Medicare Part D Prescription Drug Plans
Medicare Part D helps cover outpatient prescription medications. Coverage is available through a stand-alone prescription drug plan or through certain Medicare Advantage plans that include drug coverage.
What Is Medicare Part D?
Medicare Part D provides prescription drug coverage for Medicare beneficiaries. It is available through two types of plans: standalone Prescription Drug Plans (PDPs), which are used alongside Original Medicare and Medicare Supplement policies, and Medicare Advantage Prescription Drug plans (MAPDs), which combine Part A, Part B, and drug coverage into a single plan.
Part D plans are offered by private insurance companies that contract with Medicare. Each plan has its own formulary — a list of covered drugs — as well as its own cost-sharing structure, pharmacy network, and tier assignments. No two Part D plans are identical, and the plan that works best for one person may not be the best fit for another.
Reviewing your specific medications — including dosages, quantities, and preferred pharmacies — is essential when comparing Part D plans. A plan with a lower monthly premium may result in higher total annual costs if your medications fall on expensive tiers or are subject to prior authorization requirements.
Key Features of Part D Drug Plans
Part D plans share a general structure defined by Medicare regulations, but individual plan terms vary significantly. The following describes how Part D generally works.
Formularies
Each Part D plan maintains a formulary — a list of prescription drugs covered by the plan. Drugs are typically organized into cost-sharing tiers. Not all drugs are covered by every plan, and coverage for the same drug can vary significantly between plans. Always verify that your current medications are on a plan's formulary before enrolling.
Drug Cost-Sharing Tiers
Most Part D plans assign drugs to tiers, with different copayments or coinsurance at each tier. Tier 1 drugs (typically generic medications) generally have the lowest cost-sharing; higher tiers typically involve greater cost. Specialty and high-cost medications are often placed in the highest tiers with the highest cost-sharing. Tier placement varies by plan and formulary.
Pharmacy Networks
Part D plans contract with specific pharmacies. Using a preferred in-network pharmacy generally results in lower cost-sharing than using an out-of-network or non-preferred pharmacy. Plans may also offer mail-order pharmacy options, which can provide additional savings for maintenance medications. Confirm your preferred pharmacy's network status for each plan before enrolling.
Deductibles
Many Part D plans have an annual deductible — an amount you pay out of pocket before the plan begins covering prescription costs. Some plans waive the deductible for certain drug tiers. Medicare sets the maximum allowable deductible each year; not all plans charge the maximum. Review the deductible along with your expected drug costs to assess overall plan value.
Prior Authorization, Step Therapy & Quantity Limits
Plans may require prior authorization before covering certain drugs — meaning the plan must approve coverage before you fill a prescription. Step therapy requirements may require you to try a lower-cost drug before the plan will cover a more expensive alternative. Quantity limits may restrict how much of a drug the plan will cover within a set period. Review these restrictions for your specific medications when comparing plans.
Formulary Changes During the Year
Part D formularies can change during the plan year, subject to Medicare rules. A drug that is covered when you enroll may be removed from the formulary or moved to a higher tier during the plan year. Plans must generally provide notice before making mid-year formulary changes and must ensure continuity of coverage in certain circumstances.
Evaluating Part D Plans
A thorough comparison of Part D plans requires reviewing your specific medications, dosages, quantities, and preferred pharmacies alongside each plan's terms. A plan with a low premium is not always the lowest total annual cost.
- Whether each medication is covered: Confirm each of your current prescriptions is on the plan's formulary at a covered tier.
- Drug tier placement: Understand which tier each of your medications falls on and the associated cost-sharing for each tier.
- Estimated annual drug cost: Calculate total expected out-of-pocket drug costs for the year based on your specific medications and quantities — not just the monthly premium.
- Monthly premium: The premium you pay each month for the plan, in addition to any Medicare Part B premium.
- Deductible: Whether the plan has a deductible, and for which drug tiers it applies.
- Preferred and standard pharmacies: Whether your preferred pharmacy is in-network and whether it is a preferred or standard pharmacy — which affects your cost-sharing.
- Mail-order options: Whether the plan offers a mail-order pharmacy and any associated cost savings for maintenance medications.
- Prior authorization requirements: Whether any of your current medications require prior authorization under each plan.
- Step therapy requirements: Whether the plan requires trying an alternative medication before covering your current prescription.
