Medicare Enrollment Periods

Your Initial Enrollment Period runs for seven months: the three months before the month you turn 65, your birthday month itself, and the three months after. That is the window in which most people first sign up.

When you enrol inside those seven months changes when coverage starts, and the difference is a full month.

• Enrol in the first three months — before your birthday month — and coverage begins on the 1st of your birthday month.
• Enrol in your birthday month or in any of the three months after, and coverage begins on the 1st of the month after you sign up.

That is 42 CFR 407.25(a)(2), and it is the practical argument for enrolling early rather than on time: the same seven-month window can leave you covered on your birthday or a month later, and the difference is entirely down to which month you filed. Our Initial Enrollment Period calculator works out your exact dates, and the Turning 65 timeline sets them beside everything else that has to happen.

Many people are enrolled automatically and never file anything: if you are already receiving Social Security or Railroad Retirement benefits before 65, Parts A and B start for you. If you are not, enrolment is an action you have to take — nobody does it for you, and nobody calls to remind you.

The one common reason to delay Part B on purpose is active employer coverage. That is a genuine exception with its own rules, and the Special Enrollment Period section below is where it lives — read it before deciding to wait.

Every year from October 15 through December 7, anyone with Medicare can change how they get it. Changes made in this window take effect January 1.

In the Annual Enrollment Period you can:

• join a Medicare Advantage plan, or leave one and go back to Original Medicare;
• switch from one Medicare Advantage plan to another;
• join, switch, or drop a Part D prescription drug plan.

This is the window that matters to the most people, and the one most often let slip. The reason to look every year, even when nothing about your health has changed, is that the plans change even when you don't. Each September your plan sends an Annual Notice of Change describing what is different in January: premiums, copays, the provider network, the drug formulary. A plan that fitted you well this year can fit badly next year without you doing anything at all. Our ANOC explainer covers how to read that notice.

One thing the Annual Enrollment Period does not do: it does not give you the right to buy a Medicare Supplement (Medigap) policy regardless of your health. Medigap runs on its own rules and its own windows — see how Medigap plans work. Leaving an Advantage plan in the fall and assuming a Medigap policy will be waiting in January is a common and expensive misreading.

If you make more than one change during the window, the last one on file before December 7 is the one that takes effect.

From January 1 through March 31 there is a second, narrower window — and it is narrower in three ways people regularly get wrong.

• It is only for people already in a Medicare Advantage plan on January 1. If you are in Original Medicare, this window is not available to you.
• It allows one change. Not a season of shopping — one.
• You can switch to a different Advantage plan, or drop back to Original Medicare and add a Part D plan. You cannot use it to join an Advantage plan from Original Medicare.

Coverage from a change made here starts on the first of the month after the plan receives your request.

This window exists as a correction mechanism: you started a plan on January 1, you have now actually used it, and something is wrong — your doctor turns out to be out of network, a drug you take is not on the list the way you expected. It is a chance to fix that, once.

People conflate it with the fall Annual Enrollment Period constantly, and the cost of the confusion is real: someone who believes they can shop freely in the spring may skip the fall window that was their actual opportunity. If you are choosing between them, the fall window is the broad one. And as with the fall, dropping an Advantage plan here does not by itself create a right to buy a Medigap policy.

The General Enrollment Period is the fallback for people who missed their Initial Enrollment Period and have no Special Enrollment Period to use. It runs January 1 through March 31 each year, and coverage begins on the first of the month after you enrol — a rule that changed in 2023 and improved things considerably; it used to mean waiting until July.

This is the window that carries the penalties, and they are worth stating plainly because they are permanent.

The Part B late-enrollment penalty adds 10% to your Part B premium for each full 12-month period you could have had Part B and didn't, and you pay it for as long as you have Part B. Two years late is 20% a month, indefinitely — and because the penalty is a percentage of the standard premium ($202.90 in 2026), the dollar amount grows as the premium does. Our Part B penalty calculator works out what a specific gap costs.

The Part D late-enrollment penalty works differently: 1% of the national base beneficiary premium ($38.99 in 2026) for every full month you went without Part D or other creditable drug coverage, once a gap reaches 63 days. It also lasts for as long as you have drug coverage. The Part D penalty calculator and the Part D hub cover it in full.

Neither penalty applies if you had a valid reason to wait and a Special Enrollment Period covers you. That is the next section, and if you delayed because of employer coverage it is the one that matters to you.

A Special Enrollment Period is Medicare's acknowledgement that circumstances change between windows. Something happens; a window opens early. These are the common triggers, with the window each one usually creates.

• You moved out of your plan's service area. Typically the month of the move plus two more. Tell the plan before you move and the window can begin a month earlier.
• You lost employer or union coverage. Two months for Medicare Advantage or Part D — but Part B runs on a completely separate eight-month clock. Confusing the two is the most costly mistake in this whole section: someone who treats the two-month plan window as their Part B deadline can miss Part B entirely and pick up a lifelong penalty.
• Your plan is leaving. A carrier non-renewal or a service-area reduction opens a window around the date coverage ends, and Medigap guaranteed-issue rights usually attach — on their own separate timetable, typically 63 days around the loss. Verify before relying on it.
• Your Medicaid or Extra Help status changed. Usually the month of the change plus two. People with both Medicare and Medicaid may instead have a recurring monthly window depending on status and state.
• A FEMA disaster was declared where you live. The window runs from the incident start through two full months after it ends — and FEMA frequently extends the end date, which extends your window with it. Check the declaration rather than assuming.
• A 5-star plan serves your county. December 8 through November 30, usable once a year, and only to move into a 5-star plan.

CMS lists dozens more — for people entering or leaving an institution, for plan sanctions, for contract violations, and others.

Treat every date on this page as a starting point to verify, not a determination. Special Enrollment Period rules carry per-case and per-state variation that a trigger date alone cannot capture, and the consequence of guessing wrong is a coverage gap plus a permanent penalty. Confirm your own window with 1-800-MEDICARE, with your State Health Insurance Assistance Program, or with a licensed agent before you rely on it.

One window sits outside all of the above, runs once, and does not come back: your Medigap open enrollment period.

It lasts six months, beginning the first month you are both 65 or older and enrolled in Part B. During those six months you can buy any Medicare Supplement policy sold in your state at the same price a healthy person pays. An insurer cannot turn you down, cannot charge you more, and cannot exclude a pre-existing condition beyond a limited period, because of your health.

After it closes, that protection is gone in most states. Buying a Medigap policy later generally means medical underwriting — answering health questions, with the insurer free to charge more or decline you outright.

This is why the Medigap window belongs on an enrollment page rather than only on a plans page. The decision you make in your first months on Medicare quietly sets the terms of every later decision. Someone who chooses a Medicare Advantage plan at 65 and wants Original Medicare with a Medigap policy at 72 may find the door is now conditional on their health — and health at 72 is exactly what tends to have changed.

There are protections that survive: certain guaranteed-issue rights when coverage is lost involuntarily, and a trial right if you joined an Advantage plan when you first became eligible at 65 and leave within the first year. Several states go further, giving residents broader rights to buy Medigap later — the rules genuinely differ by state, so check your own.

The Medigap hub explains how the standardized plans compare. And whichever direction you are leaning, it is worth knowing which door stays open before you walk through the other one.