What Medigap Does, and Why the Timing Matters

Medigap fills the gaps in Original Medicare. The window when you can buy one without medical underwriting is the most important Medicare timing rule most people have never heard of.

Medigap (Medicare Supplement) is private insurance that pays some or all of the out-of-pocket costs Original Medicare leaves to you — deductibles, coinsurance, and copays. There are standardized Medigap policy types (labeled by letter — A, B, D, G, K, L, M, N, and a few historical ones), so the benefits of a given letter are the same across carriers; the price is what varies. The most important rule about Medigap is timing: you have a six-month Medigap Open Enrollment Period starting the month you are 65 or older and first enrolled in Part B. During that window, you can buy any Medigap policy sold in your state regardless of health. Outside that window — except in specific guaranteed-issue situations and a few state-by-state protections — carriers can use medical underwriting and may decline you or charge more.

The timing rule is unforgiving in most states. Knowing about it before the window opens, rather than after it closes, is the entire point of this article.

The short answer

Medigap is private insurance that you pair with Original Medicare to cover the deductibles, coinsurance, and copays Medicare leaves to you. Policies are standardized — Plan G, Plan N, and a few others are sold by many carriers, but the benefits inside each letter are set by federal rules and identical across carriers. The price is what carriers compete on. The timing rule is the part most people miss: you have a six-month Medigap Open Enrollment window that begins the month you turn 65 and are enrolled in Part B. In that window, no medical underwriting. Outside that window, in most states, carriers can require underwriting and can decline you or charge higher rates based on your health history. A few federal “guaranteed-issue” situations and several state-specific protections create additional windows, but the default outside the initial six months is underwriting. The right answer is rarely “decide later.”

What Medigap actually does

Original Medicare pays its share of approved charges and leaves the rest to you:

• A Part A deductible per benefit period
• Part A hospital coinsurance after specific day thresholds
• A Part B annual deductible
• 20% coinsurance on most Part B services, with no annual out-of-pocket maximum

Medigap policies pay some or all of that “rest” depending on the policy letter:

• Plan G — covers virtually everything except the Part B deductible. The most comprehensive policy widely sold to new Medicare enrollees today.
• Plan N — covers most cost share but leaves small copays for some office visits and emergency room visits, and does not cover Part B excess charges.
• High-Deductible Plan G — lower premium, higher deductible before benefits begin. Suited to people comfortable with a higher annual out-of-pocket exposure in exchange for a lower monthly premium.
• Plans K and L — cost-sharing structures with different out-of-pocket maximums.
• Plans A, B, D, M — less commonly sold today but still available in many states.
• Plan F — comprehensive coverage including the Part B deductible. Closed to people who first became eligible for Medicare on or after January 1, 2020 (eligibility date, not enrollment date). Available to people whose Medicare eligibility date was before January 1, 2020, even if they enrolled later.
• Plan F High-Deductible — same closure rule as Plan F.

The benefits inside each letter are federally standardized, so a Plan G from Carrier A and a Plan G from Carrier B cover the same things. Price, customer service, claims administration, and rate-increase history are where carriers differ.

Three states — Massachusetts, Minnesota, and Wisconsin — have their own Medigap standardization rules. If you live in one of those states, the policy structure looks different.

Medigap pays after Medicare. You see a Medicare provider, Medicare pays its share, your Medigap pays its share, and your remaining bill is typically very small or zero, depending on which policy letter you have.

The timing rule that matters most

The federally guaranteed Medigap Open Enrollment Period is six months long. It starts the first month you are both 65 or older and enrolled in Medicare Part B. This is a one-time window. It does not repeat.

During this window:

• You can buy any Medigap policy sold in your state.
• The carrier cannot use medical underwriting.
• The carrier cannot refuse you because of health.
• The carrier cannot charge you more because of health (some carriers may rate by tobacco use, age, or other non-health factors that are state-permitted).

Under the federal rule that applies in most states, after this window:

• Carriers may use medical underwriting (a health questionnaire, sometimes medical records).
• Carriers may decline you for pre-existing conditions or recent medical history.
• Carriers may charge higher premiums based on health.

