Each Medicare Prescription Drug Plan has its own list of covered drugs (called a formulary). Many Medicare drug plans place drugs into different "tiers" on their formularies. Drugs in each tier have a different cost.
A drug in a lower tier will generally cost you less than a drug in a higher tier. In some cases, if your drug is on a higher tier and your prescriber thinks you need that drug instead of a similar drug on a lower tier, you or your prescriber can ask your plan for an exception to get a lower copayment.
Medicare offers prescription drug coverage to everyone with Medicare. If you decide not to join a Medicare Prescription Drug Plan when you're first eligible, and you don't have other creditable prescription drug coverage, or you don't get Extra Help, you'll likely pay a late enrollment penalty.
To get Medicare drug coverage, you must join a plan run by an insurance company or other private company approved by Medicare. Each plan can vary in cost and drugs covered.