Medicare Enrollment Periods & Deadlines (2025 Guide)

Why Enrollment Timing Matters So Much

Medicare is not like signing up for a gym membership. You cannot just enroll whenever you feel like it. The federal government has set specific windows during the year when you can sign up, make changes, or switch plans — and if you miss those windows, the consequences can follow you for the rest of your life.

We are not being dramatic. Miss certain deadlines and you could face permanent premium surcharges that never go away. Enroll at the wrong time and your coverage might not start for months. Understanding when to act is just as important as understanding what Medicare covers.

Here is every enrollment period you need to know, what each one is for, and what happens if you miss it.

Initial Enrollment Period (IEP): Your First Shot at Medicare

The Initial Enrollment Period is your personal 7-month window to sign up for Medicare Part A and Part B. It revolves around your 65th birthday and is structured like this:

Starts 3 months before your 65th birthday month
Includes your birthday month itself
Ends 3 months after your birthday month

For example, if your 65th birthday is in June, your IEP runs from March 1 through September 30. If your birthday falls on the first day of the month, everything shifts one month earlier — so a June 1 birthday means your IEP runs February 1 through August 31.

When you sign up within your IEP matters. If you enroll during the first three months, your coverage starts on the first day of your birthday month. Wait until your birthday month and coverage starts the following month. Enroll in the three months after, and there can be a delay of one to three months before coverage kicks in.

The bottom line: enroll as early as possible during your IEP. The sooner you sign up, the sooner your coverage begins.

Annual Election Period (AEP): October 15 to December 7

The Annual Election Period — sometimes called Annual Open Enrollment or Fall Open Enrollment — is the big one. This is the yearly window when anyone with Medicare can make changes to their coverage. It runs from October 15 through December 7 every year, and any changes you make take effect January 1 of the following year.

During the AEP, you can:

Switch from Original Medicare to a Medicare Advantage plan
Switch from Medicare Advantage back to Original Medicare
Switch from one Medicare Advantage plan to another
Join a standalone Part D prescription drug plan
Switch from one Part D plan to another
Drop Part D coverage entirely

This is the period that generates all those Medicare commercials you see every fall. If you already have Medicare and want to review your options for next year, this is your window. Plans change their costs, formularies, and provider networks annually, so it is worth checking even if you are happy with your current plan.

Medicare Advantage Open Enrollment Period (MA OEP): January 1 to March 31

This enrollment period is specifically for people who are already enrolled in a Medicare Advantage plan at the start of the year. It runs from January 1 through March 31, and it gives you one chance to make a change if you are unhappy with your Medicare Advantage plan after it takes effect.

During the MA OEP, you can:

Switch from one Medicare Advantage plan to a different Medicare Advantage plan
Drop your Medicare Advantage plan and return to Original Medicare (and enroll in a standalone Part D plan)

You can only make one change during this period. Once you make a switch, that is it until the next Annual Election Period in the fall. Also, this period is not available to people on Original Medicare — it is exclusively for current Medicare Advantage enrollees.

General Enrollment Period (GEP): January 1 to March 31

The General Enrollment Period has the same dates as the MA OEP — January 1 through March 31 — but it serves a completely different purpose. The GEP is for people who missed their Initial Enrollment Period and did not qualify for a Special Enrollment Period.

If you turned 65, did not sign up for Part A or Part B, and do not have qualifying employer coverage that excused you from enrolling, the GEP is your safety net. You can use it to sign up for Part A and Part B. Coverage begins the month after you enroll.

Here is the catch: If you are using the GEP because you missed your IEP without a valid reason, late enrollment penalties will almost certainly apply. We will cover those penalties in detail below — but just know that the GEP is a fallback, not a free pass.

Special Enrollment Periods (SEPs): When Life Changes Your Plans

Special Enrollment Periods are exactly what they sound like — windows that open up outside the normal enrollment schedule because something significant has changed in your life. You do not have to wait for the next AEP or GEP if you qualify for an SEP.

