For Medicare beneficiaries, timing is everything when it comes to Medigap — the supplemental insurance that fills the gaps Original Medicare leaves behind. Unlike Medicare Advantage, which has a predictable Annual Enrollment Period every fall, Medigap operates under a different set of rules that catch many people off guard. The single most important window you'll ever have is your Medigap Open Enrollment Period, and once it closes, your options narrow considerably in most of the country.
Your Medigap Open Enrollment Period lasts exactly six months and begins on the first day of the month in which you are both 65 years old and enrolled in Medicare Part B. This is a federally guaranteed window, meaning every insurance company that sells Medigap policies in your state is legally required to sell you any plan they offer — at the same price they charge a healthy person your age — regardless of any pre-existing conditions you have. Whether you have diabetes, heart disease, or a history of cancer, insurers cannot deny your application or charge you a higher premium during this window. This is called guaranteed issue, and it is the strongest consumer protection in the Medigap market.
The practical implication is significant. A 65-year-old enrolling in Plan G — the most comprehensive Medigap plan available to new Medicare enrollees in 2025 — might pay $120 to $180 per month depending on their ZIP code and insurer. That same person, applying two years later after a serious diagnosis, could face premiums of $250 or more per month, or simply be turned down entirely, in states without additional protections. The six-month window is not just a bureaucratic formality; it is often the difference between affordable supplemental coverage and no coverage at all.
One common source of confusion involves people who delay enrolling in Part B because they are still covered by an employer or union health plan. If you are 65 but still working and covered by a large employer group plan, you can delay Part B without penalty — and your Medigap Open Enrollment Period will not begin until you actually enroll in Part B. So if you retire at 68 and enroll in Part B at that point, your six-month Medigap window starts then, not at 65. This is actually good news for working seniors, because it means you haven't permanently lost your protected window just because you delayed Medicare. However, you must enroll in Part B promptly when your employer coverage ends to trigger both your Part B Special Enrollment Period and your Medigap window simultaneously.
Beyond the Open Enrollment Period, federal law also provides Medigap guaranteed issue rights in specific situations called Special Enrollment Periods. These apply when something changes in your coverage circumstances through no fault of your own. For example, if you joined a Medicare Advantage plan when you first became eligible and then decide to switch back to Original Medicare within the first year, you have a guaranteed right to buy a Medigap policy. Similarly, if your Medicare Advantage plan leaves your area, goes bankrupt, or loses its Medicare contract, you are entitled to a guaranteed issue Medigap policy. If you have a Medigap policy and your insurer goes out of business, you also qualify. In each of these situations, you typically have 63 days from the date your prior coverage ends to select a Medigap plan with guaranteed issue protections.
Thirteen states have gone further than federal law by creating what are commonly called birthday rules or continuous open enrollment periods. In California, Idaho, Illinois, Kentucky, Louisiana, Maine, Maryland, Missouri, Nevada, New Jersey, New York, Oklahoma, and Oregon, beneficiaries have additional protected windows to switch Medigap plans. California's birthday rule, for instance, gives you a 30-day window each year around your birthday to switch to a Medigap plan with equal or lesser benefits without medical underwriting. New York and Connecticut go even further, requiring insurers to offer guaranteed issue Medigap coverage year-round to any Medicare beneficiary, regardless of age or health status — though premiums in those states tend to be higher as a result of that broader risk pool. If you live in one of these states, you have meaningful flexibility that most Americans simply do not have.
For beneficiaries who have already missed their Open Enrollment Period and do not qualify for a Special Enrollment Period, the path forward requires more strategy. In states without birthday rules or year-round guaranteed issue, you will need to apply for Medigap through the standard underwriting process. Insurers will review your medical history, and some conditions — including recent cancer treatment, heart attacks, strokes, or insulin-dependent diabetes — may result in denial or significantly higher premiums. In this situation, it is worth comparing Medigap costs against Medicare Advantage plans, which are required to accept all applicants during the Annual Enrollment Period (October 15 through December 7) regardless of health status. Medicare Advantage plans have $0 or low premiums in many markets but come with network restrictions and cost-sharing that Medigap eliminates.
When comparing Medigap plans, it helps to understand that the plans are standardized by letter — Plan A, Plan B, Plan D, Plan G, Plan K, Plan L, Plan M, and Plan N are the options available to new enrollees in most states. Plan F and Plan C, which covered the Part B deductible ($257 in 2025), are no longer available to people who became eligible for Medicare on or after January 1, 2020, though people who already have those plans can keep them. Plan G is now the most comprehensive option for new enrollees, covering everything except the Part B deductible. Plan N is a lower-premium alternative that requires copays of up to $20 for office visits and up to $50 for emergency room visits that don't result in inpatient admission. The benefits within each lettered plan are identical across insurers — what differs is the premium, the company's financial stability, and their history of rate increases.
To find and compare Medigap plans in your area, Medicare's Plan Finder tool at Medicare.gov allows you to enter your ZIP code and see available plans and estimated premiums. Your State Health Insurance Assistance Program, known as SHIP, provides free one-on-one counseling from trained volunteers who can walk you through your specific options based on your health situation, budget, and enrollment history. SHIP counselors do not sell insurance and have no financial stake in your decision, making them one of the most valuable and underused resources available to Medicare beneficiaries. You can find your local SHIP program through the Eldercare Locator at eldercare.acl.gov or by calling 1-800-677-1116.