- Quantity limits: Whether the plan restricts how much of a drug it will cover per fill or per time period.
What to Keep in Mind
- A low monthly premium does not necessarily mean the lowest total annual cost. High-tier drug cost-sharing can far exceed premium savings.
- Medication coverage can differ significantly between plans — two plans with similar premiums may have very different drug formularies and tier structures.
- Your pharmacy choice can significantly affect what you pay. Using a non-preferred pharmacy can result in substantially higher cost-sharing.
- Formularies can change during the plan year, subject to Medicare rules. Review your Annual Notice of Change each fall.
- You should not enroll in a standalone Part D plan if you already have Medicare Advantage that includes drug coverage — this can trigger enrollment rules and complications.
When You Can Enroll in Part D
Part D enrollment is generally limited to specific periods. Enrolling late without qualifying coverage can result in a permanent late enrollment penalty.
Initial Enrollment Period (IEP)
The 7-month window centered around your 65th birthday — three months before, the month of, and three months after — is generally when you first become eligible to enroll in a Part D plan. Enrolling during your IEP helps avoid late enrollment penalties if you do not have other creditable drug coverage.
Annual Enrollment Period (AEP)
October 15 – December 7 each year. During AEP, Medicare beneficiaries can join, switch, or drop a standalone Part D plan. Changes generally take effect January 1 of the following year. This is the primary opportunity to change your Part D coverage each year based on your current medications and available plans.
Special Enrollment Periods (SEPs)
Qualifying events — such as losing creditable prescription drug coverage, moving to a new service area, or gaining or losing eligibility for low-income subsidy programs — may trigger a Special Enrollment Period allowing you to join or switch a Part D plan outside of regular enrollment windows. The specific SEP available and its duration depend on your circumstances.
Late Enrollment Penalty Risk
If you go 63 or more consecutive days without Medicare Part D coverage or other creditable prescription drug coverage after your Initial Enrollment Period, you may owe a late enrollment penalty added permanently to your Part D monthly premium. The penalty amount is based on how long you went without creditable coverage. Employer plans, TRICARE, and certain other coverage may qualify as creditable — verify your coverage's creditable status before delaying Part D enrollment.
Medicare Part D — Frequently Asked Questions
Do I need Part D if I take no prescriptions currently?
Enrolling in Part D when you first become eligible — even if you take no prescriptions — can help you avoid the late enrollment penalty if you develop a need for prescription coverage in the future. If you go 63 or more days without creditable drug coverage after your Initial Enrollment Period, a permanent late enrollment penalty may apply when you do eventually enroll. Whether to enroll depends on your individual situation, including whether you have other creditable prescription drug coverage from another source.
Can I use any pharmacy with Part D?
Most Part D plans require you to use in-network pharmacies. Using an out-of-network pharmacy — if the plan covers it at all — typically results in higher cost-sharing or no coverage. Within the network, plans may distinguish between preferred and standard pharmacies, with lower cost-sharing at preferred pharmacies. Always confirm your preferred pharmacy's network status for any plan you are considering before enrolling.
What is a formulary?
A formulary is a list of prescription drugs covered by a Part D plan. Not all drugs are on every plan's formulary, and the tier placement of a covered drug determines your cost-sharing for that medication. Plans organize drugs into tiers — typically ranging from low-cost generics to high-cost specialty medications — with different copay or coinsurance amounts at each tier. Before enrolling in any Part D plan, verify that each of your current medications is on the plan's formulary and review its tier and associated cost.
What is creditable drug coverage?
Creditable drug coverage is prescription drug coverage from a source other than Medicare Part D that is at least as good as the standard Medicare Part D benefit. Examples may include coverage from an employer or union plan, TRICARE, or certain other sources. If your coverage is creditable, you can delay enrolling in Part D without incurring a late enrollment penalty for as long as you maintain that creditable coverage. Your plan should notify you annually whether your coverage is creditable. Keep these notices — you may need them if you later enroll in Part D.
Can I change Part D plans every year?
Yes. During the Annual Enrollment Period (October 15 – December 7 each year), you can switch from one standalone Part D plan to another, drop Part D coverage, or join a Part D plan if you are not currently enrolled. Changes take effect January 1. Reviewing your plan annually is important because formularies, premiums, and pharmacy networks can change from year to year — a plan that worked well this year may not be the best fit next year given changes to coverage or your medications.
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