Welcome to the HW forum. It's late and my brain is a bit foggy, but I think Ry has asked some good questions and given you some good advice. What is the dose that's prescribed on the prescription bottle? If someone has prescribed them for this long, then there must be a need or she is working with a doctor that's looking the other way - and these days that's getting pretty rare. So if you could explain a bit more that would help. But the first thing I would do is as Ry suggested, go with her to her doctor's appointment and tell/him her what's happening. If there is a real need for the meds, and cost is an issue, perhaps there's an alternative that could be tried. If it is an addiction issue, then have the doctor help refer you to some addiction professionals. They may be able to help with detox if it's still needed, and then with your Mom dealing with the problem. Yes, there are people her age who are addicted to prescription meds and other substnaces, but I don't know enough from what you're saying to have a sense whether your Mother is one of them.
Whatever you do, I'd suggest your direction needs to be taken from your Mother's doctors, not us in the forum. We can offer support, but we're not doctors, nor do we know all her medical history.