Original Medicare vs Medicare Advantage, and where Medigap fits

Everyone with Medicare makes the same basic choice, and many people make it without realizing it. You can get your Part A and Part B coverage through Original Medicare, the federal program itself, or through a Medicare Advantage Plan, a Medicare-approved plan run by a private company. If you never choose, you have Original Medicare. The choice is not permanent, but the rules for changing your mind are strict, and one of them, the rule for buying a Medicare Supplement (Medigap) policy later, can close a door for good.

This guide works through the choice using the government's own handbook, Medicare & You 2027, published by the Centers for Medicare & Medicaid Services (CMS) in September 2026. Open Enrollment for 2027 coverage runs from October 15 to December 7, 2026, so this is the season when the decision matters most. One caution up front: the handbook went to print before the 2027 premiums, deductibles and income limits were set, so the dollar amounts it gives, and that this guide repeats, are the 2026 figures. Our Part B premium page will carry the 2027 numbers as soon as CMS publishes them.

The two paths side by side

CMS puts the comparison in a short chart near the front of the handbook. Here are its main points, condensed.

Under both paths you keep paying the monthly Part B premium. The handbook defines "the U.S." for Original Medicare broadly: the 50 states, the District of Columbia, Puerto Rico, the U.S. Virgin Islands, Guam, the Northern Mariana Islands and American Samoa.

How Original Medicare works

Original Medicare has two parts. Part A is hospital insurance and Part B is medical insurance. You can go to any Medicare-enrolled doctor, hospital or other provider that accepts Medicare patients, you do not have to pick a primary care doctor, and in most cases you do not need a referral to see a specialist. Providers file the claims for you.

The trade-off is cost sharing with no ceiling. You pay a deductible before Medicare pays its share, then a coinsurance or copayment, and, in the handbook's words, there is no yearly limit for what you pay out of pocket unless you have other insurance such as Medigap, Medicaid, or employer, retiree or union coverage. These are the 2026 amounts the handbook lists:

Two details in that table catch people off guard. First, the Part A deductible is per benefit period, not per year. A benefit period ends once you have been out of the hospital for 60 days in a row, and a new admission after that starts a new one, so you can owe the deductible more than once in a calendar year. Second, the 60 lifetime reserve days can be used only once in your life; after day 150 you pay all costs. The higher Part B premium applies, for 2026, if your modified adjusted gross income from your 2024 tax return was above $109,000 filing individually or $218,000 filing jointly. Our IRMAA brackets page shows the full table and the Part A costs page covers the hospital amounts in more detail.

Assignment and the 15% limit

Under Original Medicare, what a visit costs you also depends on whether the doctor "accepts assignment," meaning they agree to take the Medicare-approved amount as payment in full. Most do. A doctor who accepts assignment can charge you only the deductible and coinsurance, files the claim directly, and cannot charge you for filing it.

Doctors who have not agreed to accept assignment for all services are called non-participating. They may ask you to pay the whole bill at the visit, and they can charge more than the Medicare-approved amount, though in many cases no more than 15% above it. CMS calls that cap the "limiting charge." A smaller group of doctors opt out of Medicare entirely. Medicare pays nothing for their services except in an emergency or urgent need; you would have to sign a private contract and pay them yourself. An opt-out lasts two years and renews automatically. You can check a provider on Medicare.gov's Care Compare tool before you book.

With Original Medicare you also get a Medicare Summary Notice at least twice a year, listing what was billed and what Medicare paid. It is not a bill. If it shows you are in the Qualified Medicare Beneficiary program, providers are not allowed to bill you for Part A and Part B deductibles, coinsurance or copayments.

How Medicare Advantage works

A Medicare Advantage Plan, also called Part C, is another way to get Part A and Part B coverage. You still have Medicare, but the plan, not Original Medicare, handles most of your coverage, and you use the plan's card instead of your red, white and blue Medicare card (which the handbook says to keep somewhere safe in case you need it later). Medicare pays the company a fixed amount each month for your coverage. The company must follow Medicare's rules, but each plan sets its own out-of-pocket costs and its own rules for referrals and networks, and those can change every year.

To join one, the handbook says you must:

• Have both Part A and Part B.
• Live in the plan's service area.
• Be a U.S. citizen or national, a lawful permanent resident, a Cuban-Haitian entrant, or lawfully living in the U.S. as a Compact of Free Association migrant.

You can join even with a pre-existing condition. Plans must cover all medically necessary services that Original Medicare covers, and most offer extra benefits Original Medicare does not, such as some vision, hearing and dental care, gym memberships, rides to appointments or over-the-counter items. CMS cautions that these extras are generally limited, so check exactly what a plan includes before you join. Even inside a Medicare Advantage Plan, Original Medicare still pays for hospice care, kidney transplant costs and some costs of clinical research studies.

