Medicare enrollment periods: IEP, AEP, and OEP explained
Medicare has set windows when you can sign up or change your coverage. The three most common are the Initial Enrollment Period (IEP) when you first become eligible, the Annual Enrollment Period (AEP) each fall, and the Medicare Advantage Open Enrollment Period (OEP) in the first part of the year.
Initial Enrollment Period (IEP)
Your IEP is the seven-month window around your 65th birthday: the three months before the month you turn 65, the month itself, and the three months after. This is when most people first sign up for Parts A and B and choose how to get drug coverage. Signing up on time helps you avoid late-enrollment penalties.
Annual Enrollment Period (AEP)
AEP runs from October 15 to December 7 every year. During AEP anyone with Medicare can join, switch, or drop a Medicare Advantage or Part D plan, with changes taking effect January 1. This is the main window for reviewing your coverage as plans change their costs and formularies each year.
Open Enrollment Period (OEP)
The Medicare Advantage Open Enrollment Period runs from January 1 to March 31. If you are already in a Medicare Advantage plan, you can switch to a different Advantage plan or return to Original Medicare (and add a Part D plan) once during this window. Other special enrollment periods exist for life events. For your exact dates and options, contact Medicare.gov, 1-800-MEDICARE, or your SHIP.
Special Enrollment Periods (SEPs)
Life events can open a Special Enrollment Period outside the usual windows. Common examples include losing employer or union coverage, moving out of your plan's service area, qualifying for Extra Help or Medicaid, or moving into or out of a nursing facility. Each SEP has its own timing and rules about what you can change, so the key is to act promptly when a qualifying event happens. People who have both Medicare and Medicaid, or who receive Extra Help, generally get more frequent chances to change their drug or Advantage plan.
General Enrollment Period (GEP)
If you missed your Initial Enrollment Period and do not qualify for a Special Enrollment Period, the General Enrollment Period (January 1–March 31) is your chance to sign up for Part A and/or Part B. Coverage now begins the first day of the month after you enroll. Signing up late can mean lasting late-enrollment penalties, so the GEP is a safety net rather than a preferred path.
Don't confuse the two “open enrollment” windows
A frequent point of confusion: the Medicare Advantage Open Enrollment Period (January 1–March 31) is not the same as the Medigap Open Enrollment Period. The Medigap window is a one-time, six-month period that starts when you are 65 or older and enrolled in Part B, and it is when you can buy any Medigap policy without medical underwriting. They share the words “open enrollment” but apply to different products and different timelines.
The windows at a glance
Use the table below as a quick reference, then confirm your exact dates and options with an official source. Plans and rules can change, so Medicare.gov, 1-800-MEDICARE, and your SHIP are the places to verify before you act.
| Window | When | What you can do |
|---|---|---|
| Initial Enrollment Period (IEP) | 7 months around your 65th birthday | First sign up for Part A, Part B, Part D, or a Medicare Advantage plan |
| Annual Enrollment Period (AEP) | Oct 15 – Dec 7 | Anyone can join, switch, or drop a Medicare Advantage or Part D plan |
| Medicare Advantage OEP | Jan 1 – Mar 31 | Already in an Advantage plan? Make one change or return to Original Medicare |
| General Enrollment Period (GEP) | Jan 1 – Mar 31 | Sign up for Part A/B if you missed your IEP (penalties may apply) |
| Medigap Open Enrollment | One-time 6 months at 65+ on Part B | Buy any Medigap policy without medical underwriting |
| Special Enrollment Periods | Triggered by qualifying life events | Make changes tied to the event (e.g. losing employer coverage, moving) |
Frequently asked questions
When can I first sign up for Medicare?
Most people use their Initial Enrollment Period — the seven months around their 65th birthday (three months before, the birthday month, and three months after).
What's the difference between AEP and OEP?
AEP (Oct 15–Dec 7) is when anyone can join, switch, or drop Advantage and Part D plans. OEP (Jan 1–Mar 31) is a more limited window for people already in a Medicare Advantage plan to make one change.
What if I miss my Initial Enrollment Period?
You may be able to use a Special Enrollment Period if you qualify (for example, you had employer coverage). Otherwise, the General Enrollment Period (Jan 1–Mar 31) lets you sign up for Part A/B, though late penalties may apply.
Is the Medigap Open Enrollment Period the same as the Medicare Advantage OEP?
No. The Medicare Advantage OEP (Jan 1–Mar 31) is for people already in an Advantage plan. The Medigap Open Enrollment Period is a separate one-time six-month window at 65+ to buy a Medigap policy without health underwriting.
Related guides
This is general education, not personalized advice, and plan details change every year. For all of your options, contact Medicare.gov, 1-800-MEDICARE, or your local State Health Insurance Assistance Program (SHIP) at shiphelp.org.