Medigap Basics: How Medicare Supplement Insurance Works

Last reviewed: July 2026

What Medigap is

Medigap — formally, Medicare Supplement Insurance — is private insurance that works alongside Original Medicare (Part A and Part B). Original Medicare pays a large share of most covered services, but it leaves gaps: deductibles, hospital coinsurance, and, most notably, the 20% coinsurance on most Part B services, which has no annual cap. A Medigap policy pays some or all of that cost-sharing after Medicare pays its share. If you want a refresher on how the parts fit together, see our guide to the parts of Medicare.

Although private insurers sell Medigap, federal and state law tightly standardize it. A policy covers one person (spouses each need their own), and standardized policies are guaranteed renewable — as long as premiums are paid, the insurer cannot cancel the policy because of health problems.

The standardized plan letters

In most states, Medigap comes in ten lettered plans: A, B, C, D, F, G, K, L, M, and N. The letter defines the benefits, and every policy with the same letter has identical standardized benefits no matter which insurer sells it. A Plan G from one company covers the same things as a Plan G from another; what differs is the premium and the company's service. (Massachusetts, Minnesota, and Wisconsin standardize their plans differently under long-standing waivers, so residents of those states will see a different lineup.)

A few structural points about the letters:

Plans C and F are closed to newer beneficiaries. Under the MACRA law, plans that cover the Part B deductible ($283 in 2026) cannot be sold to anyone who became eligible for Medicare on or after January 1, 2020. People who were eligible earlier may still buy them where offered, and existing policyholders can keep them.
Plans K and L are cost-sharing designs. They pay a percentage of certain benefits (50% for K, 75% for L) until you hit an annual out-of-pocket limit. Once you have met that limit and the yearly Part B deductible, they pay 100% of covered cost-sharing for the rest of the calendar year. For 2026, those limits are $8,000 for Plan K and $4,000 for Plan L.
High-deductible versions of Plans F and G carry lower premiums but pay nothing until you have paid a deductible of $2,950 in 2026, an amount CMS adjusts annually for inflation.
Plan N uses modest copayments for some office and emergency-room visits in exchange for lower premiums.
Foreign travel emergency care is included in Plans C, D, F, G, M, and N: after a $250 annual deductible, the plan pays 80% of billed charges for emergency care that begins during the first 60 days of a trip abroad, up to a $50,000 lifetime maximum.

What Medigap does not cover

Medigap fills cost-sharing gaps in Medicare-covered services. It generally does not add new categories of coverage. Standardized policies do not cover:

Prescription drugs — policies sold since January 1, 2006 cannot include drug coverage, so Medigap holders who want drug coverage pair the policy with a standalone Part D drug plan
Long-term care, such as custodial nursing home or assisted-living care
Routine dental and vision care, eyeglasses, and hearing aids
Private-duty nursing

The one-time, 6-month Medigap open enrollment window

Federal law gives each beneficiary a single Medigap Open Enrollment Period: the six months that begin the first month you are both 65 or older and enrolled in Part B. During this window, an insurer must sell you any Medigap policy it offers in your state. It cannot use medical underwriting to reject you, exclude you, or charge you more because of your health history.

Two timing details matter. First, the window is tied to Part B — if you keep working past 65 with employer coverage and delay Part B, your Medigap window does not open (or burn) until your Part B coverage begins. Second, the window does not repeat. Once it closes, it is gone. One caveat even inside the window: if you had a gap in prior creditable coverage, an insurer may make you wait up to six months before covering costs tied to a pre-existing condition, though prior continuous coverage can shorten or eliminate that wait.

For how this window interacts with the other Medicare sign-up periods, see our guide to Medicare enrollment periods.

Buying outside the window: underwriting and guaranteed-issue rights

After the open enrollment window closes, insurers in most states may medically underwrite applicants — meaning they can decline coverage or charge more based on health. Federal law preserves "guaranteed-issue rights" in specific situations, such as when a Medicare Advantage plan leaves your area or you move out of its service area, when employer coverage that supplements Medicare ends, or under the "trial right" for people who joined Medicare Advantage when first eligible at 65 and return to Original Medicare within 12 months. These rights typically come with a limited application window, often 63 days from losing the prior coverage.

State rules vary widely on top of the federal floor — a handful of states provide broader year-round or birthday-based purchase rights, and access for beneficiaries under 65 (who qualify for Medicare through disability) is not guaranteed by federal law and differs state to state. Because these rules are local and fact-specific, this is exactly the kind of question your State Health Insurance Assistance Program (SHIP) can answer for free.

Why Medigap pairs only with Original Medicare

Medigap is designed to pay secondary to Original Medicare: Medicare processes the claim first, then the Medigap policy pays its standardized share. Medicare Advantage works on an entirely different chassis — a private plan replaces how your Part A and Part B benefits are delivered, with its own networks, copays, and out-of-pocket maximum. A Medigap policy cannot be used to pay Medicare Advantage cost-sharing, and it is illegal for anyone to sell you a Medigap policy while you are enrolled in a Medicare Advantage plan, unless you are documented as switching back to Original Medicare. The two paths — Original Medicare plus Medigap plus Part D, or Medicare Advantage — are compared in detail in our guide to Medicare Advantage vs. Original Medicare.

What Medigap costs

A Medigap premium is paid to the private insurer in addition to the Part B premium you pay to Medicare. Because benefits within a letter are identical, price differences between companies for the same letter are real differences. Insurers also use different pricing methods — community-rated, issue-age-rated, or attained-age-rated — which affect how a premium changes as you get older, and premiums can also rise with inflation and other factors regardless of method. Whether a lower premium today or slower increases later matters more depends on personal circumstances, which is a comparison worth making carefully with official tools rather than marketing materials.

Where to get unbiased help

Whether Medigap fits your situation — and if so, which letter and which insurer — depends on your health, budget, travel habits, and tolerance for network rules. No article can make that call for you. Three free, non-commercial resources can:

Medicare.gov, which explains the rules and lets you compare policies sold in your area
1-800-MEDICARE (1-800-633-4227), available 24 hours a day, 7 days a week, except some federal holidays
Your SHIP counselor, who provides free, personalized, unbiased counseling in every state

Sources

This guide is for general education only and is not medical, legal, insurance, or financial advice. For decisions about your own coverage, use official sources or free help from your SHIP counselor.