Jeremy - 6 years working in a prison taught me a few things. 1) don't confront her til you have proof; 2)if she'll steal your drugs, she'll steal your money and stuff - and WORSE she'll tell those who she knows what stuff you have, and it'll be stolen by them, when she has an alibi.
Personally, I'd set up a motion camera and catch her. In fact I'd let her steal actual pills. Once you have the proof you MUST go to the cops. Otherwise if you try to handle it "in-house" you're still at risk of "an inside job". Once the police are involved, that'll put enough of a scare to hopefully cross you off the "steal here" list. Also, without some serious intervention, she's gonna keep up the same behavior, and spiral downward. think about it, you didn't do anything wrong, she did. HTH, steve