Medicare Part D: The Complete Guide (2027)

What Part D Actually Is

Medicare Part D is prescription drug coverage. Original Medicare (Part A and Part B) covers very few drugs you take at home, so Part D is how most people with Medicare pay for their prescriptions. It is run by private insurers under Medicare's rules, and every plan has its own premium, its own list of covered drugs and its own pharmacy network.

There are two ways to get it.

• A standalone Medicare drug plan. You add it to Original Medicare. You need Part A, Part B or both to join one.
• A Medicare Advantage Plan that includes drug coverage. Your hospital, medical and drug coverage all come from one plan. You need both Part A and Part B to join, and not every Advantage plan includes drugs.

Either way, you must live in the plan's service area and be lawfully present in the United States.

Part D is optional. That word does a lot of damage. Nobody makes you enroll, but if you skip it when you are first eligible and you do not have other drug coverage that counts, you can owe a penalty for the rest of your life once you do join. Most of the expensive mistakes on this page come from treating "optional" as "safe to ignore."

What You Pay

Your Part D costs come in four layers. Every plan uses some version of all four, and plans can change any of them each January.

• Monthly premium. Set by the plan. Some plans charge nothing, others charge much more. If you have Part B, this is on top of your Part B premium.
• Yearly deductible. What you pay before the plan starts paying its share. Some plans have no deductible, and some cover lower-tier drugs before the deductible is met. Medicare sets the maximum a plan can charge.
• Copayments or coinsurance. Your share of each covered prescription after the deductible. The amount depends on the drug's tier and the pharmacy you use.
• The out-of-pocket cap. Once your spending on covered drugs reaches the yearly limit, you pay nothing more for covered drugs for the rest of the calendar year.

Plans can design their cost sharing differently as long as it is worth at least as much as the standard benefit Medicare defines. That is why two plans with the same premium can produce very different bills for the same prescriptions. The premium is the least informative number on the page. What you actually pay depends on whether your drugs are on the plan's list, which tier they sit in, and which pharmacy you use.

Your plan sends two documents every fall that settle these questions for the coming year: the Annual Notice of Change, which lists what is changing in January, and the Evidence of Coverage, which spells out what the plan covers and what you pay. If they have not arrived by early fall, call the plan.

The Out-of-Pocket Cap

Part D now has a hard ceiling on what you pay for covered drugs in a calendar year: $2,100 in 2026 and $2,400 in 2027. Once you reach it, you pay $0 for covered Part D drugs for the rest of that year.

The cap replaced the old benefit structure. If you remember a "coverage gap" or "donut hole" where costs suddenly jumped mid-year, that phase no longer exists. Since 2025, the structure is deductible, then your share of each prescription, then the cap.

What counts toward the cap: what you pay for covered drugs, meaning your deductible and your copayments or coinsurance. Certain payments made on your behalf, like Medicare's Extra Help, count too.

What does not count: your monthly premiums, drugs your plan does not cover, and any IRMAA surcharge. Someone paying $60 a month in premiums still pays that premium after reaching the cap.

The cap resets every January 1. Hitting it in March means a very cheap April through December, and then the count starts over.

Insulin and Vaccines

Two cost rules sit outside the normal deductible-and-coinsurance math.

Insulin. A Part D plan cannot charge more than $35 for a one-month supply of each covered insulin, and in some cases it must charge less. You do not pay a deductible for it. The limit applies to injectable insulin, inhaled insulin, and insulin used with pumps that are not durable medical equipment. Part D also covers certain injection supplies like syringes, gauze and alcohol swabs, but supplies that contain no insulin follow your plan's normal cost sharing, not the insulin limit. Insulin used in a traditional pump that counts as durable medical equipment falls under Part B, which has a similar limit.

Vaccines. Adult vaccines recommended by the CDC's Advisory Committee on Immunization Practices, like the shingles vaccine, are generally covered under Part D at no cost to you. Some vaccines, like flu, pneumonia, hepatitis B and COVID-19 shots, are covered under Part B instead.

The Medicare Prescription Payment Plan

Every Part D plan must offer an option to spread your out-of-pocket drug costs across the calendar year in monthly bills instead of paying at the pharmacy counter.

