@JPB,
Shop! Stand alone prescription drug coverage is called Medicare Part D, of course, and the variations from one plan to another are enormous. Medicare does provide a calculator
somewhere that lets you compare annual costs based on your current prescriptions. In any case, you just have to check out the formulary for each plan. I don't know if there is any rule that there are certain prescriptions that must be covered, but it doesn't look like it.
Besides Part D, there are Medicare Part C plans which are the medicare advantage plans and the various suplimental plans, which may or may not include prescriptions. They are all different.