Medicare Part D: How to Choose the Right Prescription Drug Plan

Navigating enrollment periods, formulary tiers, and plan comparison to find the coverage that actually saves you the most money on your medications.

What Medicare Part D Covers

Medicare Part D is the federal prescription drug benefit available to anyone enrolled in Medicare. Unlike Parts A and B, which are administered directly by the federal government, Part D plans are offered by private insurance companies that contract with Medicare. This means there is no single "Medicare drug plan" — instead, you choose from dozens of competing plans in your area, each with its own premium, formulary, pharmacy network, and cost-sharing structure.

Part D plans must meet minimum standards set by Medicare, including coverage for drugs in every therapeutic category. However, individual plans have significant latitude in which specific drugs they cover, what tier they place them on, and which pharmacies are in-network. Two plans in the same zip code can produce wildly different out-of-pocket costs for the same medication, which is why comparing plans carefully — rather than simply picking the one with the lowest premium — can save you hundreds or even thousands of dollars per year.

You can get Part D coverage in two ways: through a standalone Prescription Drug Plan (PDP) that supplements Original Medicare, or through a Medicare Advantage plan (Part C) that bundles drug coverage with medical coverage. The enrollment rules and plan comparison process apply to both.

Enrollment Periods: When You Can Sign Up or Switch

Medicare Part D has specific enrollment windows, and missing them can cost you money permanently. The Initial Enrollment Period (IEP) is a seven-month window surrounding your 65th birthday — it starts three months before your birthday month, includes your birthday month, and extends three months after. If you are new to Medicare due to disability, your IEP works similarly around your 25th month of disability benefits.

The Annual Enrollment Period (AEP) runs from October 15 through December 7 every year. During this window, anyone with Medicare can join a Part D plan, switch plans, or drop coverage entirely. Changes made during AEP take effect January 1 of the following year. This is your main opportunity each year to review whether your current plan still offers the best deal for your medications.

Special Enrollment Periods (SEPs) are available in certain circumstances, such as moving to a new service area, losing employer coverage, qualifying for Extra Help (Low-Income Subsidy), or being affected by a natural disaster. If you qualify for a SEP, you can make changes outside the standard enrollment windows.

Understanding Formulary Tiers and Cost Sharing

Every Part D plan maintains a formulary — a list of covered medications organized into tiers. While plans can structure tiers differently, a common arrangement includes five tiers. Tier 1 covers preferred generics at the lowest copays, often $0 to $10. Tier 2 covers non-preferred generics, typically $10 to $25. Tier 3 includes preferred brand-name drugs at moderate copays or coinsurance. Tier 4 covers non-preferred brand-name drugs at higher cost sharing. Tier 5, the specialty tier, is reserved for very expensive medications and usually requires coinsurance of 25% to 33% rather than a flat copay.

The placement of your specific medication on a plan's formulary has a much bigger impact on your costs than the plan's premium. A plan with a $30 monthly premium that puts your cholesterol medication on Tier 4 at $90 per fill will cost you far more annually than a plan with a $45 monthly premium that covers the same drug on Tier 2 at $15 per fill. This is why the "cheapest plan" is not necessarily the plan with the lowest sticker price.

When comparing plans, use Medicare's Plan Finder tool at medicare.gov. Enter your specific medications, doses, and preferred pharmacy, and the tool will estimate your total annual cost — premiums plus out-of-pocket drug costs combined — for every plan available in your area. Sorting by total estimated annual cost rather than by premium alone will give you a much more accurate picture of which plan is actually the best value.

The Donut Hole and How the IRA Changed It

Part D has a unique coverage structure that catches many beneficiaries off guard. After you meet your annual deductible, you enter the initial coverage phase where you pay your normal copays or coinsurance. Once the combined total of what you and your plan have spent on drugs reaches a certain threshold (around $5,030 in 2026), you enter the coverage gap — historically called the "donut hole."

The Inflation Reduction Act of 2022 dramatically changed this landscape. Starting in 2025, Medicare Part D now has a $2,000 annual out-of-pocket cap for prescription drugs. Once you have spent $2,000 out of pocket in a calendar year, you pay nothing more for covered medications for the rest of the year. This is an enormous change for beneficiaries who take expensive specialty drugs or multiple brand-name medications. Before this cap existed, some beneficiaries faced thousands of dollars in donut hole costs with no ceiling on their exposure.

Extra Help: The Program Many People Miss

Medicare's Extra Help program, also known as the Low-Income Subsidy (LIS), pays part or all of Part D premiums, deductibles, and copays for beneficiaries with limited income and resources. The program has two levels: full Extra Help for those with the lowest incomes, and partial Extra Help for those who earn somewhat more. Under full Extra Help, copays can drop to as little as $0 for generic drugs and a few dollars for brand-name medications.

What many people do not realize is that the income limits for Extra Help are higher than you might assume. In 2026, individuals with annual incomes below roughly $22,000 and limited assets may qualify. Married couples with combined incomes below approximately $30,000 may also be eligible. The application is free, and you can apply through Social Security's website, by calling Social Security at 1-800-772-1213, or by contacting your State Health Insurance Assistance Program (SHIP) for free local help.

Common Enrollment Mistakes to Avoid

The most expensive mistake is not enrolling at all when you first become eligible — the late enrollment penalty never goes away and compounds over time. The second most common mistake is choosing a plan based solely on premium without checking whether your drugs are on the formulary or what tier they fall on. A $0-premium plan is not a bargain if it does not cover your medications or places them on expensive tiers.

Another frequent error is failing to re-evaluate your plan during the Annual Enrollment Period each fall. Plans change their formularies, copay structures, and pharmacy networks every year. The plan that was the best deal last year may no longer be competitive. Taking 30 minutes each October to run your medications through Plan Finder can easily save you $500 to $1,500 annually. Finally, many people do not realize they can use tools like RxGator to compare pharmacy cash prices alongside their Part D coverage — in some cases, paying cash with a discount program costs less than using your insurance copay, especially for inexpensive generics.