Original Medicare (Parts A and B) leaves real gaps: there’s no annual cap on what you can spend, and it doesn’t cover most drugs, dental or vision. Most people fill those gaps one of two ways — a Medicare Advantage plan or a Medigap supplement. They work very differently, and the choice you make at 65 can be hard to undo.
Key takeaways
- Medicare Advantage (Part C) replaces Original Medicare with a private plan — usually low premiums, a provider network, and extra benefits.
- Medigap works alongside Original Medicare to cover deductibles and coinsurance — higher premiums, but very predictable costs and any doctor that accepts Medicare.
- You can’t have both at the same time.
- Your 6-month Medigap Open Enrollment Period is the one time insurers must sell you any Medigap policy without health questions in most states.
How Medicare Advantage works
Medicare Advantage plans are offered by private insurers approved by Medicare. They cover everything Parts A and B cover, and most include Part D drug coverage. Many add dental, vision and hearing benefits, and some include gym memberships or over-the-counter allowances.
- Networks: most are HMOs or PPOs, so you’ll usually need in-network doctors and sometimes referrals.
- Costs: premiums can be low or even $0 (you still pay your Part B premium), but you pay copays and coinsurance as you use care.
- Built-in cap: every plan must have an annual out-of-pocket maximum for in-network care — something Original Medicare doesn’t have.
- Prior authorization: plans often require approval before certain services.
How Medigap works
Medigap (Medicare Supplement Insurance) is sold by private insurers to pay some or all of the costs Original Medicare leaves you with — the Part A hospital deductible, Part B coinsurance and more. Plans are standardized by letter (for example, Plan G and Plan N), so a Plan G covers the same benefits no matter which company sells it; only price and service differ.
- Doctors: you can see any doctor or hospital in the U.S. that accepts Medicare. No network, no referrals.
- Costs: higher monthly premiums, but small and predictable bills when you get care.
- Drugs: Medigap doesn’t cover prescriptions — you’ll need a separate Part D plan.
- Extras: no routine dental or vision; you’d buy those separately.
Side-by-side comparison
| Medicare Advantage | Original Medicare + Medigap + Part D | |
|---|---|---|
| Monthly premium | Often low or $0 (plus Part B) | Higher (Part B + Medigap + Part D) |
| Costs when you get care | Copays/coinsurance up to an annual cap | Low and predictable, depending on plan letter |
| Doctor choice | Network, sometimes referrals | Any provider that accepts Medicare |
| Travel within the U.S. | Often limited to emergencies outside the area | Coverage nationwide |
| Prior authorization | Common | Rare |
| Dental/vision/hearing extras | Often included | Not included |
| Drug coverage | Usually included | Separate Part D plan |
The timing rule most people miss
Your Medigap Open Enrollment Period lasts 6 months and starts the first month you’re both 65 or older and enrolled in Part B. During this window, insurers generally must sell you any Medigap policy they offer, at the same price as someone in good health, regardless of your medical history.
After that window closes, in most states insurers can ask health questions and deny you or charge more. A few states (such as New York and Connecticut) give broader rights, and some states have “birthday rules” that allow switching between Medigap plans each year — but these vary.
Why it matters: Many people choose a $0-premium Medicare Advantage plan at 65, then develop a serious condition and want to switch to Original Medicare with Medigap. By then, they may not be able to get a Medigap policy at a reasonable price. Federal “trial right” rules can help if you joined Medicare Advantage when first eligible and leave within 12 months.
Which is right for you?
Medicare Advantage may fit if you…
- Want the lowest monthly premium and are comfortable with a network
- Value dental, vision and hearing extras
- Live mostly in one area year-round
- Are relatively healthy and can handle copays up to the annual cap
Medigap may fit if you…
- Want to keep specific doctors or see specialists without referrals
- Travel often or split the year between states
- Have ongoing health conditions and want predictable costs
- Prefer to pay more monthly to avoid surprise bills
Example
Carol turns 65 in March. She spends winters in Arizona and summers in Michigan, and sees a cardiologist twice a year. A Medicare Advantage HMO in Michigan wouldn’t cover routine visits in Arizona. She enrolls in Part B, buys a Medigap Plan G during her open enrollment window, and adds a standalone Part D plan. Her monthly premiums are higher than her neighbor’s $0 Advantage plan, but she can see doctors in both states and her bills are predictable.
Frequently asked questions
Can I switch from Medicare Advantage back to Original Medicare?
Yes — during Medicare Open Enrollment (October 15 – December 7) or the Medicare Advantage Open Enrollment Period (January 1 – March 31). But getting a Medigap policy afterward may require passing health questions, depending on your state and situation.
Is a $0-premium Medicare Advantage plan really free?
No. You still pay your Part B premium, plus copays and coinsurance when you use care, up to the plan’s out-of-pocket maximum.
What’s the most popular Medigap plan?
For people new to Medicare, Plan G is among the most popular because it covers nearly everything except the annual Part B deductible. Plan N has lower premiums but some copays. (Plan F isn’t available to people who became eligible for Medicare on or after January 1, 2020.)
Can I use Medigap with Medicare Advantage?
No. It’s illegal for someone to sell you a Medigap policy if you’re in a Medicare Advantage plan, unless you’re switching back to Original Medicare.
Where can I get unbiased help?
Your State Health Insurance Assistance Program (SHIP) provides free one-on-one counseling. You can also compare plans with the Plan Finder on Medicare.gov.
Related: Turning 65: Medicare checklist · Medicare Part D in 2027
Sources: Medicare.gov (“Compare Original Medicare & Medicare Advantage”, “Medigap Open Enrollment”); CMS. Smart Premium Guide is not affiliated with Medicare or any government agency. For education only — not insurance advice. Last reviewed October 2026.
