What Medicare Part D Is and How It Works

Medicare Part D is the prescription drug coverage portion of Medicare. It is offered through private insurance companies that Medicare approves, and it helps pay for outpatient medications you pick up at a pharmacy or order through a mail-order service. Original Medicare (Part A and Part B) does not cover most outpatient prescriptions, so Part D fills that gap. If you take any medication regularly, or even occasionally, understanding how Part D works can save you significant money and prevent costly coverage gaps later.

You can get Part D coverage in one of two ways. The first is a standalone Part D plan (PDP), which pairs with Original Medicare or a Medicare Supplement plan. The second is a Medicare Advantage plan with prescription drug coverage (MAPD), which bundles medical and drug coverage into a single plan. Both options must include a formulary, which is the list of drugs the plan covers.

Eligibility, Enrollment Windows, and Late Penalties

You become eligible for Part D when you qualify for Medicare, which for most people means turning 65. You can also qualify earlier if you have certain disabilities, end-stage renal disease, or ALS. To enroll, you need to be enrolled in either Medicare Part A or Part B.

The most common time to sign up is during your Initial Enrollment Period (IEP), a seven-month window that begins three months before your 65th birthday month and ends three months after. If you miss this window and do not have other "creditable" drug coverage (coverage at least as good as Part D's standard), you may face a late enrollment penalty.

The penalty is calculated as 1% of the current national base premium for each month you went without creditable coverage, and that amount is added to your premium permanently. In 2025, the base premium is around $36.78, so even a short delay can add several dollars per month for life.

Other enrollment opportunities include the Annual Election Period (AEP), which runs from October 15 to December 7 each year, when you can switch, drop, or join a Part D plan for the following year. There is also a Medicare Advantage Open Enrollment Period from January 1 to March 31 for those already in an MAPD plan, and Special Enrollment Periods triggered by life events such as moving or losing other coverage.

Formularies, Tiers, and the Coverage Stages

Every Part D plan has a formulary, organized into tiers that determine your out-of-pocket cost. A typical structure looks like:

  • Tier 1: Preferred generic drugs, lowest copays (often under $5).
  • Tier 2: Generic drugs, slightly higher copays.
  • Tier 3: Preferred brand-name drugs, moderate copays or coinsurance.
  • Tier 4: Non-preferred brand drugs, higher cost-sharing.
  • Tier 5: Specialty drugs, the highest cost-sharing, often a percentage of the drug's full price.

Plans can change their formularies each year, so it pays to review your plan's drug list every fall. A medication you take may move tiers, lose coverage, or be replaced with a similar alternative.

Part D also uses a structured payment system with four coverage stages:

  • Deductible stage: You pay the full cost of your drugs until you meet the plan's deductible. Many plans have no deductible for Tier 1 generics.
  • Initial coverage stage: You pay copays or coinsurance, and the plan pays the rest, until your total drug costs (what you plus the plan have paid) reach a set cap. In 2025, that cap is $2,000.
  • Catastrophic stage: After the cap, you pay nothing for covered drugs for the rest of the year. This stage was expanded under the Inflation Reduction Act, eliminating the previous 5% coinsurance requirement.

Note that the "donut hole" or coverage gap stage still exists in name for accounting purposes, but under current rules there is no cost-sharing penalty while you are in it. Once your out-of-pocket spending reaches the catastrophic threshold, your payments drop to zero for covered medications.

How to Compare and Choose a Part D Plan

The best Part D plan depends on the specific drugs you take, your preferred pharmacy, and how often you fill prescriptions. The official Medicare Plan Finder at Medicare.gov lets you enter your medications, dosage, and pharmacy preference, then sorts available plans by estimated annual cost. This is the most reliable tool because it uses the plan's actual negotiated prices for your drugs in your ZIP code.

When comparing plans, look beyond just the monthly premium. A plan with a lower premium may have a higher deductible, stricter formulary, or steeper tier pricing, which can end up costing more if you take expensive brand-name drugs. Also check:

  • Pharmacy network: Using a preferred in-network pharmacy can cut copays significantly.
  • Mail-order options: Useful for maintenance medications, often at a 90-day supply discount.
  • Prior authorization and step therapy rules: Some plans require you to try a cheaper drug first before covering a more expensive option.
  • Quantity limits: Plans may cap how much of a drug you can get at one time.

If you travel or live in multiple states, confirm the plan's service area and whether it offers nationwide pharmacy networks.

Programs That Can Lower Part D Costs

If your income is limited, several programs can reduce Part D expenses. The federal Extra Help program (also called the Low-Income Subsidy) helps pay premiums, deductibles, and copays for people with limited income and resources. If you qualify, you may pay very little for generic drugs and nothing for catastrophic coverage. Eligibility is based on income and asset limits that the Social Security Administration sets, and you can apply online at ssa.gov.

Many drug manufacturers also offer patient assistance programs that provide brand-name medications at low or no cost if you qualify. In addition, some states run State Pharmaceutical Assistance Programs (SPAPs) that wrap around Part D to cover additional costs. If you take a specialty drug, ask your prescriber or pharmacist about manufacturer copay cards, though be aware that those discounts may not count toward your out-of-pocket catastrophic spending in the same way plan payments do.

Another tool under the Inflation Reduction Act is the negotiated drug price program, in which Medicare negotiates lower prices on a growing list of high-cost drugs. In 2026, a new batch of medications will join those already receiving negotiated prices, which can mean lower out-of-pocket costs on common Part D drugs.

Practical Tips for Managing Part D Year to Year

The single most useful habit is reviewing your plan every fall during the Annual Election Period, even if you are happy with it. Formularies, premiums, and your own medications can all change, and a different plan may save you hundreds of dollars. Many people who skip this review end up overpaying simply because they never switched.

Bring an updated medication list, including dosages and pharmacy details, to any plan comparison. If you have started a new medication, see how it is covered before you fill the prescription. If your doctor suggests a drug that is expensive or not on formulary, ask about therapeutic alternatives or whether an exception request is possible. Part D plans are required to have an exceptions process, and your prescriber can submit supporting documentation to request coverage.

Finally, pay attention to your Explanation of Benefits statements. They show what stage you are in, how much has been spent, and what your next refill will cost. This small step can help you avoid surprises at the pharmacy counter and plan ahead if you expect to reach the catastrophic threshold during the year.