When you move to Original Medicare (Part A and Part B), there are gaps. Part B leaves you paying 20 percent coinsurance with no annual cap. You cover those gaps one of two ways: a Medicare Advantage plan, or a Medigap policy paired with a Part D drug plan. You pick one path, not both.
This is one of the most consequential choices in Medicare, partly because switching later can be hard. Here is how the two compare.
Original Medicare plus Medigap
This path keeps you in Original Medicare and adds a private Medigap (Medicare Supplement) policy that pays some or all of what Original Medicare leaves behind.
How it works:
- You see any provider in the country that accepts Medicare. No networks.
- Medigap pays your Part A and Part B cost-sharing, depending on the plan letter you choose.
- You add a standalone Part D plan for drugs.
- There is no referral system and no prior authorization from the Medigap insurer.
Medigap policies are standardized by letter (Plan G and Plan N are the most common for new enrollees). A Plan G from one insurer covers the same things as a Plan G from another. They compete on price and service, not benefits.
The trade-off: higher monthly premium, but very predictable costs when you actually need care. For someone who travels, splits time between states, or wants to keep a specific specialist anywhere in the country, this path is hard to beat.
Medicare Advantage (Part C)
This path replaces Original Medicare with a private plan that bundles Part A, Part B, and usually Part D, often with extras like dental, vision, hearing, or a fitness benefit.
How it works:
- You use the plan network of doctors and hospitals.
- Many services require prior authorization.
- There is an annual out-of-pocket maximum, which Original Medicare alone does not have.
- The monthly plan premium is often low or zero, though you still pay your Part B premium.
The trade-off: lower fixed monthly cost and bundled extras, but network restrictions, prior authorization friction, and higher costs when you use a lot of care (up to the out-of-pocket maximum). Plans can change their networks, formularies, and benefits every year.
Side by side
| Factor | Original Medicare + Medigap | Medicare Advantage |
|---|---|---|
| Provider choice | Any provider that takes Medicare, nationwide | Plan network, often local |
| Referrals / prior auth | Generally none | Common |
| Monthly premium | Higher (Medigap + Part D) | Often low or zero plan premium (plus Part B) |
| Costs when using care | Very low and predictable | Varies, capped by out-of-pocket max |
| Drug coverage | Add standalone Part D | Usually included |
| Extra benefits (dental, vision) | Not included | Often included |
| Annual out-of-pocket cap | Built in via Medigap | Yes |
| Best for | Travelers, those wanting predictability and provider freedom | Those wanting low fixed cost and bundled extras, who stay local |
The timing trap that catches people
The single most important Medicare timing rule for this choice is the Medigap Open Enrollment Period.
It is a six-month window that starts the month you are both 65 or older and enrolled in Part B. During this window, insurers must sell you any Medigap policy they offer at the best available rate, regardless of your health. This is called guaranteed issue.
Outside this window, in most states, Medigap insurers can medically underwrite you. They can charge more, impose waiting periods for pre-existing conditions, or decline you entirely.
Why this matters for the Advantage-vs-Medigap choice: many people pick Medicare Advantage at 65 because the premium is low. If their health later declines and they want to switch to Original Medicare plus Medigap, they may find Medigap is now expensive or unavailable because they are past their guaranteed-issue window. The door that was open at 65 may be closed at 72.
A few states (for example, those with annual Medigap open enrollment or "birthday rule" protections) are more generous. Check your state rules with your SHIP counselor.
How to think about the choice
I work through these questions:
- Do you travel or live in two states? If yes, Original Medicare plus Medigap usually wins because there are no networks.
- Do your doctors take the Advantage plan you are considering? If your providers are out of network, the Advantage premium savings can disappear fast.
- How is your health, and your family history? If you expect heavy care, predictable Medigap costs can be worth the higher premium. If you are healthy and want low fixed cost, Advantage can be a good deal.
- Can you afford the Medigap premium now to lock in guaranteed issue? Buying Medigap during your open enrollment window protects future you, even if Advantage looks cheaper today.
- How much does the prior-authorization friction bother you? Advantage plans use it heavily. Some people find it stressful during serious illness.
What to do next
Use the Medicare Plan Finder on Medicare.gov to compare specific Advantage plans and Part D plans in your ZIP code.
For Medigap, get quotes for Plan G and Plan N from several insurers. Remember the benefits are identical by letter, so compare price and the insurer track record on rate increases.
Talk to your State Health Insurance Assistance Program (SHIP). It is free, unbiased counseling, unlike a commissioned agent.
For the bigger picture of moving onto Medicare, see our Medicare vs marketplace insurance guide and the open enrollment deadlines article for the Medicare enrollment calendar.
Sources
Frequently asked questions
Can I have both Medicare Advantage and Medigap?
No. Medigap only works with Original Medicare. You cannot use a Medigap policy to cover costs under a Medicare Advantage plan, and it is illegal for someone to sell you Medigap if they know you are in an Advantage plan.
When can I buy Medigap without medical underwriting?
Your best window is the six-month Medigap Open Enrollment Period that starts the month you are 65 or older and enrolled in Part B. During that window insurers cannot deny you or charge more for health reasons. Outside it, most states allow medical underwriting.
Does Medicare Advantage include drug coverage?
Most Medicare Advantage plans include Part D drug coverage. Original Medicare with Medigap does not, so you usually add a standalone Part D plan.
Which is cheaper?
Medicare Advantage usually has a lower or zero monthly plan premium but higher costs when you use care, capped by an out-of-pocket maximum. Medigap has a higher monthly premium but very low costs when you use care. The cheaper option depends on how much care you expect.
