Part D Prescription Drug Coverage

What it is

Part D is Medicare's outpatient prescription drug benefit, delivered by private insurance companies approved by Medicare. Original Medicare (Parts A and B) does not cover most prescriptions you take at home, so Part D fills that gap.

There are two ways to get it:

• Stand-alone Prescription Drug Plan (PDP): added on top of Original Medicare (and often paired with a Medicare Supplement, which has no drug coverage of its own).
• Built into a Medicare Advantage plan (MAPD): the drug coverage is bundled with the medical plan, so no separate PDP is needed.

If someone has Original Medicare + a Medigap policy, they almost always need a separate Part D plan, because Medigap does not cover drugs. If someone has an MA-Only plan (no built-in drugs), whether they can add a stand-alone PDP depends on the plan type — confirm the rules.

Key concepts

Formulary (the drug list)

Each Part D plan has a formulary — the list of drugs it covers, usually organized into tiers (generics on lower/cheaper tiers, brand-name and specialty drugs on higher tiers). Two plans can cover the same drug at very different costs, which is why matching the plan to a person's actual medications and pharmacies matters so much.

Cost-sharing structure

A Part D plan's cost depends on its premium, any annual deductible, and the copay/coinsurance for each drug tier. Because exact dollar figures and the phases of Part D change every year, consumers should verify current amounts at Medicare.gov rather than rely on prior-year numbers.

Special Needs Plans always include drugs

Unlike standard MA plans, all Special Needs Plans (D-SNP, C-SNP, I-SNP) must include Part D coverage. See Special Needs Plans — D-SNP, C-SNP & I-SNP.

Creditable coverage and the late-enrollment penalty

If a person doesn't sign up for Part D when first eligible and goes without "creditable" drug coverage (coverage at least as good as Part D — for example, certain employer or VA drug benefits) for a period of time, they can owe a late-enrollment penalty that is added to their premium for as long as they have Part D. The lesson for consumers: enroll on time, or keep documented creditable coverage.

Help paying for Part D

Lower-income beneficiaries may qualify for assistance with Part D costs, sometimes called Extra Help or the Low-Income Subsidy (LIS). People who are dual eligible (have both Medicare and Medicaid) typically receive significant help with drug and other costs (see Medicaid & Dual-Eligible Levels at a Glance and Special Needs Plans — D-SNP, C-SNP & I-SNP).

When you can enroll or change Part D

Part D enrollment follows the same major Medicare windows:

• Initial Enrollment Period around turning 65
• Annual Enrollment Period (Oct 15–Dec 7): join, switch, or drop a Part D / MAPD plan, effective Jan 1
• MA Open Enrollment Period (Jan 1–Mar 31): those leaving an MA plan can pick up a stand-alone Part D plan
• Special Enrollment Periods for qualifying life events

Full detail in Medicare & ACA Enrollment Windows.

How to choose a Part D plan (consumer view)

Rather than chase the lowest premium alone, compare plans based on:

1. Are your specific drugs on the formulary?
2. What tier are they on, and what's the copay/coinsurance?
3. Is your pharmacy preferred (in-network at the best price)?
4. Total annual cost — premium + deductible + expected drug costs, not just the monthly premium.

A plan with a slightly higher premium can be cheaper overall if it covers your medications on better tiers.

See also: Medicare Basics — Parts A, B, C & D, Medicare Advantage (Part C), Medicare Supplement (Medigap) Plans & Underwriting, Medicare & ACA Enrollment Windows, Special Needs Plans — D-SNP, C-SNP & I-SNP.