Understanding Medicare Part D: Prescription Drug Coverage Explained

If Medicare has a confusing part—and it has several—Part D is probably the most confusing. Coverage phases with different cost-sharing rules. Formularies that change every year. A donut hole that became less of a donut hole and then structurally changed again. An out-of-pocket cap that's new for 2025 and 2026. Extra Help that some people qualify for but don't know about.

And the stakes are high. If you're on expensive medications—specialty biologics, brand-name drugs, multiple chronic disease medications—Part D costs can run into the thousands. Or they can be nearly zero if you pick the right plan and understand the system.

Let's break it down from scratch with 2026 numbers.

Medicare Part D is prescription drug insurance. It's sold by private insurance companies approved by CMS. You cannot get Part D directly from Medicare—it's always through a private insurer.

There are two ways to have Part D coverage:

1. Standalone Part D plan (PDP): You buy a separate drug plan and combine it with Original Medicare (Parts A and B). You keep your medical coverage under Original Medicare and your drug coverage under the PDP. This is the route for people on Original Medicare or Original Medicare + Medigap.

2. Medicare Advantage Prescription Drug (MAPD): Most Medicare Advantage plans bundle drug coverage. If you're on MA, your drug coverage is usually already included. You cannot add a standalone PDP on top of an MAPD plan—one replaces the other.

### Who Needs Part D

Everyone on Medicare needs to think about Part D, even if they currently take no prescriptions. Here's why: if you don't enroll in Part D when first eligible and don't have other creditable drug coverage, you'll pay a late enrollment penalty every month for as long as you have Part D—and it's permanent. The penalty is 1% of the national base beneficiary premium ($38.99 in 2026) for every month without coverage.

Skip two years? That's about $9.36/month extra, forever. If you live another 20 years on Medicare, that's roughly $2,250 extra paid for a mistake you made once.

Even if you rarely use prescriptions, enrolling in a low-premium Part D plan protects you from the penalty.

### Average Part D Premiums in 2026

CMS projects the average base monthly premium for standard Part D coverage at $38.99 for 2026, up from $36.78 in 2025. But plan premiums vary widely—from under $10/month for bare-bones plans in some areas to over $100/month for plans with comprehensive formularies and low cost-sharing.

Don't pick based on premium alone. A $12/month plan that doesn't cover your $300/month specialty drug is infinitely worse than a $60/month plan that does.

Starting in 2026, Part D has been simplified from the confusing four-phase structure to essentially three phases. The old donut hole (coverage gap) has been effectively eliminated.

### Phase 1: The Deductible

In 2026, the maximum Part D deductible is $615. Plans can set their deductible lower—even to $0—but cannot exceed $615. During the deductible phase, you typically pay full drug prices until you've met the deductible amount.

Some drugs are exempt from the deductible in many plans—often Tier 1 and Tier 2 (generics and preferred generics) are covered without requiring the deductible first. Always check your specific plan's deductible structure.

### Phase 2: Initial Coverage Phase

After you've met your deductible, you enter the initial coverage phase. Here you pay 25% of the cost of covered drugs. The plan pays the other 75%.

This 25% coinsurance continues until your out-of-pocket spending hits $2,100 for 2026.

Note that what counts toward your $2,100 is your actual out-of-pocket spending—copays, coinsurance, amounts you paid during the deductible phase. The plan's share doesn't count.

### Phase 3: Catastrophic Coverage

Once you hit $2,100 in out-of-pocket drug costs, you enter catastrophic coverage for the rest of the year. In catastrophic coverage, you pay $0 for covered Part D drugs. Nothing. The plan covers everything.

This is a dramatic improvement from how Part D worked even just a few years ago. Pre-reform, the catastrophic phase still had cost-sharing (5% coinsurance or small copays). Now it's completely free.

### The Old Donut Hole: What Happened to It

Before the Inflation Reduction Act reforms phased in, Part D had a four-phase structure: deductible → initial coverage → coverage gap (donut hole) → catastrophic. The donut hole was a phase where coverage got worse before getting better again, and people paid 25% for brand-name drugs and 37% for generics.

The donut hole is effectively gone now. The reformed structure goes straight from initial coverage (25% coinsurance throughout) to catastrophic ($0) once you hit the $2,100 cap. Simpler, and much better for people on expensive medications.

### How to Hit the Cap: A Real Example

Take someone on a biologic for rheumatoid arthritis. Cost of the drug: $6,000/month.

Deductible: They pay $615 out-of-pocket first (assuming full deductible plan).

Initial coverage: They then pay 25% until their OOP reaches $2,100. After the deductible, they need to accumulate another $1,485 in OOP costs. At 25% coinsurance on a $6,000/month drug, that's $1,500/month → $1,485 to cap = approximately 1 more month.

So: by month 2 on this drug, they've hit the $2,100 cap and pay nothing for the rest of the year. Total maximum out-of-pocket: $2,100.

