How Part D Formulary Tiers Set Drug Costs

Medicare Part D is the outpatient prescription drug benefit that operates alongside the broader Medicare program, which is divided into separate parts covering hospital care, physician services, and managed-care alternatives. Part D coverage is delivered exclusively through private insurers that contract with the federal government — there is no single national drug benefit with a single national price list. Instead, each plan maintains its own formulary: a structured list of covered drugs organized into pricing tiers that determine what a member pays at the pharmacy counter.

The tier structure is the central pricing mechanism of Part D. It is not a flat copayment schedule applied uniformly across all drugs. Each tier carries a different cost-sharing design — copayment or coinsurance — and the assignment of a drug to a particular tier is the plan's primary tool for steering utilization and managing program costs. Understanding how tiers are constructed, who sets them, and where they produce unexpected results is the foundation for understanding how Part D drug costs are actually generated.

How a Formulary Tier Translates Into a Drug Price

A formulary is a list of drugs a Part D plan agrees to cover. The Centers for Medicare & Medicaid Services (CMS) requires that each formulary meet minimum coverage standards — plans must include drugs across all therapeutic categories and classes — but CMS does not dictate which specific drugs appear on any given formulary or which tier they occupy. That assignment is made by the plan's pharmacy and therapeutics committee, a body typically composed of pharmacists and physicians that reviews clinical and cost data.

Standard Part D formularies use five tiers, though plans may use more. Tier 1 is generally reserved for preferred generic drugs and carries the lowest cost-sharing, often a small flat copayment. Tier 2 covers non-preferred generics or lower-cost brand-name drugs and carries a somewhat higher copayment. Tier 3 is typically populated by preferred brand-name drugs, with a higher copayment still. Tier 4 covers non-preferred brand-name drugs, often through coinsurance rather than a flat copay, meaning the member pays a percentage of the drug's cost rather than a fixed dollar amount. Tier 5, the specialty tier, applies to high-cost drugs — biologics, specialty injectables, and similar medicines — and carries the highest cost-sharing, frequently expressed as coinsurance that can represent a substantial share of a drug's list price.

The distinction between a copayment and coinsurance is consequential at the specialty tier. A $50 copayment is a fixed charge regardless of the drug's price. A 33% coinsurance on a specialty drug that lists at $10,000 per month produces a $3,300 member obligation before any out-of-pocket cap applies. CMS sets a maximum coinsurance percentage for the specialty tier, and plans may not exceed it, but the interaction between coinsurance and a drug's list price means that the tier assignment alone does not fully determine what a member will pay without also knowing the drug's negotiated price.

Part D plans negotiate prices with drug manufacturers and pharmacy benefit managers. Those negotiated prices — and any rebates flowing back to the plan — are not publicly disclosed at the individual drug level. The price a member sees at the pharmacy reflects the plan's negotiated rate, reduced by the applicable cost-sharing structure of the assigned tier. A drug's movement from one tier to a lower tier, if it occurs, directly reduces the member's cost-sharing obligation.

CMS also requires that Part D plans cover certain "protected classes" of drugs — immunosuppressants, anticonvulsants, antidepressants, antipsychotics, antiretrovirals, and antineoplastics — under rules that limit how aggressively plans can restrict access or impose prior authorization within those classes. Drugs in protected classes must be covered broadly, but they can still be placed on higher tiers, meaning cost-sharing remains a variable even where access restrictions are limited.

Who Administers Formulary Decisions and Cost-Sharing Rules

CMS is the federal agency that sets the regulatory framework for Part D formularies. It publishes annual guidelines that plans must follow when building their formularies, reviews formulary submissions each contract year, and maintains a publicly searchable database — the Medicare Plan Finder — where formulary information is disclosed. CMS does not, however, set the tier placement of any individual drug within a plan's formulary.

The plan sponsor — an insurer that has contracted with CMS to offer a Part D benefit — holds the authority to construct its formulary and assign drugs to tiers. The plan sponsor's pharmacy and therapeutics committee makes tier placement decisions based on clinical equivalence, available generics, manufacturer pricing, and rebate negotiations. These decisions are reviewed annually and can change from one plan year to the next.

Pharmacies — both retail and mail-order — are the point of dispensing. They apply the cost-sharing rules transmitted by the plan's adjudication system at the time of a transaction. The pharmacy does not set cost-sharing; it applies the tier-based rule that the plan's systems communicate in real time when a prescription is processed.

