Medicare Enrollment Periods Explained

Medicare enrollment periods are the specific windows when you can sign up for Medicare, switch plans, or make changes to your coverage. Missing the right window can mean a delayed start date, a coverage gap, or a permanent late enrollment penalty. There are six enrollment periods to know: the Initial Enrollment Period (IEP), the Annual Enrollment Period (AEP), the Medicare Advantage Open Enrollment Period (MA OEP), Special Enrollment Periods (SEPs), the General Enrollment Period (GEP), and Medigap Open Enrollment. Each one serves a different purpose depending on whether you're new to Medicare, already enrolled, or dealing with a life change. This guide covers what each period allows, when it runs, and who it applies to.

Medicare doesn't let you sign up or make changes whenever you want. Outside of your enrollment window, you may have to wait months for the next opportunity, and in some cases you'll pay more for coverage permanently. The Centers for Medicare & Medicaid Services (CMS), the federal agency that administers Medicare, sets these windows and the rules that apply within them. Knowing which period applies to your situation is the first step to enrolling on time and avoiding unnecessary costs.

Initial Enrollment Period (IEP)

Your Initial Enrollment Period is a 7-month window built around your 65th birthday. It starts 3 months before your birth month, includes your birth month, and runs 3 months after. This is when most people first sign up for Medicare Part A and Part B. Enrolling during your IEP helps you start coverage on time and avoid a Part B late enrollment penalty. See our turning 65 timeline for a month-by-month breakdown of what to do before your birthday.

Annual Enrollment Period (AEP)

AEP runs October 15 to December 7 every year. It's the main annual window for switching Medicare Advantage plans, changing Part D prescription drug plans, or moving between Original Medicare and Medicare Advantage. Changes made during AEP take effect January 1 of the following year. Read our AEP deep-dive guide for what you can change and how to compare plans before the deadline.

Medicare Advantage Open Enrollment Period (MA OEP)

MA OEP runs January 1 to March 31 each year. It's only available to people already enrolled in a Medicare Advantage plan, and it gives you one chance to switch to a different Advantage plan or move back to Original Medicare and add a standalone Part D plan. Unlike AEP, you can only make one change during MA OEP, and it doesn't allow switching between Part D plans without also changing your Advantage plan.

Special Enrollment Periods (SEPs)

A Special Enrollment Period opens when specific qualifying life events happen, such as moving out of your plan's service area, losing employer group health coverage, or qualifying for Extra Help with prescription drug costs. Each SEP has its own length and rules, and most run 2 to 3 months from the date of the triggering event. If you're retiring and losing employer coverage, see our guide on Medicare when you retire for how that SEP works.

General Enrollment Period (GEP)

GEP runs January 1 to March 31 each year, for people who missed their Initial Enrollment Period and don't qualify for a Special Enrollment Period. Coverage that starts through GEP begins the first day of the month after you enroll, which can mean a gap without coverage. According to Medicare.gov, enrolling through GEP after a coverage gap can trigger a Part B late enrollment penalty, an additional 10% of the standard Part B premium for each full 12-month period you were eligible but didn't enroll, and that penalty typically applies for as long as you have Part B. See our guide on the Medicare late enrollment penalty for how the calculation works.

Medigap Open Enrollment

Medigap Open Enrollment is a one-time, 6-month window that starts the month you're both 65 or older and enrolled in Medicare Part B. During this window, insurers must sell you any Medigap policy they offer regardless of your health history and can't charge you more based on past or current health conditions. Outside this windowMedicare.gov notes that Medigap insurers can use medical underwriting in most states, meaning they can deny coverage or charge higher premiums based on your health. See our comparison of Medicare Advantage vs. Medicare Supplement for how Medigap fits alongside Original Medicare.