What Is Original Medicare?
When you first enroll in Medicare around age 65, you’re enrolled in what’s called Original Medicare. It has two main parts:
Covers hospital stays, skilled nursing facility care, hospice, and some home health care. Generally premium-free if you’ve had a working career and paid Medicare taxes.
Covers doctor visits, outpatient care, preventive services, tests, and procedures. Has a monthly premium (~$203/month in 2026) that you continue paying regardless of which path you choose.
Together, these two parts form Original Medicare — but here’s the critical thing most people don’t realize going in: Original Medicare does not pay 100% of your costs. It typically pays about 80% of approved costs. The remaining 20%, plus deductibles and co-pays, are your responsibility. And there is no annual out-of-pocket maximum, meaning a major illness or surgery could cost you tens of thousands of dollars in a single year.
The Two Paths: Medicare Advantage vs. Medigap
At the fork in the road, most retirees face a choice between two fundamentally different approaches to covering what Original Medicare leaves behind.
Medicare Advantage
Medicare Advantage replaces Original Medicare with a private insurance plan approved by Medicare. When you enroll, the private plan becomes your primary insurance — Medicare pays the insurer a fixed amount per person to cover you, and the plan sets its own co-pays, networks, and rules.
Most Medicare Advantage plans bundle hospital coverage, doctor coverage, and prescription drug coverage into one plan. Many also include extras like dental, vision, hearing, and wellness benefits. The defining characteristic is the premium: many plans charge $0 per month beyond what you already pay for Part B.
Medigap (Medicare Supplement)
Medigap, also called Medicare Supplement insurance, works alongside Original Medicare — it does not replace it. It’s a private insurance policy that pays most or all of what Medicare doesn’t cover. In practice: Medicare pays its 80%, and then your Medigap policy covers most of the remaining 20%, co-pays, and deductibles.
There are no provider networks. There are no referrals. As long as a doctor accepts Medicare — and the vast majority do — you are covered. The trade-off is a higher monthly premium, typically $120 to $300 or more per month, plus a separate Part D plan for prescriptions.
Medigap: Pay more monthly. Pay very little when you use care.
| Feature | Medicare Advantage | Medigap |
|---|---|---|
| Monthly premium | $0–$60/mo (beyond Part B) | $120–$300+/mo (beyond Part B) |
| Provider access | In-network only (changes annually) | Any doctor who accepts Medicare |
| Referrals required | Often yes | No |
| Prior authorization | Common for major services | Rarely required |
| Prescription drugs | Usually bundled in | Separate Part D plan needed |
| Annual OOP maximum | $3,500–$9,000+ | Most costs covered by premiums |
| Dental/Vision/Hearing | Often included as add-ons | Not included |
| Switching later | Easy to leave; hard to return to Medigap | Can switch to Advantage anytime |
| Best phase of retirement | Early retirement, healthy years | Later retirement, complex health needs |
Your Critical Enrollment Window at 65
This is the most important section of this entire guide. When you first enroll in Medicare around age 65, you have the most freedom you will ever have in this decision. Here’s why that moment is so crucial.
Under federal rules, you get a one-time six-month Medigap open enrollment period. The clock starts the first month you are both 65 or older and enrolled in Medicare Part B — not automatically at your birthday. If you’re still working past 65 and covered by an employer plan, the window only starts once you begin Part B.
During these six months, Medigap insurers generally cannot deny you coverage, charge you more due to your health, or apply waiting periods for pre-existing conditions. In plain English: no health questions, no medical underwriting. This is your clean slate moment.
How Hard Is It to Switch Later?
People often hear “you can always change your plan later” — and technically, they’re not wrong. You can switch between Medicare Advantage and Original Medicare during annual enrollment periods. But switching your Medicare structure is one thing. Getting a Medigap policy later is another thing entirely.
Generally doable during normal annual enrollment periods. Medicare Advantage plans typically don’t use medical underwriting, so pre-existing conditions don’t prevent enrollment.
Outside the protected window, insurers can ask health questions, charge 30–50% higher premiums, or deny you outright. The very health conditions that make you want Medigap are the same ones that make it hard to qualify.
Pros & Cons in Full
Who Should NOT Choose Each Plan
Neither plan is right for everyone. Understanding when each option is a poor fit is just as important as knowing when it works well.
Real Cost Examples Over Time
Let’s make this concrete. The following examples exclude Part B premiums and prescription drug costs, which apply in both scenarios — focusing only on the cost differential.
Find Your Analysis
Answer 6 questions based on your health profile and priorities. Takes about 2 minutes.
See How Healthcare Costs Affect Your Retirement Plan
Medicare decisions don’t happen in isolation. Your coverage choice affects your overall retirement budget, tax strategy, and long-term financial health.
Frequently Asked Questions
Can I switch from Medicare Advantage to Medigap later?
Technically yes, but it is often very difficult. Outside of your initial 6-month Medigap open enrollment period, insurance companies can use medical underwriting to deny coverage, charge higher premiums, or impose waiting periods for pre-existing conditions. The health conditions that make you want Medigap are often the same ones that make it hard to qualify.
Is Medigap worth the higher monthly premium?
It depends on your health, risk tolerance, and planning horizon. Medigap costs more monthly but provides highly predictable total costs. For retirees with chronic conditions, complex health needs, or those planning for a 20–30 year retirement, the predictability often outweighs the higher premium. For healthy retirees with minimal care needs, Medicare Advantage may be more cost-effective in the early years.
Why are Medicare Advantage plans sometimes $0?
Medicare pays private insurers a fixed monthly amount per enrollee (called a capitation payment). Some insurers can offer coverage for that amount alone without charging an additional premium. However, $0 premium does not mean $0 costs — you still pay co-pays, co-insurance, and deductibles when you use care, with annual out-of-pocket maximums typically ranging from $3,500 to $9,000 or more.
What happens if I miss my Medigap enrollment window?
If you miss your 6-month Medigap open enrollment period, you lose your guaranteed-issue right. After that, Medigap insurers in most states can use medical underwriting — meaning they can deny you coverage, charge higher premiums based on health history, or impose waiting periods for pre-existing conditions. Some states offer additional protections, but federal law does not guarantee a second open enrollment window.
Official Resources
For complete and up-to-date information on all your Medicare options, contact official government sources directly.