Old system: They could have paid $7,000–$15,000+ depending on how the drug fell under old rules. The reform is genuinely life-changing for people on specialty medications.

A formulary is the list of drugs a Part D plan covers, organized into tiers. Understanding formularies is how you pick the right plan and avoid nasty surprises.

### Typical Tier Structure

Most Part D plans use a 5-tier structure, though some plans use 6 tiers:

**Tier 1 – Preferred generics**: Usually $0–$10 copay. These are the cheapest generic drugs the plan wants you to use. If your drug has a Tier 1 generic equivalent, you want it here.

**Tier 2 – Non-preferred generics**: Usually $10–$25 copay. Still generics, but the plan is less enthusiastic about them.

**Tier 3 – Preferred brand-name drugs**: Usually $40–$85 copay. Brand-name drugs that the plan has negotiated favorable placement for.

**Tier 4 – Non-preferred brand-name drugs**: Usually $80–$150 copay. Brand-name drugs the plan covers but doesn't prefer.

**Tier 5 – Specialty drugs**: Usually 25–33% coinsurance with a minimum copay. These are biologics, cancer drugs, HIV medications, MS treatments—expensive specialty medications. Even at 25%, a $10,000/month drug means $2,500/month until you hit the OOP cap.

### Formulary Exceptions

If your drug isn't on a plan's formulary, or if it's on the formulary but at a tier that's expensive for you, you can request a formulary exception. Your doctor submits a statement explaining why you need that specific drug instead of alternatives. Plans must respond within 72 hours for standard exceptions and 24 hours for expedited exceptions.

Approvals aren't guaranteed. But for drugs with no therapeutic alternatives, plans often approve exceptions.

### Step Therapy: The Annoying One

Step therapy (sometimes called fail first) requires you to try a lower-tier or less expensive drug before the plan covers the one your doctor originally prescribed. So your doctor prescribes Drug A (expensive brand), but the plan won't cover it until you've tried Drug B (generic equivalent or similar class drug) and demonstrated it doesn't work for you.

For new Medicare enrollees, this can be frustrating if you've already been on a specific drug for years. There are override mechanisms—if you've been stable on a medication, your doctor can often get a step therapy override approved. But it requires documentation and takes time.

### Formularies Change January 1st Every Year

This is critical. Plans can change their formularies every year with the annual contract renewal. A drug that was Tier 3 this year might be Tier 4 or removed from the formulary entirely next year.

Every fall, review your plan's formulary for the coming year. Medicare.gov shows plan formularies. If your drugs are moving to a more expensive tier or being removed, AEP (October 15–December 7) is your window to switch to a plan that covers your drugs better.

Most people pick based on monthly premium. This is almost always the wrong approach.

### Step 1: List All Your Medications

### Step 2: Use Medicare.gov's Plan Finder

Go to Medicare.gov → Find a Part D or Medicare Advantage plan → Drug Plan (Part D). Enter your zip code and your drug list. The tool will show you your estimated annual cost under each available plan—premium + deductible + copays for your specific drugs.

This is the only right way to compare plans. Total annual cost matters, not just the monthly premium.

### Step 3: Check Your Pharmacy

Most Part D plans have preferred pharmacy networks where you pay less. Make sure your pharmacy—or a pharmacy convenient to you—is in the plan's preferred network. Paying non-preferred pharmacy prices can add $20–$100+ per refill.

Mail-order pharmacy is often cheapest for maintenance medications. Most plans offer 90-day mail-order supplies at reduced cost-sharing. If you're on multiple regular medications, the savings can be substantial.

### Step 4: Look Beyond This Year

If you're starting a new high-cost drug treatment, think about how the plan covers that drug specifically. Look at the full year cost—not just monthly premiums. A plan with a slightly higher premium that gets you to the $2,100 OOP cap faster might actually cost you less overall.

### The Low-Premium Trap

Here's a scenario that plays out constantly. Someone picks a $12/month Part D plan. Their only medication is a blood pressure drug they get for $4 at Walmart. For them, that's probably fine—the $12 plan protects them from the late enrollment penalty.

But two years later they're diagnosed with Type 2 diabetes and prescribed a GLP-1 medication. Suddenly that $12/month plan has the drug on Tier 5 with 33% coinsurance. AEP isn't for another 8 months.

You can switch plans every year at AEP. But you can't switch mid-year except with a qualifying SEP. The plan you choose in October affects you all of the following year.

Extra Help (also called the Low Income Subsidy, or LIS) is a federal program that pays some or all of your Part D costs if you have limited income and resources. It's significantly underutilized—hundreds of thousands of people who qualify aren't enrolled.

### 2026 Eligibility

You may qualify for Extra Help if your annual income is below 150% of the federal poverty level and your resources (savings, investments—not counting your home or car) are limited. For 2026:

If you're already enrolled in Medicaid, Supplemental Security Income (SSI), or Medicare Savings Programs, you automatically qualify for Extra Help.

