Medicare enrollment periods

Medicare does not work like the insurance most people had at a job, where missing open enrollment meant waiting a year. Medicare's windows carry permanent consequences: sign up late without an excuse the rules recognize, and you can face premium penalties that last the rest of your life, plus months without coverage while you wait for the next window.

The system boils down to three separate jobs with three separate clocks. First, you enroll in Medicare itself (Parts A and B) around age 65, or later if you are still covered through work. Second, you can change your private coverage (Medicare Advantage and Medicare Part D drug plans) every fall. Third, and least forgiving, you get one six-month window to buy a Medigap policy with no health questions asked.

None of this happens automatically unless you are already collecting Social Security when you turn 65, in which case Parts A and B start on their own. Everyone else has to act, and the deadlines below determine when.

The windows at a glance#

The initial enrollment period#

Your initial enrollment period (IEP) lasts seven months: the three months before the month you turn 65, your birthday month, and the three months after ¹. During it you can sign up for Parts A and B through Social Security and choose a Part D drug plan or Medicare Advantage plan.

When coverage starts depends on when in the window you act. Sign up during the three months before your birthday month and coverage begins the first day of that month. Sign up during your birthday month or the three months after, and coverage begins the first day of the month after you enroll ². Enrolling early avoids a gap, especially if employer coverage is ending at 65.

Part D has its own quiet deadline inside the IEP: once you are 63 days past the end of your window without drug coverage that Medicare considers creditable (as good as a standard Part D plan), the Part D late penalty starts accruing ³.

Sources for this section: [1] [2] [3]

Special enrollment periods#

Special enrollment periods (SEPs) exist because millions of people have good reasons not to enroll at 65, most commonly a job. Others respond to life events after enrollment.

Working past 65#

If you (or your spouse) are still working at 65 and you are covered by that employer's group health plan, you can delay Part B and Part D without any penalty ⁴. Many people in this situation still take Part A at 65, since it is usually premium-free; those contributing to a health savings account often delay Part A too, because HSA contributions must stop once any part of Medicare begins. Working in retirement covers how jobs and Medicare interact more broadly.

When the employment or the coverage ends, whichever comes first, an eight-month SEP opens for Parts A and B ⁴. The window for joining a Medicare Advantage or Part D plan is much shorter, two months after the coverage ends, and waiting longer than 63 days for drug coverage starts the Part D penalty clock. Two traps catch people here. COBRA and retiree health plans do not count as coverage from current employment, so electing COBRA does not delay the Medicare deadlines. And if the employer has fewer than 20 employees, Medicare generally becomes the primary payer at 65, meaning you usually need to enroll during your IEP even while still working; the small-employer plan can refuse to pay claims Medicare would have covered ⁴.

Other special enrollment periods#

Federal rules added in 2023 create SEPs for people who missed enrollment because of circumstances beyond their control, including a declared disaster or emergency, release from incarceration, losing Medicaid coverage, or receiving wrong information from an employer or health plan ¹. Separate SEPs let people who already have Medicare Advantage or Part D plans change them midyear after events such as moving out of the plan's service area, entering or leaving a nursing home, losing other coverage, or a plan closing down; there is also a once-a-year option to move into a plan rated five stars ⁵.

Sources for this section: [1] [4] [5]

The general enrollment period#

If you miss your IEP and no SEP applies, the fallback is the general enrollment period (GEP), January 1 through March 31 each year. Coverage begins the first day of the month after you sign up, so a March signup starts April 1 ². The GEP is where late penalties usually get assessed, and the wait can be expensive in another way: someone whose IEP ended in April has no path to coverage until the following January signup at the earliest.

Sources for this section: [2]

Annual open enrollment (October 15 to December 7)#

The window most people mean by "Medicare open enrollment" runs October 15 through December 7 every year, with changes effective January 1 ¹. Anyone with Medicare can join, drop, or switch a Medicare Advantage plan, move between Medicare Advantage and original Medicare, or change Part D plans.

This is the annual maintenance window. Plans change their premiums, drug lists, pharmacy networks, and provider networks every year, and the "Annual Notice of Change" mailed each September spells out what your plan will do differently. Comparing alternatives on medicare.gov's Plan Finder, or with a free SHIP counselor, is worthwhile even for people happy with their current plan; a drug moving to a higher tier can cost more than any premium difference.

Sources for this section: [1]

Medicare Advantage open enrollment (January 1 to March 31)#

People already in a Medicare Advantage plan get one more chance each year: from January 1 through March 31 they can make a single change, either switching to a different Medicare Advantage plan or dropping to original Medicare and adding a stand-alone Part D plan ⁵. Changes take effect the first of the following month. People on original Medicare cannot use this window to join a Medicare Advantage plan; it only works in the directions described.

Sources for this section: [5]

The late enrollment penalties#

Medicare's penalties are designed to keep healthy people from waiting until they are sick to enroll. All three are calculated differently ³.

Part B penalty#

The Part B penalty adds 10 percent of the standard premium for each full 12-month period you were eligible but not enrolled and lacked an SEP, and you pay it for as long as you have Part B ³. Suppose your IEP ended in early 2024 and you enroll during the 2026 GEP, 26 months later. That is two full 12-month periods, so a 20 percent penalty: an extra $40.58 on top of the 2026 standard premium of $202.90, or roughly $243 a month ⁶. The percentage never expires, and the dollar amount grows every time the standard premium rises.

Part D penalty#

The Part D penalty is 1 percent of the national base beneficiary premium ($38.99 in 2026) for each full month you went without creditable drug coverage after your window closed, rounded to the nearest 10 cents and added to your plan premium for as long as you have Medicare drug coverage ⁷. Someone who waited 20 months pays 20 percent of $38.99, about $7.80 a month in 2026. It sounds small, but it is recalculated against each year's base premium and never goes away; over a 20-year retirement even this example approaches $2,000, and longer gaps produce proportionally larger penalties.

Part A penalty#

Most people get Part A premium-free and can never owe this one. Those who must buy Part A and enroll late pay a 10 percent higher premium, not forever but for twice the number of years they delayed ³.

Sources for this section: [3] [6] [7]

The Medigap window is different#

Everything above concerns Medicare itself and the plans sold under Parts C and D. Medigap, the supplemental insurance that fills original Medicare's cost-sharing gaps, follows an entirely separate rule that catches many people off guard.

Your Medigap open enrollment period lasts six months, starting the first month you are both 65 or older and enrolled in Part B, and it never repeats ⁸. During those six months, insurers must sell you any policy they offer at standard rates regardless of your health. Afterward, in most states, applications go through medical underwriting: the insurer can review your conditions and medications, charge more, or decline you outright.

Federal law restores guaranteed issue rights in limited situations, generally with a 63-day window, such as when a Medicare Advantage plan shuts down or leaves your area, when retiree coverage ends, or under "trial rights" for people who joined Medicare Advantage at 65 and switch back to original Medicare within the first 12 months ⁸. A few states go further: Connecticut and New York require insurers to accept Medigap applicants year-round, and some states have "birthday rules" allowing an annual switch between comparable policies. Everywhere else, the six-month window is effectively a one-time decision about whether you may ever want Medigap.

Caution: The choice between Medicare Advantage and original Medicare with Medigap is easiest to undo early. Once your Medigap open enrollment and any trial right have passed, health problems can lock you out of Medigap in most states, no matter how many annual enrollment periods come and go.

Sources for this section: [8]

RetiredWiki. (2026, July 6). Medicare enrollment periods. https://retiredwiki.com/article/medicare-enrollment-periods