It changes when you pay, not how much. It does not lower your drug costs or save you money. It is most useful for someone who expects to hit the out-of-pocket cap early in the year with expensive prescriptions and would rather pay that amount over twelve months than in a few large pharmacy visits.

If you are enrolled and stay in the same Part D plan, your participation renews automatically for the next year. It may not be the best choice if you get or are eligible for Extra Help, because Extra Help already keeps costs low. Your plan handles enrollment, not Medicare.

Part D IRMAA: The Income Surcharge

If your income is above a certain level, you pay an extra monthly amount on top of your plan premium. This is the income-related monthly adjustment amount, usually shortened to IRMAA. It is the drug-coverage version of the same surcharge that raises Part B premiums for higher earners.

It looks back two years. Your 2026 surcharge is based on the modified adjusted gross income on your 2024 tax return. Your 2027 surcharge will be based on your 2025 return. That two-year lag is why a large Roth conversion, a big capital gain or a business sale can raise your Medicare costs two years after the fact.

Married people who file separately and lived with their spouse at any point in the year face a compressed schedule: above $109,000 and less than $391,000 pays $83.30, and $391,000 or above pays $91.00.

For 2027, the Medicare & You 2027 handbook lists the surcharge as starting above $111,000 of income on an individual return and above $222,000 on a joint return. CMS publishes the full 2027 amounts each fall, so confirm current figures on Medicare.gov.

Three mechanics catch people off guard.

• You pay it to Medicare, not your plan. It is usually deducted from your Social Security check. If you are billed instead, the bill comes from Medicare or the Railroad Retirement Board. Not paying it can cost you your drug coverage.
• Employer-paid premiums do not cover it. Even if a former employer or retirement system pays your plan premium, you pay the IRMAA separately.
• It applies inside Medicare Advantage too. If your Advantage plan includes drug coverage, the drug surcharge still applies.

If your income has dropped because of a life-changing event, like retirement, the death of a spouse, divorce or a reduction in work hours, you can ask Social Security to use more recent income. The Roth conversion and IRMAA breakdown walks through how conversions push income across these lines.

When You Can Join, Switch or Drop a Plan

Generally you stay in your plan for the full calendar year. You can only make changes during specific windows.

Special Enrollment Periods open after events like moving out of your plan's service area, losing other coverage, losing Medicaid, or qualifying for Extra Help. Two matter most for people retiring after 65.

• Losing employer or union drug coverage. When creditable coverage from work ends, you get a Special Enrollment Period to join a Part D plan, generally through the end of the second full month after the month your coverage ended.
• COBRA ending. If you had COBRA that included creditable drug coverage, you get a Special Enrollment Period to join Part D without a penalty when the COBRA coverage ends.

The Part B Special Enrollment Period breakdown covers the medical side of leaving employer coverage, which runs on different rules.

Switching does not require cancelling. Joining a new plan during a valid window automatically ends your old one when the new coverage begins. Dropping drug coverage entirely without replacing it can only be done during an enrollment window, and it starts the clock on a possible late penalty.

The Late Enrollment Penalty

This is the rule that turns "optional" into "expensive." It is a permanent addition to your monthly Part D premium for joining late without other drug coverage that counts. It is also the most commonly miscalculated rule in Medicare, because it has two separate layers that work differently.

Layer one: whether you owe anything at all

You may owe the penalty if, at any time after your Initial Enrollment Period ends, you go 63 or more days in a row without either Medicare drug coverage or other creditable prescription drug coverage. This is a yes-or-no test. A gap of 62 days or fewer triggers nothing.

Layer two: how big the penalty is

If the 63-day test is failed, the size of the penalty is counted differently, in full calendar months without coverage.

• Each full uncovered month adds 1% of the national base beneficiary premium.
• The result is rounded to the nearest $0.10 and added to your monthly premium.
• It is recalculated every year as the base premium changes, so the dollar amount can rise or fall even though your percentage stays the same.
• It generally lasts for as long as you have Medicare drug coverage, and it follows you when you change plans.

Mixing up the two layers is how people get this wrong. Days decide whether the penalty applies. Months decide how big it is.

