Almost every deadline in Medicare is forgiving. Miss the fall Open Enrollment Period and you get another one next October. Miss a Part D switch and you can usually fix it in January. Even the Part B late enrollment penalty, painful as it is, still lets you enroll.

The Medigap Open Enrollment Period is different, and it is the reason this page exists. It happens once. It is not on the fall calendar. Nothing renews it. And on the other side of it, in most of the country, a private insurer gets to look at your medical history and decide whether it wants you. This page is about timing only, not about which plan to buy. Everything below was verified against CMS, Medicare.gov, KFF and state insurance regulators on September 6, 2026.

What the Window Actually Is

Medicare.gov states the rule in one sentence: you get a six-month Medigap Open Enrollment period, which starts the first month you have Medicare Part B and you are 65 or older. CMS repeats it in the 2026 edition of Choosing a Medigap Policy and adds the part people miss: this is a one-time enrollment period, it does not repeat every year like the Medicare Open Enrollment Period.

Inside those six months, three things are true at once:

  • Any policy, no questions. An insurance company cannot refuse to sell you any of the Medigap policies it offers, and it cannot use medical underwriting to accept or deny you.
  • Best price. It cannot charge you more because you have health problems.
  • Coverage starts on time. It cannot make you wait, except for a pre-existing condition waiting period of up to six months, and even that vanishes if you had at least six months of prior continuous creditable coverage with no break longer than 63 days.

Two triggers people get wrong. First, the clock starts on Part B, not on turning 65. If you keep working past 65 with employer group coverage and delay Part B, your window has not opened yet. CMS says explicitly that when the employer coverage ends and you sign up for Part B without a penalty, your Medigap window starts then, when you can actually use it. That is one of the few places where the Part B late enrollment rules and the Medigap rules point in the same direction.

Second, if you have had Medicare under 65 because of a disability or ESRD, federal law gives you a fresh six-month window when you turn 65, regardless of whether you could buy a policy before.

What It Costs to Miss It

CMS lists three consequences of shopping outside the window: you may have to pay more, fewer policy options may be available, and there is no guarantee an insurance company will sell you a policy at all if you do not meet its medical underwriting requirements.

That last one is not theoretical. KFF reviewed 15 Medigap applications from 12 major insurers and found declinable conditions including Alzheimer’s disease, asthma requiring inhalers, cancer, congestive heart failure, diabetes with complications, end-stage renal disease, high blood pressure and stroke. Applications also ask about functional status (difficulty with activities of daily living, wheelchair use, being homebound), specific medications, pending test results, and recent hospitalizations. Several applications tell the applicant not to submit at all if any answer is yes.

Underwriting only makes coverage more expensive if it lets you in. If it does not, the alternative to Medigap is Original Medicare with no out-of-pocket cap, or a Medicare Advantage plan you may not want. Medigap premiums themselves are a separate question, covered in the Medigap cost guide and the Plan G vs Plan N comparison.

Guaranteed Issue Rights: The Federal Exceptions

Outside your open enrollment window, federal law still forces insurers to sell in a short list of situations called guaranteed issue rights, or Medigap protections. In these situations the company must sell you a policy, must cover your pre-existing conditions with no waiting period, and cannot charge you more for your health history.

Here is the current federal list, from the 2026 CMS guide:

  1. Your Medicare Advantage plan is leaving Medicare, stops serving your area, you move out of its service area, or its network changes significantly, and you switch to Original Medicare rather than another Advantage plan. You can buy Plan A, B, C, D, F or G. Plans C and F are only available if you were eligible for Medicare before January 1, 2020.
  2. You have Original Medicare plus an employer group plan (including retiree, COBRA or union coverage) that pays after Medicare, and that plan is ending. With COBRA you can buy right away or wait until the COBRA ends.
  3. You have a Medicare SELECT policy and you move out of its service area.
  4. Trial right one. You joined a Medicare Advantage plan or a PACE program when you were first eligible for Part A at 65, and within the first year you decide to switch to Original Medicare. This is the strongest one: you can buy any Medigap policy sold in your state.
  5. Trial right two. You dropped a Medigap policy to join a Medicare Advantage plan (or a Medicare SELECT policy) for the first time, you have been in the plan less than a year, and you want your old policy back. You get the same policy if the insurer still sells it, otherwise Plan A, B, C, D, F or G.
  6. Your Medigap insurer goes bankrupt or your coverage ends through no fault of your own.
  7. Your Medicare Advantage plan or Medigap insurer broke the rules or misled you, and you leave because of it.

The deadlines are tight. In most of these situations you must apply no earlier than 60 days before your coverage ends and no more than 63 days after it ends. Keep the termination letter, the claim denial and the postmarked envelope, because the insurer will want proof. CMS notes rights can last an extra 12 months in certain circumstances, which is a question for your State Insurance Department.

