Medicare Part D is the federal government’s prescription drug coverage program for people with Medicare, offered through private insurance plans approved by Medicare. It helps pay for both brand-name and generic medications, and it is available to anyone who has Medicare Part A or Part B. Enrollment is voluntary, but if you don’t sign up when you’re first eligible and don’t have other creditable drug coverage, you may pay a late enrollment penalty that adds to your premium for as long as you have Part D.

How Medicare Part D Works: Plans, Premiums, and Coverage Stages

Medicare Part D plans are sold by private insurance companies, and each plan has its own list of covered drugs, called a formulary. Formularies are organized into tiers, with lower-tier drugs (typically generics) costing less and higher-tier drugs (specialty or brand-name) costing more. You choose a plan based on the medications you take, the pharmacies in the plan’s network, and the monthly premium.

In 2025, the average monthly premium for a Part D plan is around $35 to $55, though some plans charge as little as $7 and others more than $100. Annual deductibles can vary, but the maximum deductible allowed by Medicare is $545 in 2025. Once you meet the deductible, you pay a copay or coinsurance for each prescription until you reach the initial coverage limit. After that, you enter the coverage gap, often called the “donut hole,” where you pay a percentage of drug costs until you reach catastrophic coverage. In 2025, the donut hole is effectively closed for brand-name drugs, meaning you pay no more than 25% of the cost for brand-name medications in this phase. Once your out-of-pocket costs hit $8,000 in 2025, you enter catastrophic coverage, where you pay a small copay or coinsurance for the rest of the year.

Eligibility and Enrollment Periods

You are eligible for Medicare Part D if you are enrolled in Medicare Part A (hospital insurance) or Part B (medical insurance). Most people become eligible when they turn 65, but younger individuals with certain disabilities or end-stage renal disease may also qualify. Enrollment is not automatic unless you are also signing up for a Medicare Advantage plan that includes drug coverage (known as a Medicare Advantage Prescription Drug plan, or MA-PD).

Key enrollment periods include:

  • Initial Enrollment Period (IEP): The seven-month window around your 65th birthday—three months before, the month of, and three months after. During this time, you can sign up for Part D without penalty.
  • Annual Enrollment Period (AEP): October 15 to December 7 each year. During this period, you can switch, drop, or enroll in a Part D plan for the following year.
  • Special Enrollment Period (SEP): Triggered by life events such as moving out of your plan’s service area, losing employer-sponsored drug coverage, or qualifying for Extra Help. SEPs allow you to enroll or change plans outside the standard windows.

If you miss your IEP and do not have other creditable drug coverage (such as from an employer or union), you will pay a late enrollment penalty. This penalty is calculated by multiplying 1% of the national base beneficiary premium (which is $36.78 in 2025) by the number of full months you were eligible but not enrolled. This amount is added to your monthly premium for as long as you have Part D.

What Part D Covers and What It Doesn’t

Medicare Part D covers most commercially available prescription drugs, including those for chronic conditions like high blood pressure, diabetes, asthma, and depression. Each plan’s formulary must include at least two drugs in each therapeutic category, and all plans must cover all drugs in six “protected” classes: immunosuppressants, antidepressants, antipsychotics, anticonvulsants, antiretrovirals, and anticancer drugs. However, plans can vary in which specific drugs they include within those classes.

Part D does not cover:

  • Drugs that are not approved by the FDA
  • Over-the-counter medications (unless prescribed and covered by a specific plan)
  • Drugs used for weight loss or gain, unless for a medical condition like cachexia
  • Drugs for cosmetic purposes, such as hair growth or wrinkle treatment
  • Drugs covered under Medicare Part A or Part B (e.g., drugs administered in a hospital or doctor’s office)
  • Vitamins and supplements, except for certain prenatal vitamins or fluoride preparations

Some plans may also exclude certain high-cost specialty drugs or require prior authorization, step therapy, or quantity limits to manage costs. It is critical to check a plan’s formulary before enrolling to ensure your medications are covered.

How to Choose a Part D Plan

Choosing the right Part D plan requires comparing options based on your specific medication needs and budget. Start by listing all the prescription drugs you take, including dosages and frequencies. Then, use Medicare’s Plan Finder tool at Medicare.gov to enter your drugs and see which plans cover them at the lowest total cost. The tool shows estimated annual costs, including premiums, deductibles, and copays.

Key factors to compare include:

  • Monthly premium: The amount you pay each month to maintain coverage.
  • Annual deductible: The amount you pay out-of-pocket before the plan starts paying its share.
  • Copays and coinsurance: The cost per prescription, which varies by drug tier.
  • Pharmacy network: Some plans have preferred pharmacies where you pay lower copays. If you use a mail-order pharmacy, check if it’s in-network.
  • Star rating: Medicare rates plans from 1 to 5 stars based on member satisfaction, customer service, and drug safety. Higher-rated plans often provide better service.

If you have limited income and resources, you may qualify for Extra Help (also called the Low-Income Subsidy). This program helps pay Part D premiums, deductibles, and copays. In 2025, individuals with income below $21,870 and resources under $16,600 may qualify, with slightly higher limits for couples.

Frequently Asked Questions

What happens if I don’t enroll in Part D when I’m first eligible?

If you don’t have other creditable drug coverage (coverage that is at least as good as Medicare’s) and you delay enrollment, you will pay a late enrollment penalty. This penalty is 1% of the national base beneficiary premium ($36.78 in 2025) for each full month you went without coverage. The penalty is added to your monthly premium for as long as you have Part D.

Can I get Part D if I have a Medicare Advantage plan?

Yes, but you cannot have both a stand-alone Part D plan and a Medicare Advantage plan that already includes drug coverage. If you have a Medicare Advantage plan without drug coverage, you can add a stand-alone Part D plan. However, if your Medicare Advantage plan includes drug coverage, you cannot enroll in a separate Part D plan—you must use the drug coverage provided by your Medicare Advantage plan.

Does Part D cover insulin or vaccines?

Yes, as of 2025, Part D plans cover insulin at a maximum copay of $35 for a month’s supply, thanks to the Inflation Reduction Act. Most vaccines, including the shingles vaccine (Shingrix) and the RSV vaccine, are also covered at no cost to you if they are recommended by the CDC’s Advisory Committee on Immunization Practices. However, the COVID-19 and flu vaccines are covered under Part B, not Part D.

Conclusion

Medicare Part D provides essential prescription drug coverage for millions of Americans, helping to manage the high cost of medications. Understanding how the coverage stages work, when to enroll, and what each plan covers is key to avoiding penalties and getting the most value from your plan. By comparing plans annually using Medicare’s Plan Finder and considering your specific drug list, you can select a Part D plan that fits your health needs and budget. If you have low income, explore Extra Help to reduce your costs further. Always review your plan during the Annual Enrollment Period, as formularies and premiums change each year.