What you pay in a plan

Every plan has a yearly limit on what you pay for Part A and Part B services; once you reach it, you pay nothing more for covered services that year. Some plans charge a premium on top of the Part B premium, some charge $0, and some help pay part or all of your Part B premium (CMS calls this a "Medicare Part B premium reduction"). Beyond that, your costs depend on the plan's deductibles and copayments, how often you need care, and whether you stay in network. Plans cannot charge more than Original Medicare for certain services, including chemotherapy, dialysis and days 21 to 100 in a skilled nursing facility.

Each year the plan must send you two documents. The Annual Notice of Change, due by September 30, lists changes to coverage and costs that take effect in January. The Evidence of Coverage, with a notice by October 15, gives the full details for the next year. The handbook asks you to read both, and to contact the plan if they do not arrive.

Networks and approvals

Doctors can join or leave a plan's network at any time during the year, and you generally cannot change plans because of it. The plan must still give you access to qualified doctors and specialists, help you choose a new provider and continue care already in progress, and tell you if a primary care or behavioral health provider you have seen in the past three years is leaving. If an in-network provider cannot meet your medical needs, the plan must help you get the covered service out of network at in-network cost.

You, your representative or your doctor can ask the plan in advance, orally or in writing, whether it covers a service and what you will pay. That is called an organization determination, and for some services it is the required prior authorization. If the plan says no, it must tell you in writing, and you can appeal. Starting January 1, 2027, the handbook says plans may be able to process prior authorization requests electronically and show the decisions in your patient portal.

The types of plans

One rule in that table deserves emphasis. If you join an HMO or PPO that does not include drug coverage, you cannot add a separate Medicare drug plan. You would have to rely on other drug coverage, such as employer or retiree coverage, or go without, and going without creditable drug coverage can mean a late enrollment penalty if you add Part D later.

Medical Savings Account plans work differently from the rest. The plan deposits money into a savings account you use for health costs, and coverage of Part A and Part B costs starts only after a yearly deductible. MSA plans charge no premium of their own, money left at year end carries over if you stay, and if you leave before the year ends you repay part of the latest deposit to Medicare. Special Needs Plans come in three kinds: Dual Eligible SNPs for people with both Medicare and Medicaid, Chronic Condition SNPs for people with conditions such as diabetes, chronic heart failure or dementia, and Institutional SNPs for people who live in, or need the level of care of, a nursing home or similar facility.

If you have employer or union coverage, talk to the benefits administrator before you join any Medicare Advantage Plan. CMS warns that in some cases joining one can cost you that coverage for yourself, your spouse and your dependents, and you may not get it back.

Where Medigap fits

Medicare Supplement Insurance, or Medigap, is the main way people in Original Medicare limit their out-of-pocket costs. Private insurers sell it, and it pays some of what Original Medicare leaves to you: deductibles, coinsurance and copayments. It only works alongside Original Medicare. You cannot use a Medigap policy to pay Medicare Advantage costs, and it is illegal for anyone to sell you one while you are in a Medicare Advantage Plan unless you are switching back to Original Medicare. Generally, Medigap does not cover long-term care, vision or dental care, hearing aids, eyeglasses or private-duty nursing.

The plans are standardized. In most states they are named by letter: A through D, F, G, and K through N. A plan with a given letter offers the same basic benefits wherever you live and whichever company sells it, so what differs from one company to the next is mainly the price. Massachusetts, Minnesota and Wisconsin standardize their policies differently.

The chart's footnotes add the details that change the math:

• Plans C and F are closed to anyone who became eligible for Medicare on or after January 1, 2020. If you were eligible before then, you may still be able to buy them. Plans D and G give the same benefits as C and F except the Part B deductible.
• High-deductible F and G are offered in some states. In 2026 you pay up to $2,950 in Medicare-covered costs before the policy pays.
• Plans K and L have a yearly out-of-pocket limit, $8,000 for K and $4,000 for L in 2026. Once you reach it and your $283 Part B deductible, the plan pays 100% of covered services for the rest of the year.
• Plan N pays the Part B coinsurance, but you pay up to $20 for some office visits and up to $50 for an emergency room visit that does not lead to an inpatient admission.

"Excess charges" are the amounts a non-participating doctor may bill above the Medicare-approved amount, the up-to-15% limiting charge described earlier. Only F and G pay them.

When to buy Medigap

Timing is the part of Medigap that matters most. The handbook calls your Medigap Open Enrollment Period "the best time" to buy. It lasts six months and starts the first month you have Part B and are 65 or older. After it ends, you may not be able to buy a Medigap policy, or it may cost more, unless you have a guaranteed issue right in a specific situation or your state gives extra protections. If you put off Part B because you had group coverage through your own or your spouse's current job, the six months do not start until you get Part B.