What is not on the list matters as much as what is. Voluntarily dropping Medigap, voluntarily dropping retiree coverage, losing Medicaid eligibility, and simply changing your mind about Medicare Advantage after year one are all outside federal protection. So is being under 65, which is why a qualifying life event in the commercial insurance sense has no Medigap equivalent.

Switching From Medicare Advantage Back to Medigap

This is the situation that sends most people to this page, so here is the honest answer.

Leaving Medicare Advantage is easy. You can return to Original Medicare during the fall Open Enrollment Period, or during the Medicare Advantage Open Enrollment Period that runs January 1 to March 31. Buying a Medigap policy to sit alongside it is the part that can fail.

Unless you are inside one of the trial rights above, or your plan left your area, or you moved, or you live in a state with extra protections, the Medigap insurer can underwrite you. KFF’s analysis of 2022 CMS data found that 90% of Medicare Advantage enrollees age 65 and older, about 22.4 million people, have no guaranteed issue protection to buy Medigap outside the trial period. That is the actual size of the trap.

The practical rule: apply for the Medigap policy first and get a written approval, then disenroll from the Advantage plan. Medicare.gov is explicit that Medigap coverage cannot start until your Advantage coverage ends, so the sequencing is application first, effective date second. Doing it in the other order can leave you in Original Medicare with no supplement and no out-of-pocket cap. If you have not made the choice yet, Medicare Advantage vs Medigap lays out the trade before it becomes a one-way door.

States That Give You More Than the Federal Floor

State law can go further than federal law, and a handful do. These lists come from KFF’s state-by-state collection, whose most recent published review reflects 2024 regulations, so treat them as a starting point and confirm with your own State Insurance Department.

Continuous or annual guaranteed issue for everyone 65 and up: Connecticut, Massachusetts, Maine and New York. Connecticut and New York require continuous guaranteed issue year round. New York’s Department of Financial Services confirms it directly: state law requires any insurer writing Medigap to accept an application at any time of year, with no premium distinction for health status, claims experience or medical condition. Massachusetts regulation sets an annual February 1 to March 31 window but KFF reports every insurer there offers continuous enrollment anyway. Maine’s annual one-month window only requires insurers to offer Plan A, which is the least generous plan, so the protection is real but narrow.

Minnesota is changing. KFF reports Minnesota enacted an annual guaranteed issue period for people ages 65 to 70, usable once, during the October 15 to December 7 window, slated to take effect August 1, 2026. The same law permits a premium surcharge above the standard community rate, starting at 15% in 2026 and rising over time. KFF’s own text says August 1, 2026 while one of its figure notes says August 1, 2025, so verify the effective date with the Minnesota Department of Commerce before relying on it.

Birthday rule states. KFF identifies nine: California, Idaho, Illinois, Kentucky, Louisiana, Maryland, Nevada, Oklahoma and Oregon. A birthday rule lets someone who already has a Medigap policy switch to a plan with equal or lesser benefits around their birthday without underwriting. It does not let a person without Medigap buy one. Two of these we confirmed against state sources: Nevada’s Division of Insurance describes AB 250, effective January 1, 2022, giving current enrollees at least 60 days starting the first day of their birthday month to move to a plan with the same or lesser benefits, with no rate-up or denial based on health. Maryland’s Insurance Administration rate guide references a 30-day annual open enrollment period triggered by a policyholder’s birthday, during which tobacco-rated premiums may not be used.

Anniversary and any-time switching. KFF reports Missouri lets policyholders switch to an equivalent policy from a different insurer within 30 days before or after the annual anniversary of the policy, and that Maine and Washington let existing policyholders move to a plan with equal or lesser benefits at any time of year if the coverage gap is under 90 days.

Notice the shape of all of these. The generous state rules mostly help people who already hold a Medigap policy move sideways or down. Only the four continuous or annual states help someone who has none.

The Bottom Line

There is one date that decides your Medigap options for the rest of your life, and it is the first day of the first month in which you are both 65 and enrolled in Part B. Six months from that day, in 46 states and DC, the door closes and a private insurer gets a vote.

Three practical takeaways. If you are approaching 65 and not covered by an employer plan, enroll in Part B on time and buy the Medigap plan you want to keep, because trading up later usually requires underwriting. If you are delaying Part B for employer coverage, know that your window has not opened and will open when Part B does. And if you are in Medicare Advantage past your first year and thinking about switching, get the Medigap approval in hand before you disenroll. If premiums are the obstacle, check whether you qualify for a Medicare Savings Program first, since those pay Part B premiums and cost sharing and change the math on whether you need a supplement at all. Prescription assistance programs are worth a look at the same time, because Medigap never covers drugs no matter which letter you buy.