Medicare Parts A, B, C & D Explained: What Every Senior Should Know

The Three-Legged Stool: Medicare Parts A, B, and D

Think of Medicare as a three-legged stool — you need all three legs to feel stable.

• Part A is hospital insurance. Most people pay nothing for it because they (or their spouse) paid into it through federal payroll taxes. Over 99% of people have a $0 premium for Part A.
• Part B is outpatient insurance. For 2026, the standard monthly premium is $229. If you receive Social Security, the premium is deducted automatically from your check. If not, the government bills you quarterly — setting up automatic payments can help you avoid missing a bill.
• Part D is prescription drug coverage. It does not come from the government. You buy it through a private insurance company.

Original Medicare (Parts A and B) only covers 80% of your medical expenses. The remaining 20% is your responsibility. Deciding how to cover that gap is one of the most important Medicare decisions you will make.

Covering the 20% Gap: Medicare Supplement vs. Medicare Advantage

There are two main ways to cover the 20% that original Medicare does not pay.

Medicare Supplement (also called Medigap) A Medigap plan works alongside original Medicare. Any doctor in the country who accepts Medicare will also accept your Medigap plan — you do not need to check a network. The plan uses the same "medical necessity" standard as original Medicare, so if your doctor says you need a procedure, Medicare generally will not overrule that judgment.

Medicare Advantage (Part C) Medicare Advantage is a replacement for original Medicare, run by a private insurance company. These plans are heavily advertised on television and often include extra benefits like dental, vision, and hearing. They can be a good fit if you are in good health and do not visit the doctor often. However, they come with provider networks and prior authorization requirements (see below).

Working with an independent, unbiased advisor who can compare all available plans — not just the ones that pay the highest commission — is the best way to find the right fit.

The Six-Month Enrollment Window You Cannot Afford to Miss

In 46 states, you have a six-month open enrollment window that starts when you first begin Medicare. During this window, you can sign up for any Medigap plan without answering health questions. Insurance companies cannot deny you or charge you more because of pre-existing conditions.

Once that window closes, insurers can ask about your health history. Common conditions like diabetes combined with hypertension — which affect about 35% of seniors — can result in a denial for Medigap coverage outside the enrollment window.

Many people sign up for a Medicare Advantage plan after seeing a TV commercial, not realizing they are giving up this protected window. In many cases, switching to Medigap later becomes difficult or impossible.

Special rule for people under 65 on Medicare: About 11 million people are on Medicare before age 65 due to disability. In most states, their Medigap options are limited and very expensive. However, when they turn 65, federal law gives them a fresh open enrollment window — they can enroll in a Medigap plan without health questions and cannot be charged a higher price.

Choosing a Medigap Plan: G, F, and N

Medigap plans are standardized under federal law. Every insurance company's Plan G covers the exact same hospital and medical benefits as every other company's Plan G. That means insurers compete on price alone — so shopping around can save real money.

• Plan G is the most comprehensive option for people who turned 65 on or after January 1, 2020. It covers nearly everything except the Part B deductible.
• Plan F is available to people who turned 65 before 2020. The only difference between F and G is that Plan F covers the Part B deductible (currently $283 for this year). If your Plan F premium costs more than $283 more per year than a comparable Plan G, it is worth shopping.
• Plan N is another option at a lower price point, with some cost-sharing.

Some people paying for Plan F are spending $2,435 more per year than they need to. Switching from F to G, when eligible, can mean lifetime savings of $60,000–$70,000.

Unlike Medicare Advantage plans, Medigap plans do not expire at year-end. You can generally switch Medigap plans at any time of year — though health underwriting may apply outside your open enrollment window, depending on your state.

Prior Authorizations and Networks: What Medicare Advantage Enrollees Face

Two features of Medicare Advantage plans deserve close attention, especially for people with higher health needs.

Prior authorizations With a Medicare Advantage plan, your doctor recommending a procedure is not the final word. The private insurance company may require a prior authorization — its own approval — before covering the service. According to the Kaiser Family Foundation, there were over 50 million prior authorization claims in 2023 alone. Denials can be appealed, but the process can be lengthy and exhausting.

With a Medicare Supplement plan, the standard is medical necessity. If your doctor says you need it, Medicare generally covers it without a separate insurer approval.

Provider networks Medicare Advantage plans have networks of doctors and hospitals. These networks vary by county and zip code, and doctors can leave a network mid-year — even if you are locked into the plan until the next open enrollment period. There have also been documented cases of inaccurate provider directories, sometimes called "ghost networks," where a doctor listed as in-network was not actually available.

With a Medicare Supplement plan, any doctor who accepts original Medicare accepts your plan — no network to check.

Reviewing Your Coverage Every Year

Medicare Advantage plans effectively reset every December 31. Even if your plan rolls over automatically, the benefits, costs, and network may have changed. Reviewing your plan each fall — during the Annual Enrollment Period — ensures you are on the best-fitting plan for the coming year.

For drug coverage (Part D), an annual review can also produce meaningful savings. Reviewing drug plans each fall has been shown to save enrollees significant money by matching their specific medications to the lowest-cost plan available.

For Medicare Supplement plans, the annual review is about price. Since all Plan G policies cover the same benefits, checking whether a lower-cost Plan G is available — and whether you can qualify for it — is a straightforward way to reduce premiums.

Medicaid Interruptions After a Move

If your Medicaid coverage stopped after you changed your address, the steps to fix it depend on whether you moved within the same state or to a new state.

• Same state: Contact your state Medicaid office, update your address, and reapply if needed. Coverage may have been paused because a renewal notice was sent to your old address and went unanswered.
• New state: You will need to apply to the Medicaid program in your new state. Every state runs its own program under a different name, and eligibility rules vary.

Medicaid recertification requirements have become more demanding in recent years, and local offices may have limited staff. If you are on a fixed income and rely on Medicaid, staying on top of renewal notices and address updates is essential. Confirm the specific steps with your state's Medicaid agency.