Medicare Supplement Eligibility

To buy a Medicare Supplement (Medigap) plan, you must be enrolled in Original Medicare (Part A and Part B). But meeting that basic requirement is only the first step. When you apply matters just as much as whether you qualify — and the rules vary significantly from state to state.

This guide breaks down every eligibility factor you need to understand before shopping for Medigap coverage, from the critical Open Enrollment window to guaranteed issue rights and special state protections.

Basic Eligibility: Medicare Part A and Part B

The most fundamental requirement for any Medigap plan is straightforward: you must be enrolled in both Medicare Part A and Part B (also known as Original Medicare). Medicare Advantage (Part C) enrollees cannot purchase a Medigap plan unless they first switch back to Original Medicare.

Most people become eligible for Medicare at age 65, though younger individuals with certain disabilities or End-Stage Renal Disease (ESRD) may also qualify. Having Part A and Part B active is the baseline — but the timing of your Medigap application is what truly determines your options and costs.

It is worth noting that Medigap plans do not work with Medicare Advantage. If you currently have a Medicare Advantage plan and want to purchase Medigap, you must first disenroll from Medicare Advantage and return to Original Medicare. Once your Original Medicare coverage is active, you can then apply for a Medigap policy.

The Medigap Open Enrollment Period

Your Medigap Open Enrollment Period (OEP) is the single most important window in Medicare Supplement planning. It lasts 6 months, and it begins on the first day of the month you are both:

• Age 65 or older, and
• Enrolled in Medicare Part B

During this 6-month window, you have guaranteed issue rights. That means:

• Insurance companies must sell you any Medigap policy they offer, regardless of your health history
• They cannot charge you more because of pre-existing conditions
• They cannot impose a waiting period for coverage of pre-existing conditions

This is your best opportunity to lock in comprehensive Medigap coverage at the most favorable rates. Once this window closes, the rules change dramatically.

When Does the Clock Start?

A common point of confusion: the OEP does not always start when you turn 65. If you delayed Part B enrollment — for example, because you had employer coverage — your 6-month OEP begins when Part B actually takes effect, not your 65th birthday.

This distinction matters. Someone who retires at 67 and enrolls in Part B at that point would have their Medigap OEP start at 67, not two years earlier. Conversely, if you enrolled in Part B at 65 but waited until 66 to start shopping for Medigap, your OEP may have already expired — leaving you subject to medical underwriting.

What Can You Buy During Open Enrollment?

During your OEP, you can purchase any Medigap plan available in your state. Medigap policies are standardized by letter (Plan A, Plan B, Plan C, Plan D, Plan F, Plan G, Plan K, Plan L, Plan M, and Plan N), with each letter offering a defined set of benefits. The most popular choices today are Plan G and Plan N, which balance strong coverage with reasonable premiums. Our comparison of Plan G vs. Plan N breaks down the differences in detail.

Guaranteed Issue Rights Outside Open Enrollment

Even after your OEP ends, certain life events trigger guaranteed issue rights that require insurers to sell you a Medigap policy without medical underwriting. These situations include:

• Your Medicare Advantage plan leaves your area or stops participating in Medicare
• You lose employer or union group health coverage that supplemented Medicare
• You leave a Medicare Advantage plan within the first 12 months to return to Original Medicare (a one-time "trial right")
• Your Medigap insurer goes bankrupt or your policy coverage ends through no fault of your own
• You were misled or given incorrect information when joining a Medicare Advantage or Medigap plan

In most guaranteed issue situations, you can purchase Medigap Plan A, B, C, F, K, or L. Some circumstances provide access to additional plan letters. The trial right is particularly important to understand: if you drop your Medigap plan to try Medicare Advantage for the first time, you have 12 months to return to Medigap with guaranteed issue protection. This gives you a risk-free way to test Medicare Advantage without permanently losing your Medigap eligibility.

For a deeper look at enrollment windows, see our guide on Medicare Supplement enrollment periods.

Under-65 Eligibility: Disability and Medicare

People under 65 can qualify for Medicare if they have received Social Security Disability Insurance (SSDI) benefits for 24 months, have ALS (Lou Gehrig's disease), or have ESRD. But here is the catch: federal law does not require Medigap insurers to sell policies to Medicare beneficiaries under age 65.

That said, many states have stepped in with their own protections. Roughly 30 states require insurers to offer at least some Medigap plans to disabled Medicare beneficiaries under 65. The availability, plan options, and pricing vary widely by state. In states without these protections, under-65 Medicare beneficiaries may have very limited options for supplemental coverage and might need to consider alternatives like Medicaid or employer retiree benefits.

If you are under 65 and on Medicare, check with your State Health Insurance Assistance Program (SHIP) to find out exactly what protections your state provides.

What Happens After Open Enrollment Ends?

If you miss your Medigap Open Enrollment Period and do not have guaranteed issue rights, you enter the world of medical underwriting. This means insurance companies can:

• Ask detailed health questions on your application
• Deny your application based on your health history
• Charge higher premiums for pre-existing conditions
• Impose waiting periods before covering pre-existing conditions (typically up to 6 months)

Common conditions that can lead to denial or surcharges include diabetes, heart disease, COPD, and recent cancer treatment. The specific underwriting standards vary by insurer — some are more lenient than others — but the bottom line is clear: applying during your OEP is almost always the smartest move.

It is also worth noting that even if you are approved through underwriting, you will likely pay more than someone who applied during their OEP. Pre-existing condition surcharges can add 15-25% or more to your monthly premium, and some conditions may result in a permanent rate increase rather than a temporary one.

State-by-State Variations

Medicare is a federal program, but Medigap regulation happens at the state level. This creates a patchwork of rules that can significantly affect your eligibility and options:

Because these rules have such a big impact on your options, checking your specific state's Medigap regulations should be one of the first things you do when exploring Medicare Supplement plans.

How to Check If You Are Eligible

If you are unsure whether you qualify for a Medigap plan or what protections your state offers, here are the best resources:

1. State Health Insurance Assistance Program (SHIP): Free, unbiased counseling funded by the federal government. SHIP counselors can walk you through your state's specific Medigap rules, help you understand your enrollment windows, and even assist with comparing plans. Find your local SHIP at medicare.gov.
2. Medicare.gov Medigap page: The official Medicare Supplement comparison tool lets you search for plans available in your area, compare coverage levels, and see which insurance companies sell Medigap in your zip code.
3. A licensed Medicare agent: An independent agent in your area can walk you through your state's rules and help you compare plan options side by side. Agents are typically compensated by the insurance company, so there is no cost to you for their help.

Key Takeaways

• You must have Medicare Part A and Part B to be eligible for any Medigap plan
• The 6-month Medigap Open Enrollment Period starting at age 65 (or whenever Part B begins) is your best window — guaranteed acceptance, no health questions
• Guaranteed issue rights provide additional protected opportunities after certain life events like losing employer coverage or leaving a Medicare Advantage plan
• Under-65 eligibility depends on your state — federal law does not mandate it, but roughly 30 states do
• Missing your OEP means facing medical underwriting, which can result in higher premiums or outright denial
• State rules vary significantly — some offer continuous open enrollment, birthday rules, or expanded under-65 protections
• When in doubt, contact your SHIP office or a local licensed Medicare agent for free, personalized guidance