You need more than just to be Medicare-eligible. You must also enroll in a Medicare Part D prescription plan, which you get to choose, as there are many options for your location. These plans list the meds they cover, though I don't think they list what the copays are.
Learn as much as you can about the providers you are interested in, and what your premiums would be. Also keep up with the news about Part D, because gradual changes are in the works over the next 10 years or so.
There is a plan to eliminate the 'donut hole' provision, which is all to the good. That is a period in a given year during which you yourself have to cover all costs, till you have spent up to the max, and then you get coverage again.
All this is complicated and full of bureaucracy, of course, but there is very good info on line. Find the Medicare site and read up on Medicare in general, then on Part D. Everyone will need this info eventually, or every American will, I should say.
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I'm not sure..but I'll bump your question since it's reaching the bottom of the page.