The reason is structural rather than bureaucratic. Medigap pays your share of the costs that Original Medicare leaves behind, such as the Part A deductible and the Part B coinsurance, so it needs Original Medicare underneath it to have anything to pay toward. A Medicare Advantage plan replaces that cost structure with its own copays and its own out-of-pocket maximum, so there is nothing for a Medigap policy to coordinate with.
| You have | Can add Medigap | Can add a Part D drug plan |
|---|---|---|
| Original Medicare | Yes | Yes |
| Medicare Advantage with drug coverage | No | No, it is already included |
| Medicare Advantage without drug coverage | No | Sometimes, depending on the plan type |
How it works
The two are alternatives, not layers. With Original Medicare plus Medigap you keep any doctor who accepts Medicare, pay a monthly Medigap premium, and have very little to pay at the point of care. With Medicare Advantage you accept a network and copays in exchange for a lower premium and extras such as dental, vision and a gym benefit. One route costs more each month and less when you use it; the other costs less each month and more when you use it.
What changes it
Timing is the part that catches people. Your Medigap Open Enrollment Period is the six months that start the first day of the month you are 65 or older and signed up for Part B. Inside that window a carrier must sell you a policy regardless of your health. Outside it, Medicare.gov says there is no federal guarantee that an insurance company will sell you a Medigap policy at all, and if one does, it may cost more because of past or present health problems. That is why switching from Medicare Advantage to Medigap years later is not always possible in Florida.
What to do next
If you are already in a Medicare Advantage plan and someone offers to sell you a Medigap policy, that is a red flag, not an opportunity. If you want to move to Original Medicare and add Medigap, the sequence matters: Medigap coverage cannot start until your Medicare Advantage coverage ends, so the application and the disenrollment need to line up. Talk it through before you cancel anything.
Read the full guide: Medicare Advantage versus Medigap, compared
Related questions
Can I keep my Medigap policy if I join a Medicare Advantage plan?
You can keep paying for it, but it will not pay anything while you are in a Medicare Advantage plan. Most people drop it, which is a decision worth discussing first, because getting it back later is not guaranteed.
If I leave Medicare Advantage, how long do I have to buy Medigap?
When you have the right to buy one, you have 63 days after your Medicare Advantage coverage ends, and you can apply up to 60 days before it ends.
Does Medigap include drug coverage?
No. Medigap sold today does not include prescription drugs, so people with Original Medicare and Medigap usually add a stand-alone Part D plan.
Sources
- When can I buy a Medigap policy? — Medicare.gov (CMS) · read 2026-10-13
This page explains published rules and figures, with the source for each one. It is not advice about your own situation.