Medicare Enrollment Periods Compared: IEP, AEP, OEP, SEP, and GEP

Medicare has five distinct enrollment periods, each with its own dates, rules, and restrictions. Mixing them up is one of the most common and costly mistakes people make. Choose the wrong window and you could end up locked out of the plan you want, stuck without coverage for months, or paying a permanent penalty that follows you for the rest of your life.

This guide lays all five enrollment periods side by side so you can see exactly when each one runs, what actions you can take, and which period applies to your specific situation. If you want a deep dive into the dates and deadlines for each window, read our full guide on Medicare enrollment periods and deadlines.

The Five Medicare Enrollment Periods at a Glance

Before we get into the details, here is a quick summary of each enrollment period and what it is for.

• IEP (Initial Enrollment Period): Your personal 7-month window around your 65th birthday to sign up for Medicare for the first time.
• AEP (Annual Election Period): October 15 to December 7 every year. The annual window for all Medicare beneficiaries to change plans.
• OEP (Medicare Advantage Open Enrollment Period): January 1 to March 31. A second chance for current Medicare Advantage enrollees to make one plan change.
• SEP (Special Enrollment Period): Triggered by qualifying life events like losing employer coverage or moving. Dates and duration vary.
• GEP (General Enrollment Period): January 1 to March 31. The fallback window for people who missed their IEP and need to sign up for Part A or Part B.

Now let us break each one down in detail so you understand exactly what you can and cannot do during each window.

IEP: Initial Enrollment Period

When it runs

The IEP is a personal 7-month window centered on your 65th birthday. It starts 3 months before your birthday month, includes your birthday month, and ends 3 months after. For someone turning 65 in June 2026, the IEP would run from March 1 through September 30, 2026. If your birthday falls on the first of the month, everything shifts one month earlier.

What you can do

• Enroll in Medicare Part A (hospital insurance)
• Enroll in Medicare Part B (medical insurance)
• Enroll in a Medicare Advantage plan (Part C)
• Enroll in a standalone Part D prescription drug plan
• Purchase a Medigap (Medicare Supplement) policy during the guaranteed issue window that follows Part B enrollment

Who it is for

People turning 65 who are enrolling in Medicare for the first time. This is your primary enrollment opportunity, and it is the most important one. Enrolling during the first three months of your IEP gives you the earliest possible coverage start date. Waiting until the final three months can delay your coverage by one to three months.

Key point

Missing your IEP without having qualifying employer coverage triggers late enrollment penalties that are permanent for Part B and Part D. This is the single most consequential enrollment window in all of Medicare.

AEP: Annual Election Period (Fall Open Enrollment)

When it runs

October 15 through December 7 every year. This is a fixed calendar window that does not change. Any changes you make during the AEP take effect on January 1 of the following year.

What you can do

• Switch from Original Medicare to a Medicare Advantage plan
• Switch from a Medicare Advantage plan back to Original Medicare
• Switch from one Medicare Advantage plan to another
• Join, switch, or drop a standalone Part D prescription drug plan

Who it is for

All Medicare beneficiaries. Whether you are on Original Medicare or a Medicare Advantage plan, the AEP is your annual opportunity to review and change your coverage. This is the enrollment period behind all the Medicare plan advertisements you see every fall.

Key point

You cannot use the AEP to enroll in Part A or Part B for the first time. The AEP is strictly for making changes to your existing Medicare coverage. You must already have Medicare Part A and Part B to join a Medicare Advantage plan, and you must have Part A or Part B to join a standalone Part D plan.

OEP: Medicare Advantage Open Enrollment Period

When it runs

January 1 through March 31 every year. Changes made during the OEP generally take effect the first of the month after your new plan receives your enrollment request.

What you can do

• Switch from one Medicare Advantage plan to a different Medicare Advantage plan
• Drop your Medicare Advantage plan and return to Original Medicare (and pick up a standalone Part D plan)

What you cannot do

• Join a Medicare Advantage plan for the first time (you must use the AEP or IEP for that)
• Switch from Original Medicare to a Medicare Advantage plan
• Make more than one change (you get one switch, and that is it)

Who it is for

Only people who are already enrolled in a Medicare Advantage plan as of January 1. If you are on Original Medicare, this period does not apply to you. Think of it as a safety valve for Medicare Advantage enrollees who discover their plan is not working for them after January 1.

Key point

If you drop your Medicare Advantage plan during the OEP and return to Original Medicare, be aware that Medigap underwriting rules may apply. In most states, you do not have guaranteed issue rights for a Medigap policy outside of your initial 6-month Medigap open enrollment window. This means you could be denied a supplement plan or charged a higher premium based on your health.