The most common situations that trigger a Special Enrollment Period:

Losing employer or union coverage

This is the big one. If you are 65 or older and still covered by an employer group health plan — either through your own job or your spouse's — you do not have to sign up for Part B right away. Your employer coverage counts as "creditable coverage" as long as the employer has 20 or more employees.

When that employer coverage ends — whether you retire, get laid off, or your spouse's plan changes — you get an 8-month Special Enrollment Period to sign up for Part B without any penalty. The clock starts the month after the employment ends or the coverage ends, whichever comes first.

Moving to a new area

If you move out of your current plan's service area, you qualify for an SEP to enroll in a new Medicare Advantage plan or Part D plan available in your new location. This also applies if you move back to the United States after living abroad.

Other qualifying life events

Losing Medicaid eligibility
Qualifying for Extra Help (Low-Income Subsidy) for prescription drug costs
Moving into or out of a skilled nursing facility
Your plan leaves Medicare or stops covering your area
You qualify for a Medicare Savings Program
You were enrolled in a plan that received a low quality rating (below 3 stars)

Each SEP has its own rules and timeframes, so it is important to act quickly once you realize you qualify. In most cases, you have two months from the qualifying event to make a change.

Late Enrollment Penalties: The Cost of Waiting Too Long

This is where Medicare enrollment gets serious. If you do not sign up when you are supposed to and you do not have a valid reason for delaying, Medicare will charge you a penalty — and in most cases, that penalty is permanent. It gets added to your monthly premium for as long as you have that coverage.

Part A late enrollment penalty

Most people get Part A premium-free, so this penalty only applies to those who must pay for Part A (meaning they did not accumulate 40 quarters of Medicare-tax-paying work history). If you are in that group and you delay enrollment without qualifying coverage, your Part A premium increases by 10%. You pay that higher premium for twice the number of years you could have been enrolled but were not.

For example, if you delayed enrollment by two years, you would pay the 10% penalty for four years. In 2025, the full Part A premium is $518 per month — so a 10% penalty would add $51.80 to your monthly bill.

Part B late enrollment penalty

This is the penalty that catches the most people — and it is the harshest. If you do not sign up for Part B when you are first eligible and you do not have qualifying employer coverage, your Part B premium goes up by 10% for each full 12-month period you were eligible but not enrolled. And this penalty never goes away. You pay it for as long as you have Part B.

Let's put that in real numbers. The standard Part B premium in 2025 is $185.00 per month. If you delayed enrollment by three years, your penalty would be 30% — adding $55.50 per month to your premium. That is $666 extra per year, every year, for the rest of your life.

Part D late enrollment penalty

If you go 63 or more consecutive days without Part D or other creditable prescription drug coverage after your initial enrollment period, you will pay a penalty when you eventually sign up. The penalty is calculated as 1% of the national base beneficiary premium ($36.78 in 2025) multiplied by the number of full uncovered months. Like the Part B penalty, this one is permanent.

Say you went 24 months without creditable drug coverage. Your penalty would be 24% of $36.78, which is $8.83 per month — rounded to the nearest $0.10, so $8.80 per month added to your Part D premium for life.

What Happens If You Miss Your Enrollment Window

If you missed your Initial Enrollment Period and do not qualify for a Special Enrollment Period, here is what you are looking at:

A gap in coverage. You will need to wait until the next General Enrollment Period (January 1 through March 31) to sign up for Part A or Part B. That could mean months without Medicare coverage.
Delayed coverage start. If you enroll during the GEP, your coverage does not start until July 1 of that year. So if you sign up in January, you still have to wait until July for your Part B to become active.
Permanent penalties. As described above, your premiums will be higher for the rest of your time on Medicare. These penalties compound over the years as the base premiums increase.
Limited plan options. Without Part B, you cannot enroll in a Medicare Advantage plan. Without creditable drug coverage, joining a Part D plan later means paying more for it.

