now something such as this would have you acting different?
I see the moral stand point, don't get me wrong, drugs *are* bad. I get that.
But logging onto a forum, and asking the interwebs what we *think* about it? I mean come on man. Im guessing you're a young lad yourself? I can only surmise this based on your OP.
TLDR? Doesn't matter, its culture, its life, its what happens, it shouldnt change your opinion or attitude towards her unless you have some extremely strong moral objection. Her life, let her live it and deal with things how she sees fit, and deal with your own life.