SEP: Special Enrollment Periods

When they run

Special Enrollment Periods do not have fixed calendar dates. They are triggered by specific qualifying life events and last for a limited time after that event occurs. The most common SEP durations are 2 months and 8 months, depending on the event.

What you can do

Depending on the qualifying event, you may be able to:

• Enroll in Part B for the first time without a late penalty (after losing employer coverage)
• Join, switch, or drop a Medicare Advantage plan
• Join, switch, or drop a Part D plan
• Switch from Original Medicare to Medicare Advantage or vice versa

Common qualifying events

• Losing employer or union group health coverage -- This is the most common SEP trigger. You get 8 months to enroll in Part B without penalty.
• Moving out of your plan's service area -- You can enroll in a new Medicare Advantage or Part D plan available in your new location.
• Losing Medicaid eligibility -- Triggers an SEP to make changes to your Medicare coverage.
• Your plan leaves Medicare or stops serving your area -- You receive an SEP to choose new coverage.
• Qualifying for Extra Help or a Medicare Savings Program -- Low-income beneficiaries get additional enrollment flexibility.
• Moving into or out of a skilled nursing facility -- Gives you an opportunity to change your plan.

Key point

Each SEP has its own specific rules and timeframes. Unlike the AEP or GEP, SEPs are not one-size-fits-all. Some let you enroll in Part A and Part B, while others only let you change your Medicare Advantage or Part D plan. Always confirm which actions your specific SEP allows before making decisions.

GEP: General Enrollment Period

When it runs

January 1 through March 31 every year. Coverage for those who enroll during the GEP starts on July 1 of that year. Yes, that means if you sign up on January 2, you still have to wait until July 1 for your Part B to kick in.

What you can do

• Sign up for Medicare Part A
• Sign up for Medicare Part B

What you cannot do

• Join a Medicare Advantage plan directly (you would need to wait until the next AEP after your Part A and Part B are active)
• Switch Medicare plans (this is only for initial Part A and Part B enrollment)

Who it is for

People who missed their Initial Enrollment Period and did not qualify for a Special Enrollment Period. The GEP exists as a last resort for people who need to get into Medicare but missed the normal windows. It is not a first choice. It is a safety net with consequences.

Key point

If you are enrolling through the GEP, you will almost certainly face a permanent late enrollment penalty. Your Part B premium increases by 10% for each full 12-month period you could have had Part B but did not. That penalty never goes away. Combined with the delayed July 1 start date, the GEP represents a significant gap in both coverage and cost.

AEP vs. OEP: The Most Common Source of Confusion

If there is one area where Medicare beneficiaries consistently get confused, it is the difference between the Annual Election Period and the Medicare Advantage Open Enrollment Period. Both involve Medicare Advantage plans, both happen every year, and both let you make changes. But they are very different.

The AEP (October 15 to December 7) is for everyone on Medicare. You can join Medicare Advantage for the first time, switch between plans, go back to Original Medicare, or change your Part D drug plan. You can make multiple changes during this window, though only your last enrollment choice before December 7 takes effect.

The OEP (January 1 to March 31) is only for people who are already in a Medicare Advantage plan. You get exactly one change. If you are on Original Medicare and happy with it, the OEP does not apply to you at all.

Here is a simple way to remember it: the AEP is the main event for everyone. The OEP is the do-over period exclusively for MA enrollees who realize their fall plan choice is not working.

GEP vs. OEP: Same Dates, Completely Different Purposes

Another major confusion point is that the General Enrollment Period and the Medicare Advantage Open Enrollment Period share the exact same dates: January 1 through March 31. People frequently mix them up or assume they are the same thing. They are not.

The GEP is for people who are not yet in Medicare and missed their IEP. It lets you sign up for Part A and Part B for the first time. Coverage does not start until July 1, and penalties almost always apply.

The OEP is for people who are already enrolled in a Medicare Advantage plan and want to make one change. It has nothing to do with initial enrollment in Part A or Part B.

If someone tells you there is an "open enrollment" from January to March without specifying which one, ask them to clarify. The answer determines completely different actions and consequences.

Decision Flowchart: Which Enrollment Period Do You Need?

Use this step-by-step approach to identify which enrollment period applies to your situation right now.

Step 1: Are you turning 65 soon or within a few months of your 65th birthday?

• Yes: Your IEP is your primary window. Enroll in Part A and Part B. Consider a Medicare Advantage plan or a Medigap policy with a standalone Part D plan.
• No: Continue to Step 2.

Step 2: Do you already have Medicare Part A and Part B?

