maybe you should not focus on the drugs first (cause if shes doing them and not admitting it, she may be addicted and its going to be very hard to just stop) if you really care about her.. (i mean you want her to stop, and you dont know what to do) try to get some of her trust. I know trust is an issue with drugs. But ask her how shes feeling, get to know why she feels the need for these drugs if shes doing them. Dont ask her directly at first, ask her about her life, once you gain this trust (if your willing to work with her) maybe you can confront her and tell her excately how you feel, dont tell her why its wrong, tell her what its doing to her and you. Then decide on an option is she is willing to give it up.
Goodluck, drugs are a very hard issue to deal with and most people focus on the person doing the drugs not what the drugs are doing to the person. Its a disease as is being an alocholic, its hard to quit over night, its a process.
Take care,
Liz