Medicare Enrollment Periods: When to Enroll and When to Make Changes

There's no single "Medicare enrollment period." I hear that assumption from clients constantly, and it's one of the most common reasons people end up with a penalty or an unexpected gap in coverage.

The window that applies to you depends on your situation: whether you're enrolling in Medicare for the first time, adding Part B after you finally drop employer coverage, buying a Medigap plan, or changing your Medicare Advantage or Part D plan. Each of those has its own calendar, its own rules, and its own consequences for getting it wrong.

Missing the correct window can mean a lifelong premium penalty, a medically underwritten Medigap application, or months without coverage. Here's how to tell which one is yours.

Medicare Enrollment Periods at a Glance

If you're brand new to Medicare, start with the Initial Enrollment Period below. If you already have Medicare and simply want to change coverage, skip ahead to Medicare Open Enrollment or Medicare Advantage Open Enrollment.

Initial Enrollment Period: Your First Chance to Enroll

When It Begins

Your Initial Enrollment Period (IEP) runs for seven months: the three months before the month you turn 65, your birthday month, and the three months after. This is generally your best opportunity to enroll in Part A (hospital insurance) and Part B (medical insurance) without a penalty. See our full breakdown of Medicare age requirements for how eligibility works in less common situations.

When Your Coverage Starts

If you enroll before your birthday month, coverage generally starts during your birthday month. If you enroll during or after your birthday month, coverage generally starts the following month. If your birthday falls on the first of the month, your eligibility actually begins one month earlier than usual — a quirk that catches a lot of people off guard.

Do You Need Medicare at 65 If You're Still Working?

Not always — but this is where I tell clients to slow down rather than assume. Whether you can safely delay Part B depends on:

• Current employer coverage — coverage based on active employment, yours or a spouse's, generally lets you delay Part B without penalty.
• Employer size — if the employer generally has fewer than 20 employees, Medicare usually pays first after age 65. Delaying Part B could leave claims unpaid or only partially covered.
• COBRA and retiree coverage — these generally do not count as coverage based on current employment and do not protect your Part B enrollment rights the way active employer coverage does.
• HSA contributions — enrolling in premium-free Part A can trigger up to six months of retroactive coverage, which can create an HSA contribution problem. If you're contributing to an HSA, talk to a tax or benefits professional before enrolling in any part of Medicare.

Part A and Part B Late Enrollment Penalties

Most people get Part A premium-free based on work history, so there's usually no penalty risk there. Part B is different: if you go 12 or more months past your eligibility without Part B or other qualifying coverage, your premium goes up 10% for every full 12-month period you waited — and that increase applies for as long as you have Part B, not just once.

Part D has a separate late enrollment penalty if you go 63 consecutive days or more without Medicare drug coverage or other creditable prescription coverage after becoming eligible.

Special Enrollment Period for Employer Coverage

Who Qualifies

You generally qualify for a Part B Special Enrollment Period (SEP) if you have coverage based on current employment through your own employer, your spouse's employer, or — in some disability situations — a family member's employer.

How Long You Have

The Part B SEP generally lasts as long as that employment coverage continues, plus roughly eight months after it ends or your employment ends, whichever comes first.

Coverage That Usually Doesn't Protect You

This is where I see the most expensive mistakes. The following generally do not preserve your Part B SEP rights:

• COBRA continuation coverage
• Retiree health coverage
• Marketplace or individual health insurance

If you're relying on one of these and assuming you have time, you may already be past your real deadline. It's worth confirming your specific timeline before your employer coverage changes.

General Enrollment Period

The General Enrollment Period (GEP) runs every year from January 1 through March 31. It exists for people who missed their Initial Enrollment Period and don't qualify for a Special Enrollment Period.

Coverage generally begins the month after you enroll, and a late enrollment penalty may still apply if you went without Part B for an extended period.

The GEP should be treated as a fallback, not a plan. If you can enroll during your IEP or a SEP instead, do it — the GEP still generally carries a penalty risk that the other windows don't.

Medigap Open Enrollment Period

For many people, this is the most important Medicare enrollment period. Read our full Medigap Open Enrollment guide for state-by-state detail, but here's what matters most.

When the Six-Month Window Begins

It starts the first day of the month you're both age 65 or older and enrolled in Medicare Part B. If you delay Part B because of qualifying employer coverage, this window is delayed right along with it — it begins when your Part B coverage actually starts, whenever that is.

Why This Window Matters

During these six months:

• You can buy any Medigap plan sold to new applicants in your state
• The insurer can't deny your application based on your health
• The insurer can't charge you more because of your health history
• No medical underwriting is required

Base premiums can still vary by other state-permitted factors — age, gender, tobacco use, ZIP code, household discounts — but health can't be one of them during this window.

What Happens After It Ends?

