Medicare Open Enrollment: Every Deadline You Need to Know
Medicare has more than one enrollment window, and mixing them up is one of the most common (and costly) mistakes people make. Here's each one, what it's for, and who it applies to.
Initial Enrollment Period (IEP) — around your 65th birthday
A seven-month window: three months before the month you turn 65, your birthday month, and three months after. This is your first chance to enroll in Part A, Part B, a standalone Part D plan, or a Medicare Advantage plan. Enrolling in the earlier part of this window generally gets your coverage started sooner — waiting until after your birthday month can delay when coverage actually begins, so don't wait if you don't have to.
Annual Enrollment Period (AEP) — October 15 to December 7, every year
This is "Medicare Open Enrollment" in the sense most people mean it. Anyone with Medicare can use this window to switch from Original Medicare to Medicare Advantage (or back), change Medicare Advantage plans, or change Part D plans — for coverage that starts January 1 of the following year. This is the one window worth marking on your calendar every single year, even if you're happy with your current plan: formularies, premiums, and networks can all change year to year, so a plan that was your best fit last year isn't guaranteed to still be the best fit.
Medicare Advantage Open Enrollment Period (MA OEP) — January 1 to March 31
A narrower, second window — but only for people who are already enrolled in a Medicare Advantage plan. You get one chance during this period to switch to a different Medicare Advantage plan or drop MA entirely and return to Original Medicare (picking up a standalone Part D plan in the process). It does not let you switch between standalone Part D plans if you're on Original Medicare the whole time — that only happens during AEP or a qualifying Special Enrollment Period.
Special Enrollment Periods (SEPs) — triggered by specific life events
Outside the windows above, certain events open a temporary enrollment window of their own: moving out of your plan's service area, losing employer or union coverage, your plan losing its Medicare contract, gaining or losing Medicaid or Extra Help eligibility, and several others. The available window length varies by the specific triggering event, so if something like this happens to you, check with Medicare or your plan promptly rather than assuming you have to wait for AEP.
The penalty for waiting too long
If you go without Part D or other "creditable" drug coverage (coverage at least as good as Medicare's) for 63 days or more after you were first eligible, you can be charged a late enrollment penalty — a percentage added permanently to your monthly Part D premium, based on how many months you went without coverage. It's not one-time; it stays on your premium for as long as you have Part D. If you have creditable coverage elsewhere (an employer plan, for instance), you generally avoid the penalty as long as you enroll in Part D within 63 days of losing that other coverage.
The practical takeaway
Mark the fall AEP window every year regardless of satisfaction with your current plan, know that MA OEP exists as a narrower do-over if you're already in an MA plan, and don't let a gap in drug coverage run past 63 days if you can help it.