A few more rules from the handbook:

• You generally need Part A and Part B to buy Medigap, and you pay the Medigap premium to the insurer on top of the Part B premium.
• Each policy covers one person. Spouses buy separate policies.
• Prices for the same lettered plan vary between companies and may rise as you age. Some states limit Medigap premiums.
• A policy generally starts the first of the month after you apply.
• Federal law generally does not require insurers to sell Medigap to people under 65, though some states do.
• You cannot have drug coverage in both Medigap and a Medicare drug plan. You can buy Medigap and a separate drug plan from the same company, but you may make two premium payments.
• Some states sell Medicare SELECT, a Medigap policy with a hospital network. You have 12 months to change your mind and switch to standard Medigap.

Switching later: the rules that matter

You are not locked into your first choice, but you can switch only at certain times. These are the windows the handbook lists for Medicare Advantage.

The January to March window only works in one direction. During it you cannot move from Original Medicare into a Medicare Advantage Plan, join a drug plan if you have Original Medicare, or switch drug plans in Original Medicare. If you use it to return to Original Medicare and join a drug plan, you do not need to contact the Medicare Advantage Plan; the disenrollment happens automatically when the drug plan enrollment goes through. Our open enrollment page and the enrollment periods guide cover these dates alongside the Part A and Part B windows.

Going back to Original Medicare and Medigap

Switching from Medicare Advantage back to Original Medicare is always possible in the right window. Getting Medigap again is the hard part, because outside your six-month open enrollment you may not be able to buy a policy, or it may cost more. The handbook describes two protections:

• The trial right. If you drop a Medigap policy to join a Medicare Advantage Plan for the first time, you have a single 12-month period to get that Medigap policy back, if the same company still sells it, when you return to Original Medicare. If it is not available, you can buy certain other Medigap policies, depending on state law and on whether you were new to Medicare on or after January 1, 2020.
• Joining at 65. If you joined a Medicare Advantage Plan when you first became eligible for Part A at 65, you can buy certain Medigap policies sold in your state if you switch to Original Medicare within the first year in the plan.

Two practical points. A Medigap policy usually does not cancel itself when you join a Medicare Advantage Plan, so you have to contact the insurer to cancel it. And if you drop Medigap and later want it back after the trial period, CMS warns you may not be able to get the same policy, or in some cases any policy. If someone tries to sell you Medigap while you are staying in a Medicare Advantage Plan, the handbook asks you to report it to your State Insurance Department.

Military retirees and TRICARE

For military retirees the choice runs through TRICARE. The handbook says that if you have TRICARE, you generally must sign up for Part A and Part B when you are first eligible for Medicare to keep it; active-duty members and their families are the exception and do not need Part B to keep TRICARE. Our TRICARE For Life guide explains how TRICARE and Medicare work together, and the who pays first checker covers other insurance. The handbook lists the TRICARE For Life contact as 1-866-773-0404 (TDD 1-866-773-0405). CHAMPVA has the same requirement: you must sign up for Part A and Part B to keep it, and its number is 1-800-733-8387.

What drives the decision

The handbook does not recommend one path over the other, and neither does this guide. What it does is list what your costs depend on under each. Read side by side, those lists suggest the questions worth answering before Open Enrollment closes:

• Your doctors. Are the doctors and hospitals you use in the plan's network? At Medicare.gov/plan-compare you can add your providers and see whether they are in network, or you can ask the provider directly.
• Travel and second homes. Original Medicare works anywhere in the U.S.; most plans limit non-emergency care to their service area.
• The worst year, not the average one. Original Medicare alone has no yearly cap. A Medicare Advantage Plan has one, and Medigap or other coverage can fill most of Original Medicare's gaps.
• Your drugs. Check each plan's drug list, and remember the HMO and PPO rule on separate drug plans.
• Other coverage. Employer, retiree, union, TRICARE, CHAMPVA or Medicaid coverage can change the answer completely.
• Your Medigap window. If you are within six months of starting Part B at 65 or older, you are in what the handbook calls the best time to buy Medigap.

If your income and resources are limited, look at the Medicare Savings Programs and Extra Help before comparing anything else, since they can help pay some of your Medicare costs. The help paying for Medicare guide walks through who qualifies.

Free, unbiased help

You do not have to make this choice alone. The handbook points to free help:

• 1-800-MEDICARE (1-800-633-4227), TTY 1-877-486-2048, and the plan comparison tool at Medicare.gov/plan-compare.
• Your State Health Insurance Assistance Program (SHIP), for free, personalized help; find yours at shiphelp.org.
• Your State Insurance Department for Medigap rights and prices; the directory is at content.naic.org/state-insurance-departments.
• The CMS booklet Choosing a Medigap Policy, at Medicare.gov/publications.

Whichever path you take, mark October 15 to December 7 on your calendar every year. Plans change their costs, benefits and networks each year, and that window is when you can respond.