When you first become eligible for Medicare — typically at age 65 — you face a foundational decision: how will you supplement Original Medicare? The two main options are Medicare Supplement Insurance (Medigap) and Medicare Advantage (Part C). These approaches differ in structure, cost, predictability, and flexibility. Understanding those differences before enrollment matters because switching later can be more difficult than many people realize.
What Original Medicare Leaves Uncovered
Original Medicare — Parts A and B — covers a broad range of hospital and outpatient services but leaves significant cost-sharing in place. There is no annual out-of-pocket maximum under Original Medicare alone.
Medicare Supplement (Medigap): Standardized Gap Coverage
Medigap policies are sold by private insurers but are standardized by CMS. Each plan type must provide identical core benefits regardless of which insurer sells it. The most comprehensive plan available to individuals newly eligible for Medicare is Plan G, which covers Part A and B coinsurance, the Part A deductible, skilled nursing facility coinsurance, and 80% of foreign travel emergency costs.
Note: Plan F, which additionally covers the Part B deductible, is no longer available to individuals who became eligible for Medicare on or after January 1, 2020.
What Medigap does not cover: prescription drugs. Anyone with a Medigap policy must enroll separately in a Medicare Part D plan.
Medigap's primary advantage is predictability. After paying the monthly premium, most covered medical costs are handled. There are generally no provider networks — you may see any physician or hospital that accepts Medicare.
Medicare Advantage (Part C): Integrated Coverage
Medicare Advantage plans contract with CMS to deliver Part A and Part B benefits through a private insurer. Most plans bundle Part D drug coverage and many include extra benefits — dental, vision, or hearing — that Original Medicare does not cover.
Many Medicare Advantage plans carry a $0 monthly premium, though enrollees continue to pay their Part B premium. The trade-offs include provider networks, prior authorization requirements, and copayments — though by law MA plans must cap annual out-of-pocket costs for in-network services.
The Enrollment Window and the Underwriting Problem
The most consequential timing issue is the Medigap Open Enrollment Period (OEP) — a one-time, six-month window that begins the month you are both age 65 and enrolled in Medicare Part B. During the OEP, federal law grants you guaranteed issue rights: any insurer must sell you a policy and cannot charge more based on your health history.
After the OEP closes, this federal protection generally ends.
If you enroll in Medicare Advantage at 65 and later want to switch to Medigap, you will typically face medical underwriting in most states. The health condition that motivates the desire to switch may be precisely what prevents approval.
A small number of states — including Connecticut, Maine, Massachusetts, and New York — have enacted stronger protections providing year-round guaranteed issue rights.
Annual Enrollment Opportunities
Medicare Advantage enrollees may switch plans during the Annual Enrollment Period (AEP): October 15 through December 7 each year. There is also a Medicare Advantage Open Enrollment Period from January 1 through March 31. Returning to Original Medicare during either period does not automatically restore Medigap guaranteed issue rights.
Getting Help
State Health Insurance Assistance Programs (SHIPs) — federally funded, no-cost counseling programs in every state — provide unbiased guidance. Contact information is at Medicare.gov or 1-800-MEDICARE.
Educational purposes only. Not financial, tax, or legal advice.