Example: skipping Part D entirely

Frank's Initial Enrollment Period ended on April 30, 2025. He had no drug coverage from anywhere and did not join a plan. During open enrollment in the fall of 2026 he joined a drug plan, with coverage starting January 1, 2027.

He went without creditable coverage from May 2025 through December 2026. That is 20 full months, so his penalty is 20% of the base premium.

• 2027: 0.20 × $41.33 = $8.27, rounded to $8.30 a month, added to his premium every month.
• In 2028 it is recalculated using the 2028 base premium. Still 20%, different dollar amount.

$8.30 a month looks small. It is also permanent. Over 20 years of retirement, and with the base premium generally rising, it adds up to roughly $2,000 or more for coverage he did not have.

Example: the 63-day line after retiring

Linda had creditable drug coverage through her employer until she retired on March 31.

Losing that coverage gives her a Special Enrollment Period through May 31, the end of the second full month after her coverage ended. Coverage starts the first of the month after the plan gets her request.

• If she enrolls in May, her Part D coverage starts June 1. Her gap is April 1 to May 31, which is 61 days. Under 63, so no penalty.
• If she lets May 31 pass, her window is closed. Her next chance is open enrollment in the fall, with coverage starting January 1. Her gap now runs April through December, far past 63 days, and the penalty is counted in full uncovered months: 9 of them. In a year when the base premium is $41.33, that is 0.09 × $41.33 = $3.72, rounded to $3.70 a month, added for as long as she has Part D.

Missing one deadline moved her from no penalty to a penalty she pays for life, and from two months without drug coverage to nine.

Creditable Coverage: The Exception That Saves You

Creditable prescription drug coverage is non-Medicare drug coverage expected to pay, on average, at least as much as standard Medicare drug coverage. If you have it, you can delay Part D without building a penalty.

Coverage that can count includes drug coverage from a current or former employer or union, TRICARE, the Indian Health Service, the Department of Veterans Affairs, and most Federal Employees Health Benefits plans. "Can count" is the operative phrase. Whether a specific plan is creditable is decided by that plan, not by its type.

The plan has to tell you in writing. The sponsor of your drug coverage must disclose whether it is creditable:

• Before your Part D Initial Enrollment Period
• Before the coverage takes effect
• Whenever its creditable status changes, or the coverage ends
• Before October 15 each year, ahead of Medicare's open enrollment period
• Whenever you ask

Keep those notices. When you eventually join Part D, the plan asks about your prior coverage. You can generally attest to having had creditable coverage without sending documents up front, but if the penalty is ever questioned, the notice is the cleanest proof you have. If you do not tell your new plan about your prior creditable coverage, you may be charged a penalty you never should have owed.

Not all drug coverage is creditable. Some high-deductible employer plans are not. Most drug coverage in older Medigap policies is not. A plan that is creditable this year can stop being creditable next year, which is exactly why the notice arrives every fall.

Disputing a Penalty You Do Not Owe

After you join a plan, it will send you a letter if it believes you owe a late enrollment penalty. If you disagree, you can ask for a review, called a reconsideration.

• Deadline: no later than 60 days from the date on the letter. Use the reconsideration request form that comes with it.
• Evidence: you must provide proof that supports your case, like creditable coverage notices from an employer, union, VA or other plan.
• Who decides: an independent review entity, not your plan. It generally decides within 90 days, though that can be extended for good cause.
• While you wait: you pay the penalty. If it is removed, you are generally refunded.

Missing the 60 days is not automatically fatal. You can ask for more time and explain why your request was late. The faster route is not missing it: when the letter arrives, find your creditable coverage notices and file.

The breakdown of the letter that proves you do not owe a drug penalty covers what that notice looks like and where to find it.

Part D and Other Coverage

How Part D fits with coverage you already have is where most coordination mistakes happen.

Employer or union coverage. Your employer or union must tell you each year whether its drug coverage is creditable. Call your benefits administrator before you join any Medicare drug plan. Joining Part D can cause you to lose your employer or union health coverage, possibly for your dependents too, and you may not be able to get it back.

