Medicare Drug Coverage: How Formulary Tiers Decide What You Pay (and Which Plan to Pick)

If you take any prescription medications, the formulary is the most important thing to check before choosing a Medicare plan, and it is the thing most people never look at. The formulary is the list of drugs your plan covers and the tier each drug sits on, which determines what you pay. Two plans in the same zip code with identical premiums can differ by thousands of dollars per year for the exact same medications, purely because of how their formularies are built. This guide walks through the tier system, the coverage rules layered on top of it, and the step-by-step way to check your specific drugs before you commit to a plan.

Bryce's Take

This is the guide I wish every client read before our call, because the formulary is where the real money is and almost nobody checks it. I have sat with clients comparing two plans with identical premiums where one cost $1,500 more per year purely because of how it tiered their blood thinner and their inhaler. The premium is the number the marketing shows you. The formulary is the number that actually hits your bank account. When we talk, the first thing I do is run your exact medication list through every plan in your zip code and sort by total annual cost, not premium. It takes fifteen minutes and it is the single most valuable part of choosing a drug plan.

What a formulary actually is

A formulary is the list of prescription drugs a Medicare Part D plan or Medicare Advantage plan with drug coverage agrees to cover, along with the cost-sharing tier assigned to each drug. Every plan files its formulary with the Centers for Medicare and Medicaid Services each year. Within federal rules that require coverage of at least two drugs in most therapeutic categories and full coverage of certain protected classes (such as antidepressants, antipsychotics, anticonvulsants, immunosuppressants, antiretrovirals, and cancer drugs), plans have wide latitude to decide which specific drugs to include and where to place them. That latitude is why formularies vary so much, and why the plan with the lowest premium is frequently not the plan with the lowest total cost for your medications.

The tier system, explained

Part D plans organize covered drugs into tiers. Lower tiers cost you less; higher tiers cost more. Most plans use five tiers, though the exact structure varies. Here is what each tier typically means and what you can expect to pay per fill.

The tier is not a property of the drug. It is a decision the plan makes. This is the single most important idea in this guide: the same medication can sit on completely different tiers depending on which plan you choose, and that decision is worth hundreds or thousands of dollars a year.

Coverage rules layered on top of tiers

Beyond the tier, plans apply three coverage rules that affect whether and how you can get a drug. Prior authorization requires your plan to approve the drug before it will cover it, usually for expensive drugs or those with abuse potential. Step therapy requires you to try a lower-cost alternative first and demonstrate it did not work before the plan covers the drug your physician originally prescribed. Quantity limits cap how much of a drug you can get in a given period. A drug can be on a favorable tier but still be hard to access because of these rules, so when you review a formulary, look at both the tier and any restrictions flagged next to your specific drugs.

Why the same drug costs different amounts on different plans

Drug manufacturers negotiate with plans for preferred formulary placement, much like a food company negotiates shelf position with a grocery chain. A plan that strikes a favorable deal on a particular brand-name drug places it on Tier 3; a plan without that deal places the same drug on Tier 4 with higher cost-sharing. Neither plan is wrong. They simply made different business arrangements. Here is a simplified illustration of how a single hypothetical medication regimen can play out across two plans with the same monthly premium.

Same premium, same drugs, same zip code, and a difference of roughly $1,300 for the year. This is why comparing plans by premium alone is the most common and most expensive mistake in Medicare drug coverage. The premium is a small, visible number. The formulary difference is a large, hidden one.

Specialty drugs, Tier 5, and the 2025 out-of-pocket cap

If you take a specialty drug, a biologic for rheumatoid arthritis, an oral cancer therapy, a multiple sclerosis drug, or a similar high-cost medication, it almost certainly sits on Tier 5, priced as coinsurance rather than a flat copay. Historically this meant thousands of dollars in annual out-of-pocket costs. The Inflation Reduction Act changed this: starting in 2025, Medicare Part D has a hard annual out-of-pocket cap of $2,000. Once your covered drug cost-sharing reaches $2,000 in a plan year, you pay nothing more for covered drugs for the rest of the year, regardless of how expensive your medications are. For anyone on specialty drugs, this is the most important change to Medicare in decades, but the tier and formulary placement still determine how quickly you reach that cap and whether the drug is covered at all.

How to check your specific drugs before you enroll

This is the step that separates a good plan choice from an expensive one, and it takes about fifteen minutes. Do not choose a plan until you have done this.

1. Write down every medication you take, including the exact name, dosage (for example, 20 mg), and how often you take it.
2. Go to the Medicare Plan Finder at medicare.gov and enter your zip code and your medication list exactly.
3. Add your preferred pharmacy, and check both retail and mail-order pricing, since they often differ.
4. Let the tool calculate the estimated total annual cost for each plan, which combines premium plus your actual drug costs.
5. Sort by total annual cost, not by premium. Note any drugs flagged with prior authorization, step therapy, or quantity limits.
6. Confirm each of your drugs is actually on the formulary. A drug that is not covered at all will show as excluded or full price.

The plan with the lowest total annual cost for your specific medications is the right financial choice, and it is frequently not the plan with the lowest premium. Redo this comparison every year during the Annual Enrollment Period (October 15 to December 7), because formularies change annually and a plan that was optimal last year may have moved your drug to a higher tier.

When formularies change, and what protects you

Formularies are set for a plan year but can change in limited ways mid-year, and they are rewritten entirely for each new year. If your plan removes your drug from its formulary or moves it to a higher tier mid-year, you are generally protected for the rest of the calendar year if you were already taking it, and you may gain a special enrollment opportunity in some circumstances. The bigger risk is the annual reset: each fall, your plan mails an Annual Notice of Change describing formulary changes for January. Read it. A plan quietly moving your maintenance medication from Tier 2 to Tier 4 can add hundreds of dollars to your year, and the Annual Enrollment Period is your one clean window to switch to a plan that covers it better.

What to do if your drug is not covered

If a medication you need is not on a plan's formulary, or is on a tier that makes it unaffordable, you have several options. You can request a formulary exception, which requires your physician to document that the drug is medically necessary and that covered alternatives are inappropriate; plans must respond within 72 hours for standard requests and 24 hours for expedited ones. You can ask your physician whether a therapeutically equivalent drug on a lower tier would work for you. You can request a tiering exception to have a non-preferred drug covered at a lower cost-sharing level. Or, during the Annual Enrollment Period, you can switch to a plan whose formulary covers your drug on a better tier, which is often the cleanest solution when your medication regimen is stable and well-defined.

Bryce Casson

Licensed Independent Medicare Broker

Bryce Casson is an independent Medicare insurance broker who works with every major carrier. He does not represent any single insurer, which means his recommendations are based on what actually fits each client's situation, not on commissions or quotas.