Medicare Part D: How Prescription Drug Coverage Works

Medicare Part D covers prescription drugs. It is offered through private insurance companies approved by Medicare and comes with its own enrollment rules, deadlines, and choices. If you take regular medications — or expect to — understanding Part D before you enroll can save you significant money.

How Part D Works

Part D plans are sold by private insurance companies that have been approved and regulated by Medicare. You choose a plan, pay a monthly premium, and the plan helps cover the cost of your prescriptions according to its specific formulary (the list of drugs it covers) and cost-sharing structure.

What Part D covers varies by plan, but all plans are required to cover at least a standard set of drugs across common categories. Each plan publishes its formulary — the list of drugs it covers and at what cost tier — so you can check before enrolling whether your specific medications are covered.

Standalone Part D vs. Medicare Advantage

How you get Part D depends on how you receive your Medicare coverage:

• Original Medicare (Parts A and B): You add a standalone Part D plan separately. You choose one plan from those available in your area.
• Medicare Advantage (Part C): Most Medicare Advantage plans include drug coverage already. Check your plan's specific drug formulary — not all Advantage plans cover the same medications.

If you have Original Medicare and do not add Part D, and you do not have other qualifying drug coverage (such as from an employer or the VA), you may face a late enrollment penalty when you do eventually sign up.

The Late Enrollment Penalty

If you go 63 or more consecutive days without creditable drug coverage after first becoming eligible for Part D, Medicare adds a penalty to your monthly premium. This penalty is calculated based on how long you went without coverage and stays with you permanently.

The penalty is avoidable. If you have qualifying coverage from another source — an employer plan, VA benefits, TRICARE — you do not need to enroll in Part D right away. But confirm in writing that your other coverage is creditable before deciding to delay.

Choosing a Part D Plan

Not all Part D plans are the same, and the plan that works best for you depends on which drugs you take. Before enrolling:

1. Make a list of every prescription drug you currently take, including the dosage
2. Go to the plan finder at medicare.gov and enter your drugs
3. The tool will show you which plans cover your medications and estimate your total annual cost — premium plus out-of-pocket — so you can compare apples to apples
4. Check that your preferred pharmacy is in-network for any plan you consider

The lowest monthly premium is not always the lowest total cost. A plan with a slightly higher premium but lower copayments for your specific drugs may cost less overall.

The Extra Help Program

If your income and resources are limited, you may qualify for Extra Help — a federal program that reduces Part D premiums, deductibles, and copayments significantly.

Extra Help is sometimes called the Low-Income Subsidy (LIS). Eligibility is determined by income and assets, and applications are accepted year-round through the Social Security Administration at ssa.gov/extrahelp.

If you think you might qualify, apply — the savings can be substantial, and eligibility does not require a complicated process.

Reviewing Your Coverage Each Year

Part D plans can change their premiums, formularies, and cost-sharing each year. A plan that covered your medications well this year may not be the best option next year.

During the annual Open Enrollment Period (October 15 through December 7), review your plan's Annual Notice of Change and compare it to other available plans using the medicare.gov plan finder. Changes take effect January 1.