Massachusetts, Minnesota, and Wisconsin operate under their own Medigap standardization rules and may have different protections — if you live in one of those states, confirm the specifics with your state SHIP or department of insurance.

There are exceptions — federal guaranteed-issue rights apply in specific situations like an employer plan ending or a Medicare Advantage plan leaving your service area (Medicare.gov — Guaranteed Issue Rights). Some states have additional protections — annual birthday rules, anniversary rules, year-round guaranteed issue, or under-65 guaranteed issue — that go beyond the federal minimum.

But the default rule outside the six-month initial window, in most states, is underwriting.

Why this matters for the comparison

The Medigap timing rule is one of the few asymmetries in Medicare that shapes the entire path decision:

• Choosing Original Medicare + Medigap + Part D during your initial enrollment uses your guaranteed-issue window. You have a low-risk on-ramp into the Original Medicare path.
• Choosing Medicare Advantage at initial enrollment and switching to Original Medicare later does not preserve your Medigap guaranteed-issue window. In most states, that later switch requires underwriting.

This is not a recommendation to choose one path over the other. It is an honest description of how the rules work. The asymmetry is real, and many people learn about it after the window has closed.

Two states are widely known for being more accommodating: New York and Connecticut have year-round guaranteed issue for Medigap policies. Several states have annual birthday or anniversary rules that allow some switching protection. The specifics are state-by-state. Your state SHIP can tell you exactly what your state’s rules are.

How this applies to you

If you are turning 65. Your six-month Medigap Open Enrollment window starts the month you are enrolled in Part B. Use it deliberately. Even if you choose Medicare Advantage now, understand that this window is your one federally guaranteed opportunity to add Medigap without underwriting in most states.

If you are working past 65 with employer coverage and delaying Medicare. Your Medigap Open Enrollment window does not start at 65 — it starts when you enroll in Part B, whenever that is. You preserve the guaranteed-issue window for later.

If you are on Medicare Advantage and considering switching to Original Medicare. Confirm whether you have any federal guaranteed-issue right (for example, a trial right within your first year on MA) or any state-specific protection. If neither applies, plan for underwriting.

If you live in New York, Connecticut, or a state with strong protections. Your switching flexibility is greater than the federal minimum. Confirm the specifics with your state’s department of insurance or SHIP.

If you are helping a parent. The Medigap timing rule is one of the most consequential pieces of information you can give them before their initial enrollment.

Cost-side considerations

Medigap premiums vary by:

• Policy letter (Plan G typically costs more than Plan N)
• Carrier
• Age (most states allow attained-age, issue-age, or community-rated pricing — these structures change how premiums rise over time)
• Tobacco use (in many states)
• ZIP code or rating area
• Gender (in some states)

Two policies with the same letter and the same premium today can have different rate-increase histories. Asking carriers — or asking a state SHIP — about rate-increase history is a reasonable question.

Medigap pairs with a standalone Part D plan for prescription drug coverage. The OM + Medigap + Part D combination typically has a higher monthly premium total than a Medicare Advantage plan, with the trade-off being predictability of out-of-pocket costs and broader provider access. Whether that trade-off is right depends on your situation.

What this is not

It is not a recommendation to choose Medigap over Medicare Advantage. Both paths are legitimate. The Medigap timing rule is a structural fact, not a verdict.

It is not the same as Medicare. Medigap is private supplemental insurance regulated by your state’s department of insurance and federally standardized by CMS. It is not part of Medicare itself.

It is not available alongside Medicare Advantage. You cannot have a Medigap policy and an MA plan at the same time. Medigap is paired with Original Medicare.

It is not a substitute for SHIP counseling or a licensed agent. Both can walk you through your specific state’s rules and which carriers serve your area.

It is not legal advice. If your situation is unusual — under-65 disability, multi-state moves, employer-coverage transitions — talk to your state SHIP before assuming the rules apply uniformly.

Timing is the lever. Knowing the window exists is most of the protection.

• The Two Medicare Paths and What Each One Asks of You
• What Original Medicare Covers — and What It Does Not
• The Six-Month Medigap Window — companion piece focused on the window itself
• Switching Later: What People Often Miss