COBRA. If your COBRA includes creditable drug coverage, you can delay Part D and get a Special Enrollment Period when COBRA ends. Be careful applying that to the medical side: COBRA does not protect you from the Part B late penalty the way active employer coverage does, and once you are eligible for Medicare, COBRA may pay only a small share of your medical bills.

Medicare Advantage. If you are in an Advantage plan that includes drug coverage, your drugs come through that plan. Joining a standalone drug plan while you are in most Advantage plans will generally move you out of the Advantage plan and back to Original Medicare. Check before you enroll in anything.

Older Medigap policies with drug coverage. You can keep the policy, but you cannot have drug coverage in both Medigap and a Medicare drug plan. If you join a drug plan, tell your Medigap insurer, which removes the drug coverage and adjusts your premium. You cannot add it back later. Most Medigap drug coverage is not creditable, which means relying on it can build a penalty.

Federal employees and retirees (FEHB). Federal Employees Health Benefits plans usually include creditable drug coverage, so you do not need Part D. You can join Part D and keep your FEHB plan. Medicare pays first for retired federal employees. FEHB pays first for current employees.

Postal retirees (PSHB). The Postal Service Health Benefits Program works differently. If you are a postal retiree or eligible family member enrolled in PSHB and eligible for Part D, your PSHB prescription coverage is generally provided through a Part D employer group plan run by your PSHB plan, and you are enrolled in it automatically. Opting out of that Part D plan generally means losing your PSHB prescription coverage without lowering your premium. Check with OPM or your PSHB plan before changing anything.

Veterans' benefits. You can join a Medicare drug plan, but you cannot use VA drug coverage and Part D for the same drug at the same time.

CHAMPVA. You can join a Medicare drug plan, but if you do, you lose access to the Meds by Mail program, which provides maintenance drugs at no cost.

TRICARE. You do not need a separate Medicare drug plan. Most people with TRICARE who are entitled to Part A must have Part B to keep TRICARE drug benefits. If you do join a Medicare drug plan, it pays first and TRICARE pays second.

Medicaid. If you have Medicare and full Medicaid, Medicare covers your prescriptions, and you qualify for Extra Help automatically.

Formularies, Tiers and Coverage Rules

Every plan has its own formulary, the list of drugs it covers. All plans must cover a wide range of drugs, including most drugs in certain protected classes like those that treat cancer, HIV, depression, psychosis and seizures, and drugs that prevent organ transplant rejection. Beyond that, formularies differ, which is why the only reliable way to compare plans is to enter your own prescriptions into Medicare's Plan Finder.

Tiers. Formularies sort drugs into tiers, and each tier has a different cost. Lower tiers, often generics, generally cost you less. A drug's tier can matter more than the premium.

Coverage rules. Plans can attach conditions to specific drugs:

• Prior authorization: you or your prescriber must contact the plan before the prescription is filled, and may need to show the drug is medically necessary.
• Step therapy: you may need to try one or more similar, lower-cost drugs first.
• Quantity limits: a cap on how much you can get at one time.

Formularies can change during the year. Plans can add or remove drugs and change tiers mid-year if they follow Medicare's guidelines, and your coinsurance can rise when a manufacturer raises a price. Your plan must notify you of formulary changes that affect drugs you take. That notice is your cue to talk with your prescriber about an alternative or ask for an exception.

The transition fill. When you join a new plan, within the first 90 days you may get a one-time 30-day supply of a drug you have been taking that the new plan does not normally cover, or that it normally requires prior authorization or step therapy for. It buys time to switch drugs or request an exception. It is not ongoing coverage.

Each month you fill a prescription, your plan sends an Explanation of Benefits. Check it for mistakes.

Exceptions and Appeals

If your plan will not cover a drug, or covers it at a price you think is wrong, you have formal rights, and they start before you ever buy the drug.

Coverage determination. This is the plan's first formal decision about your benefits: whether it covers a drug, whether you met its requirements, or how much you pay. The pharmacy does not make this decision. The plan does. If your pharmacy cannot fill a prescription, the pharmacist should give you a notice explaining how to contact your plan. If you do not get one, ask for it.