The system is not forgiving. Medicare assumes you know the rules, and there is very little room for "I did not realize I needed to sign up." If there is one takeaway from this article, it is this: do not assume someone will remind you. Mark the dates. Set the alarms. Talk to a licensed agent or your local SHIP (State Health Insurance Assistance Program) counselor if you have questions.

Still Working at 65? Here Is What You Need to Know

More Americans are working past 65 than ever before, and the Medicare enrollment rules for people with employer coverage are frequently misunderstood. Getting this wrong can cost you thousands. Here is how it works.

If your employer has 20 or more employees

Your employer group health plan is considered your primary insurance, and Medicare is secondary. In this situation, you can safely delay enrolling in Part B without penalty. Many people in this position enroll in premium-free Part A (there is no downside) but hold off on Part B until they retire or lose their employer coverage.

When your employment or employer coverage ends, you get the 8-month Special Enrollment Period to sign up for Part B. No penalty. No gap if you act promptly.

If your employer has fewer than 20 employees

This is where people get tripped up. If the employer has fewer than 20 employees, Medicare becomes your primary insurance at 65 — even if you are still working and have employer coverage. In this case, you should sign up for both Part A and Part B during your IEP. If you delay Part B, your employer plan may not cover you properly, and you will face the late enrollment penalty when you eventually do sign up.

Key tips for people still working at 65

Always sign up for Part A if it is free. There is no cost and no downside for most people. The one exception: if you are contributing to a Health Savings Account (HSA), enrolling in any part of Medicare makes you ineligible for new HSA contributions. Stop contributing to your HSA at least six months before enrolling in Part A to avoid tax penalties, since Part A can be backdated up to six months.
Get proof of employer coverage in writing. When you eventually enroll in Part B, you will need to fill out CMS form L564 (Request for Employment Information) and have your employer complete it. This proves you had qualifying coverage and protects you from the penalty.
COBRA does not count as employer coverage. This surprises many people. COBRA continuation coverage is not considered creditable employer coverage for Medicare purposes. If you retire at 65 and elect COBRA instead of signing up for Part B, you will face the late enrollment penalty.
Retiree coverage is not the same as active employer coverage. If you have retiree health benefits from a former employer, that typically does not give you a Special Enrollment Period. You still need to sign up for Part B during your IEP.
Do not rely on your HR department to know Medicare rules. Many HR representatives are not well-versed in Medicare coordination. Verify the information you receive with Medicare directly (1-800-MEDICARE) or through your State Health Insurance Assistance Program.

A Quick Reference: All Enrollment Periods at a Glance

Initial Enrollment Period: 7 months around your 65th birthday. Sign up for Part A, Part B, Part D, or Medicare Advantage.

Annual Election Period: October 15 to December 7. Switch Medicare Advantage or Part D plans. Changes effective January 1.

Medicare Advantage Open Enrollment: January 1 to March 31. Current MA enrollees can make one plan change or return to Original Medicare.

General Enrollment Period: January 1 to March 31. For people who missed their IEP. Coverage starts July 1. Penalties likely apply.

Special Enrollment Periods: Triggered by qualifying life events (losing employer coverage, moving, etc.). Timeframes vary — usually 2 to 8 months depending on the event.

The Bottom Line

Medicare enrollment is not something you can figure out later. The windows are specific, the penalties are real, and the rules are unforgiving for people who miss deadlines. Whether you are turning 65, retiring, or helping a parent navigate this process, the single most important thing you can do is understand which enrollment period applies to your situation — and act before it closes.

If you are approaching 65, start your research at least six months before your birthday. If you are still working with employer coverage, make sure you understand the 20-employee threshold and get your documentation in order. And if you have already missed a window, contact Medicare or a licensed agent as soon as possible — there may be a Special Enrollment Period or other option that applies to you.

The deadlines are not flexible, but with the right information and a little planning, you can navigate every one of them without a problem.

Medicare Enrollment Periods and Deadlines: Every Window You Need to Know