You have Medicare. You pick up your prescription at the pharmacy. The pharmacist tells you it is not covered. Or worse, it is covered but costs $200 a month when you expected $15. This situation happens to Medicare beneficiaries constantly, and it almost always comes down to one thing: the formulary.

A formulary is the list of prescription drugs your Part D plan covers, organized into tiers that determine your cost. Understanding how formularies work is essential for anyone on Medicare who takes regular medications, which is the majority of beneficiaries.

How Part D Formularies Are Structured

Every Medicare Part D prescription drug plan, whether it is a standalone plan or part of a Medicare Advantage plan, has its own formulary. These lists vary significantly from plan to plan. A drug that is covered on one plan's formulary at a $10 copay might be on a different plan's formulary at $80, or not covered at all.

Formularies are typically organized into tiers, usually four to six. Tier one covers generic drugs at the lowest cost. Tier two covers preferred brand-name drugs at a moderate cost. Higher tiers cover non-preferred brands and specialty drugs, with costs that can be substantial. Specialty drugs, often used for serious conditions like cancer, rheumatoid arthritis, or multiple sclerosis, frequently sit on the highest tier with coinsurance rather than fixed copays, meaning you pay a percentage of a very high drug price.

When a drug is on your plan's formulary does not guarantee it will always be covered in the same way. Plans can change their formularies mid-year for certain reasons, including removing drugs that have a new generic equivalent available. If your plan changes your medication's tier or removes it from the formulary, you should receive a notice, and you have rights to a coverage determination and appeal if you believe the change is inappropriate.

Strategies to reduce what you pay at the pharmacy are worth knowing, and our guide on slashing your pharmacy bills covers the most effective approaches available to Medicare beneficiaries.

The Coverage Gap and Catastrophic Coverage

Part D has a coverage structure that surprises many new beneficiaries. After you and your plan spend a combined amount on covered drugs, you enter what was historically called the coverage gap or donut hole. For 2025 and beyond, legislative changes have significantly reduced the out-of-pocket exposure in this phase, but the structure itself still exists and affects how costs are calculated throughout the year.

Once your true out-of-pocket spending on covered drugs reaches the catastrophic threshold for the year, your cost-sharing drops dramatically. You pay either a small copay or a modest coinsurance percentage. This catastrophic protection matters enormously for people taking high-cost specialty medications, who can reach this threshold relatively early in the year and then see their monthly drug costs drop substantially for the remaining months.

Tracking your year-to-date Part D spending matters for this reason. Log into your plan's member portal or call your plan to ask where you stand relative to the coverage phases. Knowing that you are approaching the catastrophic threshold might affect the timing of prescription refills or whether to ask your doctor about a 90-day supply.

What to Do When Your Drug Is Not Covered

If a medication your doctor prescribes is not on your plan's formulary or is placed at a tier that makes it unaffordable, you have several options. Start by asking your doctor whether a therapeutically equivalent medication is available that sits on a lower tier. Generic alternatives or drugs in the same class often produce the same clinical result at a fraction of the cost.

You can request a formulary exception, which asks your plan to cover the drug at a lower tier or cover a non-formulary drug because it is medically necessary. Your doctor must submit a supporting statement explaining why the specific medication is required and why alternatives are not appropriate for your condition. Plans are required to respond to exception requests within a defined timeframe.

If your plan denies the exception, you have the right to appeal. The appeals process for Part D follows a structured timeline with multiple levels of review, including independent review and administrative law judge hearings if needed. This process protects beneficiaries from being denied coverage for genuinely necessary medications.

During the annual Medicare open enrollment period, which runs from October 15 to December 7 each year, you can switch Part D plans. This is the most practical long-term solution if your current plan consistently creates cost problems for the medications you take regularly. The Medicare Plan Finder tool at Medicare.gov allows you to enter your specific medications and compare actual out-of-pocket costs across every plan available in your area. Running this comparison annually rather than staying on your current plan by default can save hundreds of dollars per year for beneficiaries who take multiple medications.