Each Medicare 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. Sometimes, if your prescriber thinks you need a drug that's on a higher tier, you or your prescriber can ask your plan for an exception to get a lower copayment.
A Medicare drug plan can make some changes to its formulary during the year within guidelines set by Medicare. If the change involves a drug you're currently taking, your plan must do one of these:
Provide written notice to you at least 60 days prior to the date the change becomes effective.
At the time you request a refill, provide written notice of the change and a 60-day supply of the drug under the same plan rules as before the change.