A pharmacy benefit manager (PBM) often operates between the plan sponsor and the pharmacy network. The PBM negotiates drug prices with manufacturers, manages the pharmacy network, and processes claims. The PBM's negotiated rebates and pricing arrangements directly affect the plan's costs, which in turn influence tier placement decisions, but the PBM operates as a contracted administrator rather than a regulatory body.

Where Tier Rules Break Down or Produce Unexpected Results

Formulary changes mid-year are a common source of confusion. CMS rules permit plans to make certain formulary changes during the plan year — including removing a drug from the formulary or moving it to a higher tier — subject to advance notice requirements. A drug that carried a Tier 2 copayment in January may be reassigned to Tier 4 coinsurance by mid-year if the plan follows the required notification process. Members already stabilized on that drug face a higher cost mid-treatment without having changed plans.

The non-interference provision in Part D law prohibits the federal government from directly negotiating drug prices with manufacturers on behalf of the program, though the Inflation Reduction Act of 2022 created a limited negotiation process for a defined set of high-cost drugs. Outside that new mechanism, price negotiation remains the domain of individual plan sponsors and their PBMs, which means the same drug can carry different negotiated prices — and therefore different effective coinsurance amounts — across competing plans in the same service area.

Step therapy protocols add another layer of friction. Plans may require that a member try and fail a lower-tier drug before the plan will cover a higher-tier alternative. This prior authorization requirement is distinct from the tier structure itself, but it interacts with it: a drug may technically appear on the formulary at Tier 3 while simultaneously requiring step therapy documentation before access is granted. The formulary lookup tool shows the tier and the cost-sharing; it does not always make step therapy requirements equally visible.

The out-of-pocket cap, which under the Inflation Reduction Act applies at a defined threshold beginning in 2025, limits total annual cost-sharing for covered drugs. However, the cap applies only to covered drugs on the plan's formulary. A drug that is not on the formulary — or one that requires an exception that is denied — does not accumulate toward the cap. Members whose drugs are not covered must pursue the plan's exceptions and appeals process, which operates on a separate administrative track from the formulary itself.

Just as late enrollment in Part B generates a permanent premium penalty, delayed or missed enrollment in Part D carries its own late enrollment penalty — a separate calculation applied to the national base beneficiary premium, compounding for each month of uncovered drug benefit eligibility. The Part D penalty is distinct from the Part B penalty in both its formula and its trigger, but both reflect Medicare's structure of time-sensitive enrollment windows.

What the Plan's Evidence of Coverage Shows — and What It Omits

Each Part D plan is required to publish an Evidence of Coverage (EOC) document, typically released in the fall before the coverage year begins. The EOC describes the plan's cost-sharing structure by tier — the copayment or coinsurance percentage for each tier during each phase of the benefit (deductible, initial coverage, and catastrophic). It also describes the plan's exception and appeals process.

The EOC does not list every drug on the formulary or every drug's tier assignment. That information is contained in the plan's formulary document, which is a separate file, and in the Medicare Plan Finder's drug-pricing tool, which allows lookup by specific drug name and dosage. The EOC tells a member the cost-sharing rules that apply at each tier; the formulary document tells a member which tier a specific drug occupies.

The EOC also discloses the plan's prior authorization and step therapy requirements in general terms but does not enumerate every drug subject to those requirements. A member who looks only at the EOC will see the tier-based cost-sharing structure without necessarily seeing the utilization management rules that govern access to specific drugs within that tier.

Annual Notice of Change (ANOC) documents, mailed each fall, summarize changes from the prior year — including tier reassignments, formulary additions and deletions, and cost-sharing changes. The ANOC is the primary disclosure mechanism for mid-contract changes that take effect at the start of the new plan year. Changes that occur during the plan year are communicated through separate notices, which plans are required to send but which arrive outside the annual review cycle that the ANOC is designed to support.

The formulary tier is, in practice, a pricing instruction embedded in a coverage decision. The same molecule — the same active ingredient at the same dose — can carry materially different cost-sharing obligations depending solely on which plan's formulary it appears in and which tier that plan has assigned it to. The tier structure is the mechanism by which a program that covers tens of millions of beneficiaries and thousands of drugs produces individualized prices at the pharmacy counter, and it is rebuilt, at least in part, every contract year.

Note: This explains how a retirement system works. It is not financial, tax, or legal advice, it is not specific to any individual's retirement, and it is not a substitute for a licensed financial, tax, or legal professional. Rules, ages, and dollar limits change by year — check the cited sources.