What Medicare Part D Is
Medicare Part D is the prescription drug benefit available to people enrolled in Medicare. It was added to Medicare by the Medicare Prescription Drug, Improvement, and Modernization Act of 2003 and took effect in 2006. Unlike Medicare Parts A and B, which are administered directly by the federal government, Part D is delivered exclusively through private insurance plans that have been approved and regulated by the Centers for Medicare and Medicaid Services (CMS).
Enrollment in Part D is voluntary, though failing to enroll when first eligible while not having other qualifying drug coverage triggers a permanent late enrollment penalty. Part D is available to anyone enrolled in Medicare Part A or Part B, regardless of income.
Part D drug coverage is available through two types of plans. A stand-alone Prescription Drug Plan (PDP) provides drug coverage only and is used alongside Original Medicare (Parts A and B) and, if desired, a Medigap supplement. A Medicare Advantage Prescription Drug Plan (MA-PD) bundles Part A, Part B, and Part D coverage into a single private plan. A beneficiary enrolled in a Medicare Advantage plan with drug coverage does not need a separate stand-alone PDP.
Stand-Alone PDPs vs. Medicare Advantage with Drug Coverage
Stand-alone PDPs are the appropriate choice for beneficiaries who want to remain in Original Medicare and supplement it with either a Medigap policy or standalone coverage. A PDP adds drug coverage to Original Medicare without requiring any change to how medical benefits are delivered.
Medicare Advantage plans with drug coverage (MA-PDs) replace Original Medicare with a private plan that bundles all coverage — including prescription drugs — into a single product. These plans typically include network restrictions and may require referrals or prior authorization for some services. When comparing a stand-alone PDP to an MA-PD, the comparison involves the full scope of coverage, network access, and total cost-sharing, not just the drug benefit in isolation.
A beneficiary enrolled in a Medicare Advantage plan without drug coverage who wants to add drug coverage must switch to an MA-PD plan or drop Medicare Advantage entirely and enroll in Original Medicare with a stand-alone PDP. They cannot add a stand-alone PDP to a Medicare Advantage plan without drug coverage in most circumstances.
How Formularies Work
Every Part D plan maintains a formulary — a list of covered prescription drugs. CMS establishes minimum formulary requirements, but each plan's specific formulary is set by the plan and can differ substantially from other plans in the same market area. A drug covered by one Part D plan may not be covered by a different plan, or may be covered at a different cost tier.
Part D plans must cover at least two drugs in each therapeutic category and class defined by the United States Pharmacopeia (USP) guidelines. For certain protected drug classes — including immunosuppressants for transplant recipients, antidepressants, antipsychotics, anticonvulsants, antiretrovirals for HIV/AIDS, and antineoplastics — plans must cover substantially all drugs in the class, not just the two-drug minimum.
Formularies can change during the plan year. When a plan makes changes that affect currently enrolled members — removing a drug from the formulary or moving it to a higher cost tier — federal rules generally require that the plan provide notice at least 60 days in advance and ensure that affected members can access the drug at the previous tier's cost-sharing until they can transition to an alternative or change plans during the next enrollment period.
A Part D plan's formulary is reviewed and can change at each plan year. A drug that was covered at a low tier this year may be moved to a higher tier, placed under prior authorization, or removed from the formulary entirely at renewal. Reviewing the formulary for any plan being considered during the Annual Enrollment Period — specifically checking whether current medications are covered and at what tier — is a necessary step before renewing or selecting coverage. The Medicare Plan Finder at medicare.gov/plan-compare allows beneficiaries to enter their medications and compare formulary coverage across available plans.
Drug Cost Tiers
Part D plans organize covered drugs into cost tiers, with different cost-sharing amounts assigned to each tier. Tier structures vary by plan, but most Part D plans use a five-tier structure:
| Typical Tier | Drug Type | Cost-Sharing |
|---|---|---|
| Tier 1 | Preferred generic drugs | Lowest copay, often $0 to $5 |
| Tier 2 | Non-preferred generics | Low copay |
| Tier 3 | Preferred brand-name drugs | Moderate copay |
| Tier 4 | Non-preferred brand-name drugs | Higher copay or coinsurance |
| Tier 5 / Specialty | Specialty and high-cost drugs | Highest cost-sharing, often coinsurance percentage |
Placing a generic drug in the plan's formulary at a lower tier than the brand-name equivalent is standard. The plan determines which drugs are placed in which tiers through its formulary management process, subject to CMS oversight. Beneficiaries who need a drug covered at a high tier may request a formulary exception to have the drug covered at a lower tier's cost-sharing if clinical circumstances warrant.
