Most people approaching 65 think the big Medicare decision is when to enroll. It isn’t. The bigger one is what to bolt onto Original Medicare once you’re in — a Medigap supplement or a Medicare Advantage plan. They look like two flavors of the same product. They’re actually two different systems, and the door to switch from one to the other doesn’t stay open forever.
Here’s the plain-English version of what each one is, what the trade-off looks like, and the rule that catches most people off guard.
What You’re Actually Choosing Between
Original Medicare (Parts A and B) covers a big chunk of your hospital and doctor costs — but not all of it. There’s a deductible. There’s a 20% coinsurance with no annual cap. So almost everyone adds something on top.
You have two main paths:
- Medigap (Medicare Supplement) — A private policy that pays the gaps Original Medicare leaves behind. You keep Original Medicare. You pay a monthly premium for the Medigap plan. You typically pay a separate Part D premium for prescriptions. Most doctors who take Medicare take Medigap.
- Medicare Advantage (Part C) — A private all-in-one plan that replaces Original Medicare for the people enrolled in it. Premiums are often very low, sometimes zero. Drug coverage is usually built in. But you generally have to use the plan’s network of doctors and hospitals.
One is a supplement that stretches Original Medicare further. The other is a swap.
The Trade-Off in One Sentence
Medigap usually costs more upfront and less when something goes wrong. Medicare Advantage usually costs less upfront and more when something goes wrong — in copays, in network rules, in prior authorizations.
If you rarely see a doctor, Medicare Advantage can look like a great deal. If you have a chronic condition, see specialists, or travel a lot, Medigap’s predictability tends to age better.
The Rule That Catches People Off Guard
Here’s the part nobody tells you when you’re comparing premiums on a Sunday afternoon: in most states, switching from Medicare Advantage to Medigap later can be hard or expensive.
When you first sign up for Medicare at 65, you get a one-time Medigap Open Enrollment window — six months long — where insurers have to sell you a policy at the standard rate, regardless of your health history. That’s the easy door.
After that window closes, in most states Medigap insurers can use medical underwriting. They can ask about your health, charge you more, or decline you outright. So the person who picks Medicare Advantage at 65, develops a serious condition five years later, and tries to move to Medigap for the predictable bills can find themselves stuck.
A handful of states have stronger rules — New York, Connecticut, Massachusetts, and a few others — that limit underwriting. Most don’t. This is worth checking before you decide, not after.
Questions to Walk Through Before You Pick
- Do you already have doctors and specialists you want to keep? Check whether they take Original Medicare (almost all do) and whether they’re in any Advantage plans you’re considering.
- Do you split time between two states or travel often? Original Medicare plus Medigap travels with you. Most Advantage plans are tied to a regional network.
- Do you have a chronic condition or expect significant care this year? Run the numbers on a heavy-use year, not an average year.
- What are the Medigap underwriting rules in your state after the open enrollment window closes? Your State Health Insurance Assistance Program (SHIP) can tell you for free.
- Are you the type who wants flat predictable premiums, or are you comfortable with copays and prior authorizations in exchange for a lower base cost?
None of these questions have a universal right answer. They have your right answer.
Where to Get Unbiased Help
Two places worth knowing about: Medicare.gov has a Plan Finder that lets you enter your prescriptions and pharmacies and see real cost estimates. SHIP counselors give free, unbiased Medicare guidance in every state — they don’t sell plans. A local independent agent who works with multiple carriers (not just one) is also useful, especially if you want help comparing the fine print on Advantage plans.
Lumeway’s Retirement bundle includes a Medicare Plan Comparison Worksheet that walks you through doctors, prescriptions, premiums, deductibles, and out-of-pocket maximums side by side. It’s an organizational tool to help you stop holding all the variables in your head while three insurance company websites are open in different tabs.
The Retirement bundle includes 15 step-by-step worksheets for Medicare, Social Security, pension elections, and beneficiary updates. Browse planning tools at lumeway.co.
The right Medicare choice isn’t the cheapest one in May. It’s the one that still works in November.
This post is for informational purposes only and does not constitute legal, financial, or medical advice. Medicare rules and Medigap underwriting protections vary by state and change from year to year. Verify current rules at Medicare.gov or with your State Health Insurance Assistance Program (SHIP) before making enrollment decisions.