I think if you try to hold her medicine, she will just find some from another source. At one point with my RABF, we decided he would be open with me. I wanted him to stop the lying, so he took me with him when he traded meds with a "friend." Of course, what he was showing me that he was taking and what he was really taking were two different things. He admitted later that he had been taking much more than I ever saw. He was also getting meds off the internet and from different docs/pharmacies.
It's impossible to control their usage. Believe me, I've tried. She is only going to stop when she is ready to stop. The begging and pleading and telling her what an addict she is doesn't help either. My RABF always denied to me that he was an addict. Later, after he came clean, he admitted that he knew he had a problem. He was just denying it to himself and me. However, he was not yet ready to stop denying he had a problem. Do you see what I mean?
I've learned the hard way--over years and years, that I have no control over addicts. No matter how hard I try to save them, they have to do it themselves. Sometimes, I imagine myself "handing" the addict over the God/angels/higher power.