• No, and I missed my IEP: Check if you qualify for a Special Enrollment Period. If not, your next chance is the GEP (January 1 to March 31), with coverage starting July 1.
• Yes: Continue to Step 3.

Step 3: Do you want to change your existing Medicare plan?

• Yes, and it is October through early December: Use the AEP (October 15 to December 7) to make any plan changes you need.
• Yes, it is January through March, and I am in an MA plan: Use the OEP to make one plan change or drop back to Original Medicare.
• Yes, but it is not one of those windows: Check whether you qualify for a Special Enrollment Period based on a recent life change. If not, you will need to wait for the next AEP.

Step 4: Have you recently experienced a qualifying life event?

• Yes: You likely have a Special Enrollment Period. Act quickly because most SEPs have strict time limits, typically 2 to 8 months from the qualifying event.
• No: You will need to wait for the next applicable enrollment period. Mark the AEP dates (October 15 to December 7) on your calendar.

Side-by-Side Comparison: What Each Period Allows

Here is a direct comparison of what you can and cannot do during each enrollment period.

Enroll in Part A and Part B for the first time

• IEP: Yes
• AEP: No
• OEP: No
• SEP: Yes (Part B, if losing employer coverage)
• GEP: Yes (with penalties and delayed coverage start)

Join or switch Medicare Advantage plans

• IEP: Yes
• AEP: Yes
• OEP: Switch only (must already be in an MA plan; one change allowed)
• SEP: Yes (depending on the qualifying event)
• GEP: No

Join, switch, or drop a Part D drug plan

• IEP: Yes
• AEP: Yes
• OEP: Only when paired with returning to Original Medicare
• SEP: Yes (depending on the qualifying event)
• GEP: No

Return to Original Medicare from Medicare Advantage

• IEP: Not applicable (you are just getting started)
• AEP: Yes
• OEP: Yes (one change only)
• SEP: Yes (depending on the qualifying event)
• GEP: No

Common Mistakes to Avoid

After helping thousands of people navigate Medicare, these are the enrollment period mistakes we see most often.

1. Thinking the AEP is for initial enrollment. The AEP is for changing plans. If you have never enrolled in Medicare, the AEP will not help you sign up for Part A and Part B. You need your IEP, an SEP, or the GEP.
2. Confusing the OEP with the GEP. Both run January 1 through March 31 but serve entirely different populations. The OEP is for current MA enrollees. The GEP is for people who missed their IEP and need to sign up for Part A or Part B.
3. Assuming COBRA coverage excuses you from enrolling. COBRA does not count as employer group coverage for Medicare purposes. If you are 65 or older and elect COBRA instead of signing up for Medicare, you will face late enrollment penalties when you do enroll.
4. Waiting too long during an SEP. Special Enrollment Periods have strict deadlines. The 8-month window for employer coverage loss goes by faster than people expect. Once it closes, your next option is the GEP with penalties.
5. Not reviewing plans during every AEP. Medicare Advantage plans and Part D plans change their costs, formularies, and provider networks every year. Even if you love your plan, something may have changed. Reviewing your options annually during the AEP can save you hundreds of dollars. Learn more about how to switch Medicare plans during enrollment periods.

Coverage Start Dates by Enrollment Period

When your coverage actually starts depends on which enrollment period you use and when within that window you enroll. Here is what to expect.

• IEP: Enroll in the first 3 months and coverage starts the 1st of your birthday month. Enroll during your birthday month and coverage starts the following month. Enroll in the last 3 months and coverage may be delayed 1 to 3 months.
• AEP: All changes take effect January 1 of the following year, regardless of when during the AEP you make them.
• OEP: Changes generally take effect the 1st of the month after the plan receives your enrollment.
• SEP: Varies by type. The Part B employer SEP allows coverage to start the month after you enroll. Other SEPs may have different start dates.
• GEP: Coverage always starts July 1, no matter when during January through March you enroll.

The Bottom Line

Medicare enrollment is not a single event. It is a system of five interconnected windows, each designed for a specific purpose and a specific group of people. Understanding which period applies to your situation is the difference between seamless coverage and months of gaps, penalties, and frustration.

If you are approaching 65, your IEP is everything. Do not miss it. If you already have Medicare and want to make changes, the AEP in the fall is your main opportunity. If you are in a Medicare Advantage plan that is not working, the OEP gives you one more chance at the start of the year. If life throws you a curveball, check whether you qualify for an SEP. And if you missed your window entirely, the GEP is there as a last resort, though it comes with real financial consequences.

The rules are rigid, but they are not mysterious. Now that you know what each enrollment period does and who it is for, you can plan ahead and make confident decisions about your Medicare coverage.