The Part D Cost Phases
Part D cost-sharing is structured in phases based on cumulative drug spending during the plan year. The structure of these phases has changed over time, most significantly through the Inflation Reduction Act of 2022, which eliminated the catastrophic coverage phase cost-sharing for beneficiaries and capped annual out-of-pocket drug costs.
The general phases of Part D cost-sharing are:
- Deductible phase: Most Part D plans have an annual deductible during which the beneficiary pays the full cost of covered drugs (up to a maximum deductible amount set by CMS each year) before the plan begins paying. Some plans waive the deductible for lower-tier drugs.
- Initial coverage phase: After the deductible is met, the beneficiary pays their plan's standard cost-sharing (copays or coinsurance) for covered drugs, and the plan pays the remainder.
- Catastrophic coverage: After the beneficiary's total out-of-pocket drug costs reach the annual cap (established by the Inflation Reduction Act), the beneficiary pays nothing for covered Part D drugs for the remainder of the plan year. This out-of-pocket cap is a significant change from the prior structure, which required cost-sharing even in the catastrophic phase.
The specific dollar thresholds for each phase are set by CMS and published annually. Current thresholds are available on the CMS Part D general information page.
IRMAA: Income-Related Premium Adjustments
Higher-income Medicare beneficiaries pay a premium surcharge on top of their Part D plan's base premium. This income-related monthly adjustment amount (IRMAA) is determined by Social Security based on the beneficiary's modified adjusted gross income from two years prior. IRMAA applies to Part D in addition to the IRMAA that applies to Part B premiums.
IRMAA is tiered — the surcharge increases with income above the threshold. Beneficiaries whose income decreases significantly due to a qualifying life event (retirement, reduction in work hours, loss of a spouse) may request a reconsideration of their IRMAA determination based on more recent income information rather than the two-year-prior income used by default.
The Late Enrollment Penalty
Beneficiaries who do not enroll in Part D when first eligible — and who do not have creditable prescription drug coverage from another source, such as an employer plan — are subject to a late enrollment penalty if they later enroll. The penalty is calculated as 1 percent of the national base beneficiary premium for each full month the beneficiary went without Part D or creditable coverage, and it is added permanently to the monthly premium for as long as the beneficiary has Part D coverage.
Creditable coverage is drug coverage that is expected to pay at least as much as Medicare's standard drug coverage on average. Employer-sponsored drug coverage is typically creditable. When a beneficiary loses creditable coverage (for example, by retiring and losing employer coverage), they have a Special Enrollment Period of 63 days to enroll in Part D without incurring the late enrollment penalty.
Employers and other entities that provide prescription drug coverage to Medicare-eligible individuals are required by CMS to notify those individuals annually whether the coverage is creditable. This notice must be provided before October 15 of each year (the start of the Medicare Annual Enrollment Period) and at certain other times. Retaining documentation of creditable coverage helps demonstrate continuous coverage and avoid late enrollment penalty disputes.
Low Income Subsidy (Extra Help)
The Low Income Subsidy (LIS), also known as Extra Help, is a federal program that reduces Part D costs for Medicare beneficiaries with limited income and resources. Extra Help covers some or all of the Part D premium, deductible, and cost-sharing amounts, depending on the beneficiary's income and resource levels.
Beneficiaries who receive Medicaid, Medicare Savings Program assistance, or Supplemental Security Income (SSI) are typically automatically eligible for Extra Help. Others may apply through the Social Security Administration. The Social Security Administration's Extra Help application is available at ssa.gov/medicare/part-d.
Beneficiaries who qualify for Extra Help are also entitled to a Special Enrollment Period each month to switch Part D plans, rather than being limited to the Annual Enrollment Period like other beneficiaries. This allows them to change to a plan that best meets their needs at any time.
Medicare Part D provides prescription drug coverage through private plans approved by CMS, available either as stand-alone PDPs paired with Original Medicare or as integrated coverage within Medicare Advantage plans. Each plan maintains its own formulary organized in cost tiers, which can change year to year. Cost-sharing is structured in phases, with the Inflation Reduction Act establishing a new out-of-pocket cap in catastrophic coverage. Missing the enrollment window without creditable coverage triggers a permanent late enrollment penalty. The Medicare Plan Finder at medicare.gov/plan-compare is the authoritative tool for comparing available Part D plans by formulary coverage and cost.