Medicare Supplement Insurance: What It Covers and What It Doesn't

Tojocu Editorial Team | Last verified: April 2026

Medicare Supplement Insurance, commonly referred to as Medigap, is a private health insurance policy designed specifically to integrate with traditional federal Medicare. Medigap policies function to pay the out-of-pocket costs and deductibles that Original Medicare does not cover, providing retirees with highly predictable healthcare expenditures. Specific requirements, rates, and availability vary by state and insurer.

The Purpose of Medigap

Original Medicare, administered by the federal government, consists of Part A (hospital insurance) and Part B (medical insurance). While Original Medicare covers a substantial portion of healthcare costs for individuals aged 65 and older, it is not comprehensive. It features significant financial gaps, including deductibles, copayments, and coinsurance [1].

For example, Medicare Part B typically covers 80 percent of the approved cost for outpatient services and doctor visits. The patient is legally responsible for the remaining 20 percent. For a major surgery or ongoing cancer treatments, that 20 percent coinsurance can easily result in tens of thousands of dollars in out-of-pocket expenses [2]. Medigap policies are purchased from private insurance companies to fill these specific financial voids, paying the remaining 20 percent and shielding the retiree from catastrophic medical debt.

Standardized Plan Structures

To simplify the purchasing process and protect consumers, the federal government requires that all Medigap policies be strictly standardized. Medigap policies are identified by letters A through N [3].

Every Medigap policy with the same letter offers the exact same basic benefits, regardless of which private insurance company sells it or where the consumer resides. For instance, a Plan G policy sold by a massive national carrier in Texas provides the exact same medical coverage as a Plan G policy sold by a small regional carrier in Ohio [1]. The only difference between the two policies is the monthly premium charged by the respective insurance companies and their customer service ratings. Plan F and Plan G are historically the most comprehensive options, though federal legislation has restricted the availability of Plan F for newly eligible Medicare beneficiaries to curb unnecessary doctor visits.

Coverage Exclusions

While Medigap covers the financial gaps of Part A and Part B, it does not cover services that Original Medicare explicitly excludes. A standard Medigap policy provides zero coverage for long-term custodial care, such as nursing home stays or daily in-home aides [2]. Furthermore, Medigap policies do not cover routine dental care, vision care, eyeglasses, hearing aids, or private-duty nursing. Crucially, modern Medigap policies do not cover retail prescription drugs; beneficiaries must purchase a separate Medicare Part D policy to secure pharmaceutical coverage.

Eligibility and the Open Enrollment Period

To purchase a Medigap policy, an individual must be actively enrolled in both Medicare Part A and Part B. The timing of the purchase is the most critical factor in securing coverage.

The Medigap Open Enrollment Period is a one-time, six-month window that begins on the first day of the month in which an individual is both 65 or older and enrolled in Medicare Part B [3]. During this precise six-month window, the applicant possesses "guaranteed issue rights." This means the private insurance company cannot use medical underwriting. The carrier cannot refuse to sell the applicant any Medigap policy it offers, nor can it charge a higher premium due to pre-existing health conditions like heart disease or diabetes [1].

If an individual attempts to purchase a Medigap policy after this six-month window closes, they lose their guaranteed issue rights. The insurance company can require a full medical questionnaire and retains the legal right to deny coverage entirely or charge exorbitant premiums based on the applicant's current health status.

Premium Calculation Methodologies

Private insurance carriers utilize three distinct methodologies to calculate and adjust Medigap premiums over time. Consumers must understand these pricing structures before purchasing a policy, as they dictate long-term affordability [2].

**Community-Rated:** Under this structure, every individual holding the same policy in a specific geographic area pays the exact same monthly premium, regardless of age. The premium is not increased simply because the policyholder grows older, though it may increase due to inflation.

**Issue-Age-Rated:** The premium is calculated based on the applicant's exact age at the time they purchase the policy. A buyer who purchases at age 65 will pay a lower premium than a buyer who purchases at age 70. Once purchased, the premium will not automatically increase strictly because the policyholder ages, making this a highly predictable option.

**Attained-Age-Rated:** The premium is initially based on the buyer's current age but is mathematically designed to increase automatically every year as the policyholder grows older [3]. While attained-age policies often feature the lowest initial premiums for 65-year-olds, they frequently become the most expensive option as the policyholder enters their 80s on a fixed income.