A formulary is a plan’s list of covered drugs. Every stand-alone Part D plan and Medicare Advantage plan with drug coverage has one. The list can include brand-name drugs, generics, and biologic medicines, along with similar alternatives called biosimilars.

How Drug Tiers Work

Plans group covered drugs into tiers that determine how much you pay. A common structure includes:

  • Lower-cost generics
  • Preferred brands
  • Non-preferred drugs
  • Specialty drugs

Each plan sets its own tiers. A drug in a higher tier generally costs more. Your price can also depend on the pharmacy and whether you pay a fixed amount or a percentage of the cost.

Required Drug Coverage

Plans must cover a broad range of medications and most drugs in six protected classes: cancer medicines, HIV/AIDS drugs, antidepressants, antipsychotics, seizure medicines called anticonvulsants, and drugs that suppress the immune system after an organ transplant.

Coverage rules can still apply in permitted circumstances.

Can Formularies Change?

Yes. Plans can make changes during the year under Medicare rules when new drugs or medical information become available or when a generic or biosimilar enters the market. The plan must notify affected members as required.

What If Your Drug Is Not Covered?

If a drug is absent from the formulary, ask your prescriber about a covered alternative or request a formulary exception with a supporting medical explanation. If the drug is listed but requires the plan’s approval, called prior authorization, or trying another drug first, called step therapy, it has a coverage restriction; use the authorization process or ask for an exception to that rule.

A tiering exception asks the plan to charge you the lower amount from another tier for a covered drug, when its rules allow this. Some tiers do not qualify. This request cannot add an excluded drug to the covered list.

When new coverage begins, ask whether a medicine you already take qualifies for a temporary supply, called a transition fill, while you and your prescriber resolve a formulary or restriction problem. Not every rejected drug qualifies. Medicare explains restrictions, exceptions and transition fills. A denial notice gives the appeal route and deadline.

For each medication, record whether it is listed, its tier, and any authorization or quantity rule. Those three answers explain why two pharmacy rejections may need different solutions.

Medicare policy sources last checked: October 8, 2026.