Anyone remember the smint thread?
I took all the drugs at that age OP. If she was taking them last night, waking her up to discuss it will cause nothing but a shitstorm. I'd let her wake naturally, and stand with the bag in your hand - let her fill the silence... Point out that she doesn't know what it's mixed with, that she's being fucking stupid, that there is no such thing as a 'safe' drug as no one ever knows what's in it. See if you can get her to open up, find out if she uses habitually at parties or if this was a one off; for the latter you'll probably be ok, the former I'd escort her to the gp for a chat and see if there's any help available.
Either way, if you're the one funding her, I'd stop all access to money; not as a punishment, still buying makeup, magazines, wee treats, just not letting her have actual cash access so money can't go on drugs. Obviously harder if she has a job and her own money...
Oh, and tell her that I'm 29. I stopped all drugs when I was 20. My friends didn't, I only have 3 friends still alive from that time; the rest are all dead, either by overdose, alcohol addiction which followed the drug lifestyle, or in a few cases suicide - directly linked to their mental health issues caused by drugs. One died at the age of 27 because his body simply packed up, multiple organ failure by chronic ketamine use. I have more dead friends, than I have digits on my fingers and toes.