### What Extra Help Covers

Extra Help covers Part D premiums, deductibles, and most cost-sharing. With full Extra Help:

For someone on multiple medications, this can be worth thousands of dollars per year.

### How to Apply

Apply through Social Security Administration—online at SSA.gov, by calling 1-800-772-1213, or at a local Social Security office. You can also apply through your state Medicaid office.

Apply even if you're not sure you qualify. The income and resource calculations have nuances—not everything counts as a resource. Your state's SHIP (State Health Insurance Assistance Program) counselor can help you determine eligibility for free.

### The Automatic Enrollment

If you're already on Medicaid, SSI, or a Medicare Savings Program, CMS will automatically enroll you in a benchmark Part D plan. But you're allowed to choose a different plan that might better cover your specific medications. Automatic enrollment isn't always optimal—compare the auto-assigned plan against others using Medicare.gov's drug plan finder.

About 89% of Medicare Advantage plans include Part D drug coverage—these are called MAPD plans. If you're on a MAPD, your drug coverage operates under the same general Part D rules (formularies, the $2,100 OOP cap, coverage phases) but administered by your MA plan.

### One Card, One Plan

The practical advantage is simplicity. One insurance card, one premium payment (often $0 or very low), medical and drug coverage bundled. For people who don't want to manage multiple separate policies, this is appealing.

### MA Drug Coverage vs Standalone PDP

The trade-off is flexibility. If you're on an MA plan and the drug coverage isn't great—maybe your specialty drug is on Tier 5 with high cost-sharing—you can't swap just the drug plan without swapping the whole MA plan. With Original Medicare + standalone PDP, you could switch your drug plan at AEP while keeping your medical coverage unchanged.

### Creditable Coverage Notice

Every year, MA plans must tell members whether the drug coverage is creditable—meaning as good as standard Medicare drug coverage. Almost all MAPD plans qualify. If yours doesn't (rare but possible), you'd need a standalone PDP to avoid late enrollment penalties, which you can't add to an MAPD. This situation is uncommon but worth knowing.

### When You Leave a MAPD

If you switch from an MAPD back to Original Medicare during AEP or MA OEP, you can and should enroll in a standalone Part D plan simultaneously. Your MAPD drug coverage ends when MA coverage ends (December 31 for AEP changes). Don't create a gap.

Like Part B, Part D has income-related surcharges for higher earners. These are called IRMAA (Income-Related Monthly Adjustment Amount) surcharges and they're determined by your income from two years prior.

### 2026 Part D IRMAA Surcharges

| 2024 Income (Single) | 2024 Income (Joint) | Monthly Part D Surcharge | |---|---|---| | Up to $106,000 | Up to $212,000 | $0 | | $106,001–$133,000 | $212,001–$266,000 | $14.50 | | $133,001–$167,000 | $266,001–$334,000 | $37.60 | | $167,001–$200,000 | $334,001–$400,000 | $60.50 | | $200,001–$500,000 | $400,001–$750,000 | $83.70 | | Above $500,000 | Above $750,000 | $91.00 |

The IRMAA surcharge is on top of whatever premium your Part D plan charges. If you're in the highest IRMAA bracket and paying $38.99 for your plan, your actual monthly Part D cost is $38.99 + $91.00 = $129.99.

### Appealing IRMAA

IRMAA is based on 2024 income for 2026 calculations. If your income has significantly dropped since then—due to retirement, job loss, divorce, or death of a spouse—you can appeal using Form SSA-44. CMS will use your more recent income instead. This can save hundreds per year.

After seeing how this system works for a lot of people, the same mistakes come up again and again.

### Mistake 1: Not Enrolling When First Eligible

The late enrollment penalty is permanent. Even if you take no medications now, enroll in a low-premium plan to protect yourself.

### Mistake 2: Picking Based on Premium Alone

A $10/month premium plan that doesn't cover your drugs costs more than a $50/month plan that does. Use Medicare.gov's drug plan finder to compare total annual cost.

### Mistake 3: Not Checking the Formulary Every Year

Formularies change January 1st. Read your Annual Notice of Change. If your drugs are moving to a higher tier or being dropped, switch during AEP.

### Mistake 4: Ignoring Mail-Order Savings

For maintenance medications, mail-order pharmacy typically offers 90-day supplies at reduced cost-sharing. If you're paying Tier 3 or Tier 4 copays for a drug you take daily, the difference between retail 30-day fills and mail-order 90-day supplies can be $50–$150 per refill.

### Mistake 5: Not Applying for Extra Help

If your income is under $22,590 (individual) or $30,660 (couple), apply for Extra Help. Hundreds of thousands of eligible people aren't enrolled. The application takes about 15 minutes at SSA.gov.

### Mistake 6: Assuming Your MA Plan's Drug Coverage Is Optimal

Just because drug coverage is bundled into your MA plan doesn't mean it's the best match for your specific drugs. During AEP, use the drug plan finder to see if switching to an MA plan with different drug coverage (or switching to Original Medicare + standalone PDP) would save money.