Medicare Supplement Insurance — commonly called Medigap — fills the cost gaps that Original Medicare leaves behind, covering expenses like the 20% coinsurance under Part B and hospital deductibles. But unlike the Affordable Care Act marketplace plans, Medigap policies sold to people under 65 or outside specific protected enrollment windows are not required by federal law to ignore your health history. That means preexisting conditions can directly affect whether you can get a Medigap plan, how much you pay for it, and which plans are available to you. Understanding exactly when you are protected — and when you are not — can be the difference between affordable supplemental coverage and being locked out of it entirely.
The single most important protection for most beneficiaries is the federal Medigap Open Enrollment Period. This is a one-time, six-month window that begins the first month you are both age 65 or older and enrolled in Medicare Part B. During this window, any insurance company selling Medigap policies in your state must sell you any plan they offer, cannot charge you more because of your health status, and cannot make you wait for coverage of a preexisting condition — with one narrow exception. If you did not have creditable coverage (such as employer insurance) for the six months before enrolling in Part B, an insurer may impose a waiting period of up to six months for preexisting conditions, but they still cannot deny you the policy outright. This federal guarantee is your strongest protection, and it only comes around once under standard rules.
Once that six-month window closes, federal law no longer requires insurers to offer you guaranteed issue Medigap coverage in most circumstances. This is where medical underwriting enters the picture. An insurer can ask detailed questions about your health history on an application — conditions like atrial fibrillation, chronic obstructive pulmonary disease, diabetes with complications, obesity, or a history of cancer are among the conditions that commonly trigger either a higher premium or an outright denial. In states that allow it, insurers may also offer a policy but exclude coverage for a specific preexisting condition for a set period. The practical result is that a 70-year-old with Type 2 diabetes trying to switch from a Medicare Advantage plan back to Original Medicare plus a Medigap policy may find that no insurer in their state will sell them a Plan G at a standard rate — or at all.
There are, however, federally guaranteed Special Enrollment Periods that trigger guaranteed issue rights outside the standard window. These apply in specific situations: if your Medicare Advantage plan leaves your area or goes bankrupt, if you move out of your plan's service area, if you lose employer-sponsored coverage, or if you enrolled in Medicare Advantage when you first became eligible for Medicare and want to switch back to Original Medicare within the first year. In these cases, you have a 63-day window to enroll in certain Medigap plans — typically Plan A, Plan B, Plan D, Plan G, Plan K, or Plan L — without medical underwriting. The 63-day clock starts the day your prior coverage ends, so acting quickly is essential. Missing that window by even a few days can mean losing the protection entirely.
State law adds another layer of protection that varies significantly depending on where you live. Thirteen states have enacted what are known as birthday rules or broader guaranteed issue protections that go beyond federal minimums. In California, for example, beneficiaries have a 30-day window each year around their birthday to switch to a Medigap plan with equal or lesser benefits without medical underwriting — regardless of their health status. The same birthday rule applies in Idaho, Illinois, Kentucky, Louisiana, Maine, Maryland, Missouri, Nevada, New Jersey, Oklahoma, and Oregon. New York and Connecticut go even further, requiring insurers to offer Medigap plans on a guaranteed issue basis year-round to anyone enrolled in Medicare Part B, regardless of age or health history. If you live in one of these states, your options for switching or enrolling in Medigap are meaningfully broader than for beneficiaries in states like Texas or Florida, where no such additional protections exist.
The plan you choose also matters when it comes to preexisting condition rules. In 2026, the most popular Medigap plans among new enrollees are Plan G and Plan N. Plan G covers nearly all out-of-pocket costs under Original Medicare except the Part B deductible, which is $257 in 2026. Plan N covers the same gaps but requires copays of up to $20 for office visits and up to $50 for emergency room visits that don't result in inpatient admission. Both plans are subject to medical underwriting outside guaranteed issue periods in most states. Plan A, the most basic option, covers only core benefits and is the plan insurers are most commonly required to offer during Special Enrollment Periods. If you are in a guaranteed issue situation, you may not have access to the more comprehensive plans, which is another reason to enroll during your initial open enrollment window if at all possible.
For beneficiaries who are still working past 65 and covered by an employer group health plan, the timing of Medigap enrollment requires careful planning. If you delay Part B enrollment because of active employer coverage, your six-month Medigap Open Enrollment Period does not begin until you actually enroll in Part B — which could be years later. That means you can preserve your guaranteed issue rights by enrolling in Part B and a Medigap plan when you retire, even if you are 68 or 72 at the time. The key is that the window is tied to your Part B enrollment date, not your 65th birthday. However, if you enrolled in Part B at 65 and then dropped it or never enrolled in a Medigap plan, that original window is gone and you would be subject to underwriting.
If you are approaching your initial enrollment period or facing a life event that may trigger a Special Enrollment Period, the most practical step is to contact your State Health Insurance Assistance Program, known as SHIP. Every state has a SHIP office that provides free, unbiased counseling from trained advisors who can walk you through your specific situation, compare plan premiums in your area, and confirm whether you qualify for guaranteed issue rights. You can find your state's SHIP contact through the Medicare.gov website or by calling 1-800-MEDICARE. Additionally, your state's insurance commissioner office regulates Medigap insurers and can confirm which state-specific protections apply to you — a particularly important step if you live in one of the birthday rule states and want to understand exactly how and when to use that window.