Three kinds of exceptions. You or your prescriber can ask the plan to:

• Cover a drug that is not on its formulary
• Waive a coverage rule, like prior authorization, step therapy or a quantity limit
• Charge you a lower-tier price for a higher-tier drug, if you cannot take any of the lower-tier alternatives for the same condition

For any exception, your prescriber must provide a statement explaining the medical reason the plan should approve it. Requests can be standard or expedited. A request is expedited when you have not gotten the drug yet and waiting could put your life or health at risk. Standard decisions generally come within 72 hours of the plan receiving the prescriber's statement, and expedited decisions within 24 hours.

If the plan says no, you can appeal. The first level is a redetermination by the plan, which you generally request within 65 days of the decision notice. If the plan still denies it, the case can go to an independent review entity and further levels beyond that. You can appoint a representative, including a family member, doctor or financial advisor, and your State Health Insurance Assistance Program can help at no cost.

Extra Help

Extra Help is a Medicare program that pays some or all of Part D premiums, deductibles and coinsurance for people with limited income and resources. Three facts make it more important than its low profile suggests: it eliminates the Part D late enrollment penalty, it lets you change drug plans far more often than everyone else, and many people who qualify never apply.

The income limits are higher if you still work, live in Alaska or Hawaii, or have dependents living with you.

Resources include your IRAs. Checking and savings accounts, stocks, bonds, mutual funds and Individual Retirement Accounts all count. Your home, car, household items, burial plot, up to $1,500 per person set aside for burial expenses, and life insurance policies do not.

Some people qualify automatically: anyone with Medicare and full Medicaid, anyone getting help from their state Medicaid program to pay Part B premiums, and anyone receiving Supplemental Security Income. They get a purple letter from Medicare and do not need to apply.

What it does once you have it:

• No late enrollment penalty.
• Low drug costs. In 2027, generally no more than $5.80 for each generic and $14.40 for each brand-name drug at a participating pharmacy.
• Monthly plan changes. If you have Medicaid or Extra Help, you may be able to switch drug plans, or leave Medicare Advantage for Original Medicare plus a drug plan, once each month, effective the first day of the next month.

Extra Help is not available in Puerto Rico, the U.S. Virgin Islands, Guam, the Northern Mariana Islands or American Samoa, which have separate programs.

Part D Under 65

If you qualify for Medicare before 65 because of a disability, End-Stage Renal Disease or ALS, you can join Part D on the same terms, with your own Initial Enrollment Period tied to when you first became eligible.

The late enrollment penalty works the same way under 65, with one important difference: a penalty you build before 65 does not follow you past it. When you turn 65, you get a new Initial Enrollment Period, and any late enrollment penalty from before 65 is eliminated. Months without creditable coverage after that new window ends can build a new one.

Common Mistakes

Skipping Part D because you take no prescriptions. The penalty counts months without creditable coverage, not months you needed drugs. Many plans charge little or nothing in premium, which is often cheaper than a lifetime penalty.

Counting the penalty in days. Days decide whether the penalty applies at all. Full calendar months decide how big it is.

Assuming any drug coverage protects you. Only creditable coverage does. Check the notice, every year.

Losing the creditable coverage notice. It is the cleanest proof you owe nothing if a plan charges a penalty.

Joining a standalone drug plan while in Medicare Advantage. In most cases that moves you out of your Advantage plan.

Joining Part D without calling your employer first. It can end your employer coverage, and possibly your family's.

Choosing a plan by premium. Your drugs, their tiers and your pharmacy decide your real cost. Run your actual prescriptions through Plan Finder.

Ignoring the Annual Notice of Change. Your plan can change its drug list, tiers, deductible and pharmacy network every January. Doing nothing in the fall accepts all of it.

Forgetting IRMAA after a big income year. A Roth conversion or asset sale can raise your Part D premium two years later.

Not applying for Extra Help. Retirees with modest income often assume they will not qualify. Apply anyway, and remember that IRAs count as resources.

FAQ

Related Knowledge Blasts

Short, plain-language breakdowns of the rules behind this guide:

Does Your Coverage Count as Creditable Drug Coverage? →

The Letter That Proves You Do Not Owe a Medicare Drug Penalty →

The Medicare Part B Special Enrollment Period When Employer Coverage Ends →

How Roth Conversions Interact With IRMAA →

The January to March Window to Leave Medicare Advantage →