You can still apply for Medigap after the six months close, but approval isn't guaranteed. Most states allow medical underwriting at that point, meaning the insurer can ask health questions and potentially deny the application. If you are accepted, an insurer may impose a waiting period of up to six months for coverage of a pre-existing condition in certain situations. Prior creditable coverage may reduce or eliminate that waiting period. Guaranteed-issue rights or state-specific protections may still apply in certain circumstances, so it's worth checking your situation specifically rather than assuming you're out of options.

Medigap Open Enrollment Is Not Annual

I want to say this plainly because the confusion is so common: this window happens once, tied to your age and Part B enrollment. It is not the same as the fall Medicare Open Enrollment Period, and it does not come back every year. There is no federal annual enrollment period that lets everyone change Medigap plans without underwriting.

If you're approaching the end of your Medigap Open Enrollment Period, compare plans and prices before the window closes.

Medicare Open Enrollment: October 15 – December 7

Medicare officially calls this the Medicare Open Enrollment Period. Insurance agents and carriers also commonly refer to it as the Annual Election Period, or AEP. During this period, you can:

• Join a Medicare Advantage plan from Original Medicare
• Switch from Medicare Advantage back to Original Medicare
• Join, switch, or drop a Part D prescription drug plan
• Change Medicare Advantage plans

Changes take effect January 1. You can submit more than one change during this period — whichever election you make last before December 7 is the one that takes effect.

What to Review Every Fall

Insurance companies mail an Annual Notice of Change (ANOC) each fall explaining how your plan is changing for the next year. Don't assume your plan is staying the same. Check:

• Premium
• Drug formulary
• Pharmacy network
• Provider network
• Copayments and coinsurance
• Maximum out-of-pocket limit
• Prior-authorization requirements

Important Warning About Switching to Original Medicare

Medicare Advantage Open Enrollment: January 1 – March 31

This period is only for people already enrolled in a Medicare Advantage plan. It's not a second chance to join Medicare Advantage from Original Medicare — that requires Medicare Open Enrollment or a qualifying SEP.

If you're already in a Medicare Advantage plan, you can use this window once to:

• Switch to a different Medicare Advantage plan
• Return to Original Medicare
• Add a standalone Part D plan if you return to Original Medicare

Changes generally take effect the first day of the month after you make them.

Other Medicare Special Enrollment Periods

Beyond the employer-coverage SEP, other qualifying events can open a Special Enrollment Period, including:

• Moving outside your plan's service area
• Losing employer or union coverage
• Losing Medicaid eligibility
• Gaining or losing Extra Help with Part D costs
• Entering or leaving a skilled nursing facility or other institution
• Your plan leaving Medicare or ending service in your area
• Being affected by an emergency or disaster
• Receiving incorrect or misleading information about your plan

The permitted action and the length of the window both depend on which qualifying event applies to you — there isn't one standard SEP timeline. If you think you qualify, it's worth confirming the specifics of your situation before assuming a deadline.

Which Medicare Enrollment Period Applies to You?

If you're turning 65: use your Initial Enrollment Period.

If you're working past 65: confirm whether your current employer coverage actually lets you delay Part B without penalty.

If your employer coverage is ending: use your Part B Special Enrollment Period, and coordinate your Part B, Medigap or Medicare Advantage, and Part D effective dates so there's no gap.

If you missed Medicare enrollment entirely: use the General Enrollment Period unless another SEP applies to you.

If you want to buy Medigap: check whether you're in Medigap Open Enrollment, have a guaranteed-issue right, or will need to answer health questions.

If you want to change Medicare Advantage or Part D coverage: use Medicare Open Enrollment, Medicare Advantage Open Enrollment, or a qualifying SEP, depending on your situation.

How to Enroll in Medicare

• Apply through Social Security online, by phone, or in person
• Confirm your Part A and Part B effective dates
• Decide between Original Medicare and Medicare Advantage
• Add a Part D plan if you choose Original Medicare and do not have other creditable drug coverage
• Apply for Medigap if you choose Original Medicare and want help with Medicare's deductibles and coinsurance
• Don't cancel existing coverage until your new coverage is confirmed

Common Medicare Enrollment Mistakes

• Assuming Medicare Open Enrollment and Medigap Open Enrollment are the same thing
• Delaying Part B while covered only by COBRA
• Missing your Medigap Open Enrollment Period
• Forgetting to review your Annual Notice of Change
• Dropping Medicare Advantage before confirming you can buy Medigap

Bottom Line

Medicare enrollment periods serve different purposes. Your Initial Enrollment Period is generally your first opportunity to enroll in Parts A and B, while a Special Enrollment Period may let you delay Part B if you have qualifying coverage through current employment. The General Enrollment Period is a fallback if you miss those windows. Medigap has its own one-time six-month Open Enrollment Period, while Medicare Advantage and Part D changes generally happen during the October 15–December 7 Medicare Open Enrollment Period. Before you change coverage, confirm your eligibility, your effective dates, your drug coverage, and your ability to get Medigap without underwriting.