What Is Medigap? Medicare Supplement Insurance Explained

Original Medicare (Parts A and B) covers a large share of hospital and medical costs, but not all of it — deductibles, coinsurance, and copayments can still add up quickly, with no annual out-of-pocket cap. Medigap, also called Medicare Supplement Insurance, is private insurance designed specifically to cover those remaining gaps.

What Medigap Actually Covers

Depending on the plan, Medigap can cover Part A hospital coinsurance and up to 365 extra hospital days after Medicare benefits are used up, Part B coinsurance or copayments, the first three pints of blood, and in some plans, foreign travel emergency care that Original Medicare doesn't cover at all.

The Standardized Plans

Medigap plans are standardized by federal law into lettered categories (Plan A, G, N, and others), meaning Plan G from one insurer covers exactly the same benefits as Plan G from any other insurer — only the premium price differs. This makes comparison shopping mostly about price and insurer service quality, not benefit differences, which is unusual in insurance.

Medigap vs Medicare Advantage

This is the distinction that trips people up most. Medigap works alongside Original Medicare and a separate Part D drug plan — you keep Original Medicare and the supplement just covers the leftover costs. Medicare Advantage (Part C) replaces Original Medicare entirely with a private plan that often bundles drug coverage. You cannot have both Medigap and Medicare Advantage at the same time.

When to Enroll

The best time to buy a Medigap policy is during your 6-month Medigap Open Enrollment Period, which starts the month you're both 65+ and enrolled in Part B. During this window, insurers must sell you any Medigap policy at the standard rate regardless of health conditions. Miss it, and insurers in most states can use medical underwriting to charge you more or deny coverage outright.