Medicare Enrollment Periods Explained: IEP, AEP, OEP, SEP, and Medigap

Medicare enrollment periods are one of the most confusing parts of the entire Medicare system — and one of the most consequential. Miss the wrong window by even a single month and you could face a permanent penalty on your premium, a gap in coverage, or be locked out of the plan you actually want for an entire year.

This guide covers every Medicare enrollment period: when each one opens, what you can and can't do during it, and what happens if you miss it. If you're turning 65, coming off employer coverage, or simply trying to figure out whether you can make a change right now, this is your reference.

Quick Reference: All Medicare Enrollment Periods

Period	When	Who It's For	What You Can Do
IEP (Initial)	7 months around your 65th birthday	People turning 65 (or newly Medicare-eligible)	Enroll in Parts A, B, choose Advantage or Supplement + Part D
Medigap OEP	6 months from Part B effective date	New Part B enrollees	Enroll in any Medigap plan, no medical underwriting
AEP (Annual)	Oct 15 – Dec 7 every year	All Medicare beneficiaries	Switch, add, or drop Medicare Advantage and Part D plans
OEP (MA Open)	Jan 1 – Mar 31 every year	Medicare Advantage enrollees only	Switch MA plans or return to Original Medicare + Part D
SEP (Special)	Triggered by qualifying life events	Anyone with a qualifying event	Varies by event — enroll, switch, or drop coverage

The Medicare Year at a Glance

Initial Enrollment Period (IEP)

The Initial Enrollment Period is the first window most people have to sign up for Medicare. It spans 7 months total: the 3 months before your 65th birthday month, your birthday month itself, and the 3 months after.

For example, if your birthday is July 15, your IEP runs from April 1 through October 31.

What you can do during your IEP:

• Enroll in Medicare Part A (hospital insurance)
• Enroll in Medicare Part B (medical insurance)
• Choose a Medicare Advantage (Part C) plan as your coverage delivery method
• Enroll in a standalone Part D prescription drug plan (if you stay on Original Medicare)
• Enroll in a Medicare Supplement (Medigap) plan — your Medigap OEP starts when Part B begins

When your coverage starts depends on when you enroll:

• Enroll in the 3 months before your birthday month → coverage begins the 1st of your birthday month
• Enroll during your birthday month → coverage begins the 1st of the following month
• Enroll 1–3 months after your birthday month → coverage begins the 1st of the month after you enroll

The penalty for missing your IEP: If you don't have qualifying coverage elsewhere (such as active employer insurance through a job you or your spouse still holds), missing Part B enrollment triggers a 10% permanent premium surcharge for every 12-month period you went without coverage. On a 2025 standard Part B premium of $185/month, two years late means paying $37/month more — for the rest of your life. The Part D late enrollment penalty compounds similarly: 1% of the national base premium for every month you went without creditable drug coverage.

If you're still working and covered by an employer plan when you turn 65, you can delay Medicare without penalty. Once that coverage ends, a Special Enrollment Period opens. This is one of the most important distinctions to understand — talk to a broker before making this decision.

Medigap Open Enrollment Period

The Medigap Open Enrollment Period is separate from all the other enrollment windows and is arguably the most valuable — and most overlooked — consumer protection in the Medicare system.

It lasts 6 months from the date your Part B coverage becomes effective. During this window, any insurance company selling Medigap plans in your state must sell you any plan they offer at the same price they charge a healthy person — regardless of your health history, pre-existing conditions, or current medications.

Outside this window, Medigap insurers in most states can use medical underwriting. That means they can reject your application, charge significantly higher premiums based on health history, or add coverage waiting periods for pre-existing conditions.

In Florida, there is an additional protection called the birthday rule: once per year, during the 30 days following your birthday, you may switch to a Medigap plan with equal or lesser benefits from any carrier — without underwriting. This is a Florida-specific right that gives Medigap enrollees an annual window to shop for lower premiums on the same plan letter.

The bottom line: if you're enrolling in Medicare for the first time, your Medigap OEP is your one guaranteed shot at getting any Supplement plan you want at the best available rate. Don't let it close without making an informed decision.

Annual Enrollment Period (AEP)

The Annual Enrollment Period — October 15 through December 7 every year — is the main window that applies to all Medicare beneficiaries. This is when the insurance industry releases new plan designs for the following year, and beneficiaries can review their options and make changes.

What you can do during AEP:

• Switch from one Medicare Advantage plan to a different Medicare Advantage plan
• Switch from Medicare Advantage back to Original Medicare (Parts A + B)
• Switch from Original Medicare to a Medicare Advantage plan
• Enroll in, switch, or drop a standalone Part D prescription drug plan

What you cannot do during AEP:

• Enroll in a Medigap plan with guaranteed issue rights (unless you have a qualifying SEP)
• Make changes that take effect before January 1 of the following year

All AEP changes take effect January 1. If you make a change on October 15 and then change your mind on December 6, the last change you submit before December 7 is the one that counts.

