Medicare Supplement Plans (Medigap): What They Cover and How to Choose

What Medigap Is and How It Works

Medicare Supplement insurance — universally known as Medigap — is private health insurance sold specifically to fill the cost-sharing gaps left by Original Medicare (Parts A and B). When you have both Original Medicare and a Medigap policy, Medicare pays its share of your covered healthcare costs first, and then your Medigap policy pays its share of whatever is left, depending on the specific plan you have.

Medigap is regulated by both the federal government and individual states. Federal law standardizes the benefits within each plan letter, which means a Plan G sold by one insurer covers exactly the same services as a Plan G sold by a competitor. The only thing that legitimately varies between insurers for the same plan letter is the premium. This standardization makes comparison shopping straightforward once you understand what each letter covers.

Medigap policies only work with Original Medicare. They cannot be used alongside Medicare Advantage (Part C) plans. If you enroll in Medicare Advantage, a Medigap policy provides no benefit and insurers are prohibited from selling you one knowingly.

The Gaps Medigap Is Designed to Fill

Original Medicare Part A (hospital insurance) and Part B (medical insurance) cover a wide range of services but leave enrollees responsible for several categories of cost-sharing that can be significant, particularly for people with serious or ongoing health needs.

The primary cost-sharing gaps in Original Medicare include:

• Part A deductible: Applied per benefit period for hospital stays, not per year. A person hospitalized multiple times in a year could face this deductible more than once.
• Part A coinsurance for extended hospital stays: After a certain number of inpatient days, a daily coinsurance amount begins and increases for very long stays.
• Part B deductible: An annual deductible before Part B coverage begins.
• Part B coinsurance: After the deductible, Original Medicare pays 80 percent of approved costs for most Part B services; the enrollee owes the remaining 20 percent. For large medical bills, 20 percent can be substantial.
• Skilled nursing facility coinsurance: After a certain number of days in a skilled nursing facility, daily coinsurance applies.
• Foreign travel emergency coverage: Original Medicare provides very limited coverage outside the United States.

The Standardized Plan Letters

Federal regulations define ten standardized Medigap plan types, each identified by a letter. Not every plan is available in every state, and Massachusetts, Minnesota, and Wisconsin have their own alternative standardization systems. For the remaining states, the plans available as of 2025 are:

*Plan N covers Part B coinsurance but may require copays of up to $20 for some office visits and up to $50 for emergency room visits that don't result in inpatient admission.

Plans C and F are no longer available to people who became eligible for Medicare on or after January 1, 2020. People enrolled in these plans before that date may keep them.

Most Common Plans: G, N, and the Discontinued F

Plan G has become the most widely sold Medigap plan for new Medicare enrollees since Plan F was discontinued for new eligibles. Plan G covers nearly everything Plan F covered except the Part B deductible, which enrollees pay once per year. The trade-off is typically a lower monthly premium relative to Plan F. For most enrollees, Plan G provides the closest to comprehensive gap coverage available today.

Plan N is a popular lower-premium alternative to Plan G. It covers the same core benefits but introduces the possibility of small copays for some office visits and emergency room visits that do not result in a hospital admission. It also does not cover Part B excess charges — the additional amount that providers who do not accept Medicare assignment can bill beyond the Medicare-approved amount. Plan N works best for people who primarily see providers who accept Medicare assignment, which is the large majority of Medicare providers.

When to Enroll: The Open Enrollment Window

The Medigap open enrollment period is the six-month window that begins on the first day of the month in which you are both age 65 or older and enrolled in Medicare Part B. During this window, insurers are required to sell you any Medigap plan they offer at standard rates, regardless of your health history. They cannot charge you more due to pre-existing conditions, and they cannot deny you coverage.

This six-month window is one of the most important deadlines in Medicare planning. Missing it does not permanently eliminate your ability to buy Medigap, but it fundamentally changes the terms under which you can buy it.

Medical Underwriting Outside Open Enrollment

Outside of the initial open enrollment window, most states allow Medigap insurers to use medical underwriting when you apply. This means the insurer reviews your health history and can charge higher premiums, exclude coverage for pre-existing conditions, or deny your application entirely based on your health status.

A small number of states — including New York, Massachusetts, Connecticut, and a few others — require guaranteed issue for Medigap on a year-round basis, meaning insurers cannot deny coverage based on health status regardless of when you apply. If you live in one of these states, the urgency of the initial enrollment window is lower.

There are also federally guaranteed "special enrollment" periods that trigger guaranteed issue rights in specific circumstances — such as losing other coverage involuntarily, leaving a Medicare Advantage plan, or your insurer going bankrupt. These are narrow exceptions, not a substitute for enrolling during the initial window.

Medigap vs. Medicare Advantage: Not the Same

Medicare Advantage (Part C) and Medigap are two fundamentally different ways to supplement Original Medicare, and they cannot be combined.

Medicare Advantage replaces Original Medicare with a private plan that typically includes additional benefits (often including prescription drugs, dental, and vision) but restricts you to a plan network and may require referrals and prior authorizations. Medigap supplements Original Medicare without replacing it, allowing you to see any provider who accepts Medicare nationwide with minimal or no cost-sharing beyond your monthly premium.

The right choice depends on your health, your preferred providers, your travel patterns, and your financial situation. Medigap tends to work well for people who want predictable costs and broad provider access. Medicare Advantage can be a lower-cost option for people who are comfortable with a network-based plan and want the additional benefits. The Medicare.gov Medigap overview provides authoritative detail on both options.

What Medigap Costs and How Premiums Are Set

Medigap premiums vary by insurer, location, plan letter, and the pricing method the insurer uses. There are three ways insurers are allowed to price Medigap premiums:

• Community-rated (no-age-rated): Everyone in the same geographic area pays the same premium regardless of age. Premiums may increase due to inflation but not due to aging.
• Issue-age-rated: Premiums are based on the age at which you first bought the policy. Younger buyers lock in lower rates that do not increase solely due to aging.
• Attained-age-rated: Premiums increase as you get older in addition to any general inflation increases. This is the most common pricing method and typically starts with the lowest premium but increases the most over time.