Medicare Enrollment Periods Explained

Initial Enrollment Period

This is your first and most important enrollment window. It runs for 3 months before the month you turn 65, the month of your birthday, and 3 months after — 7 months total.

What you can do: Enroll in Part A, Part B, a Part D drug plan, and a Medigap plan. This is your guaranteed issue window for Medigap — no health questions, no underwriting.

When coverage starts: If you enroll in the 3 months before your birthday month, coverage starts on the first day of your birthday month. If you enroll in your birthday month or after, coverage starts 1–3 months later.

Special Enrollment Period

If you or your spouse are still working and covered by employer insurance when you turn 65, you can delay Medicare enrollment without penalty and enroll later during a Special Enrollment Period when that coverage ends.

SEP for employer coverage: You have 8 months from when your employer coverage ends (or you stop working, whichever comes first) to enroll in Part B without penalty.

Other SEP triggers include: Moving to a new area, losing Medicaid eligibility, leaving a Medicare Advantage plan that leaves your area, and other qualifying events.

Medigap Open Enrollment Period

This is your guaranteed right to buy any Medigap plan in your state without medical underwriting. It starts the month you’re both 65+ and enrolled in Part B, and lasts 6 months.

This window does not repeat. After it closes, switching Medigap plans in most states requires answering health questions and can result in denial or higher premiums.

Missouri exception: Missouri’s Anniversary Rule gives you an annual window to switch Medigap carriers without underwriting — but this is for switching between carriers, not for initial enrollment after your open enrollment window closes.

Annual Enrollment Period

Every fall, you can make changes to your Medicare Advantage or Part D plan. Changes take effect January 1 of the following year.

What you can do: Switch Medicare Advantage plans, switch Part D plans, join or drop a Medicare Advantage plan, or switch between Original Medicare and Medicare Advantage.

Important: AEP does NOT give you guaranteed issue rights for Medigap. If you want to switch from Medicare Advantage to a Medigap plan, you may face underwriting in most states (except Missouri’s Anniversary Rule window, which is separate).

Medicare Advantage Open Enrollment

If you’re enrolled in a Medicare Advantage plan, you can switch to a different MA plan or return to Original Medicare (and add a standalone Part D plan) during this window.

You cannot use this period to enroll in a Medigap plan with guaranteed issue rights — that still requires underwriting in most states.