Why AEP matters every year, even if you're happy: Plans change their premiums, cost-sharing structures, and drug formularies every January. A plan that was the best fit in 2024 may not be the best fit in 2025. Every fall, around late September, your plan sends you an Annual Notice of Change (ANOC) letter. That letter tells you exactly what's changing. Reading it — or having a broker review it for you — is the single most important Medicare task of the year.

Medicare Advantage Open Enrollment Period (OEP)

The Medicare Advantage Open Enrollment Period runs January 1 through March 31 every year. It's specifically for people already enrolled in a Medicare Advantage plan who realize — after the AEP changes took effect — that their new plan isn't working for them.

What you can do during the MA OEP:

• Switch from your current Medicare Advantage plan to a different Medicare Advantage plan
• Drop your Medicare Advantage plan entirely and return to Original Medicare — and add a standalone Part D drug plan

What you cannot do during the MA OEP:

• Switch from Original Medicare to Medicare Advantage (this requires AEP or a SEP)
• Add or switch a standalone Part D plan if you're staying in Medicare Advantage
• Enroll in Medigap with guaranteed issue rights (unless a qualifying SEP applies)

Changes made during the OEP take effect the first day of the month after the plan receives your enrollment request. If your new plan receives your application on February 10, your coverage switches March 1.

You can only use the MA OEP once per calendar year. It's a one-change window, not a revolving door.

Special Enrollment Periods (SEPs)

Special Enrollment Periods are triggered by qualifying life events and allow you to make Medicare changes outside of the standard enrollment windows. SEP availability, duration, and what you can do varies by the specific event.

Common qualifying events and their SEPs:

Qualifying Event	SEP Window	What You Can Do
Losing employer or union coverage	8 months from coverage loss date	Enroll in Part B, choose a plan
Moving out of your plan's service area	60 days from the move	Switch to a plan available in new area
Your plan loses its Medicare contract	60 days from contract end notice	Switch to any available plan
Gaining Medicaid (dual eligibility)	Ongoing — monthly changes allowed	Switch plans, access Special Needs Plans (D-SNPs)
Moving into/out of a skilled nursing facility	60 days from move	Switch plans
Enrolling in/losing Extra Help (LIS)	Ongoing — monthly changes allowed	Switch Part D plans
Returning to the U.S. after living abroad	60 days from return	Enroll in Part B, choose a plan

One SEP worth knowing for Florida residents specifically: the Five-Star SEP allows you to switch to a Medicare Advantage or Part D plan that earned a 5-star quality rating from CMS — once per year, anytime from December 8 through November 30. Not every county has a 5-star plan available, but when one exists, this SEP gives you a mid-year escape hatch to access it.

Important note about Medigap and SEPs: Most SEPs do not automatically grant guaranteed issue rights for Medigap plans. There are specific federal and state-level guaranteed issue situations (such as losing Medigap coverage because your insurer became insolvent, or returning to Original Medicare after a Medicare Advantage plan leaves your area). Outside of those specific triggers, Medigap is subject to underwriting after your initial OEP ends.

The Most Common Enrollment Mistakes

Knowing the enrollment windows is only useful if you know the pitfalls. These are the mistakes we see most often:

• Assuming Medicare is automatic at 65. If you're not receiving Social Security benefits yet, you must actively enroll in Part B. Part A may be automatic, but Part B requires an application — and missing it means penalties.
• Confusing "still working" with "still covered." Working past 65 only delays Medicare without penalty if your employer coverage is through your own current employment (or your spouse's). COBRA, retiree coverage, or coverage from a former employer does NOT count as qualifying coverage for delaying Part B.
• Waiting until December 7 to enroll during AEP. Plans can hit capacity, and a late application can result in processing delays. Early October is the ideal time to review and enroll.
• Ignoring the ANOC letter in September. This is the one document Medicare sends that actually tells you what's changing on your plan. Most people throw it away.
• Letting the Medigap OEP close without a decision. You can always wait and try to enroll in Medigap later — but after the OEP, insurers can decline you based on health history. In Florida, the birthday rule provides some ongoing protection, but the OEP is your strongest window.

Not Sure Which Enrollment Period You're In?

The rules around Medicare enrollment are genuinely complex, and the consequences of a wrong move — a permanent penalty, a coverage gap, or a year locked into a plan that doesn't fit — are serious. That's why getting clarity before you act matters.

At Core Insurance Solutions, we help people in Lakeland and across Polk County figure out exactly where they stand: which enrollment window applies to their situation, what their options are, and what the tradeoffs look like across every available plan. We work with all major carriers, and our consultations are completely free — we're compensated by whichever carrier you enroll with, at no extra cost to you.

Whether you're turning 65, coming off employer coverage, unhappy with your current plan, or just trying to make sense of a letter you received — reach out to us and we'll figure it out together.