The Centers for Medicare and Medicaid Services runs the Medicare program you receive and sets the rules for how it works

The Centers for Medicare and Medicaid Services (CMS) is the federal agency inside the U.S. Department of Health and Human Services that administers Medicare. CMS decides which services Medicare covers, how much it pays doctors and hospitals, what your premiums and deductibles are, and how the program changes from year to year. When you have a question about why Medicare denied a claim or what your coverage includes, you are ultimately dealing with rules CMS wrote.

CMS also runs Medicaid, the joint federal-state program for lower-income people, and the Children's Health Insurance Program (CHIP). But for Medicare beneficiaries, CMS is the organization that determines your benefits, sets payment rates, and handles appeals when you disagree with a coverage decision.

You do not interact with CMS directly most of the time. Instead, you work with your insurance company (for Medicare Advantage plans) or with Medicare contractors who process claims (for Original Medicare). But CMS is the authority behind those organizations — it writes the rules they follow and oversees whether they follow them correctly.

Key Takeaways

  • CMS is a federal agency that runs Medicare and decides what services are covered, what you pay, and how much doctors and hospitals receive.
  • You typically interact with your insurance company or Medicare contractors, not CMS directly, but CMS sets the rules they must follow.
  • CMS updates Medicare coverage and payment rules every year, which can change your costs and what services are available to you.
  • If you disagree with a Medicare decision, you can file an appeal, and CMS has a formal process for reviewing those appeals.
  • CMS publishes information about coverage decisions, payment rates, and program changes on Medicare.gov, where you can search for specific services or conditions.

How CMS decides what Medicare covers

CMS uses a formal process to decide whether Medicare will pay for a new drug, device, or treatment. The agency reviews scientific evidence, considers whether the service is reasonable and necessary for treating a medical condition, and publishes its decision. This process can take months or years, especially for expensive or new treatments.

Once CMS makes a coverage decision, it applies nationwide — Medicare either covers something in all 50 states or it does not. However, CMS also allows regional Medicare contractors to make some coverage decisions for their area if a service is not covered nationally. These are called Local Coverage Determinations (LCDs), and they can vary by region. You can search for LCDs on the CMS website to see whether a specific service is covered where you live.

CMS also publishes a list called the Medicare Benefit Policy Manual, which spells out exactly what is and is not covered under each part of Medicare. Your doctor's office or your insurance company can look this up, but you can also search it yourself on Medicare.gov if you want to know the official reason a service was or was not covered.

What CMS controls about your costs

CMS sets the amount Medicare pays for most services — a rate called the Medicare Physician Fee Schedule for doctor visits and procedures. This is not the same as what you pay out of pocket. CMS decides the total amount Medicare will reimburse; your insurance company or Original Medicare then determines your share based on your deductible, copay, or coinsurance.

CMS also sets the yearly amounts for your Part B premium (doctor and outpatient care), your Part A deductible (hospital care), and your Part D premium cap (prescription drugs). These amounts change most years. CMS announces the new amounts in the fall, and they take effect January 1. If you receive Social Security, your Part B premium is usually deducted automatically from your check.

For Medicare Advantage plans, CMS sets a payment rate to each insurance company for each beneficiary, and the company then decides what copays and coinsurance to charge you. This is why two Medicare Advantage plans in the same area can have very different costs — CMS sets the baseline payment, but the insurance company decides how to structure your out-of-pocket expenses.

How to find CMS coverage information yourself

The main place to search CMS coverage decisions is Medicare.gov. You can use the search box to look up a specific drug, device, or service. The site will tell you whether it is covered under Part A (hospital), Part B (doctor and outpatient), or Part D (prescription drugs), and it will explain any limits or conditions.

If you need more detail, you can visit the CMS Coverage with Evidence Development page, which lists services that Medicare covers only under certain research conditions or with specific documentation. You can also search for National Coverage Determinations (NCDs), which are coverage decisions that explore everywhere in the country.

For appeals and coverage disputes, CMS maintains a formal process. If your claim is denied, you receive a notice explaining why. You can request a reconsideration, and if you disagree with that decision, you can file a formal appeal. The CMS website has a section on appeals that walks through each step and tells you the time limits for each one.

CMS updates and rule changes that affect you

CMS publishes new rules and updates to Medicare coverage throughout the year. Major changes are announced in the fall for the following year, but smaller updates can happen anytime. These changes might affect your premium, your deductible, which drugs are covered, or which doctors are in your plan's network.

You can stay informed by visiting Medicare.gov and signing up for email updates, or by reading the notices your insurance company sends you. Insurance companies are required to tell you about coverage changes before they take effect, usually in October for changes starting January 1. If a change affects you significantly, you have the right to switch plans during the annual enrollment period (October 15 to December 7).

CMS also publishes an annual report called the Medicare Trustees Report, which discusses the financial health of the Medicare trust funds. While this is technical reading, it can help you understand whether Medicare is facing funding challenges that might affect future benefits or costs.

When to contact CMS directly

Most of the time, you should contact your insurance company or your doctor's office first with questions about coverage or claims. However, there are situations where contacting CMS directly makes sense. If you believe your insurance company or a Medicare contractor is not following CMS rules, or if you want to file a formal complaint about how you were treated, you can reach CMS through Medicare.gov.

CMS also has a phone line for beneficiaries: 1-800-MEDICARE (1-800-633-4227). This line can answer general questions about Medicare coverage and rules, though for specific claims or plan questions, your insurance company is usually faster. The line is open 24 hours a day, 7 days a week.

If you are appealing a coverage denial or a claim rejection, you may need to submit documents directly to CMS or to a CMS contractor. The appeal notice you receive will tell you where to send your appeal and what important date you have to meet. Missing a important date can mean you lose your right to appeal, so follow the instructions on the notice carefully.

How CMS differs from your insurance company

Your insurance company (whether it is a Medicare Advantage plan or a Medigap policy) is a private business that contracts with CMS. The insurance company collects your premiums, processes your claims, and decides which doctors and hospitals are in your network. CMS sets the rules the insurance company must follow, but the insurance company makes day-to-day decisions about your care.

If you have a problem with your insurance company — for example, a claim was processed incorrectly or you were treated unfairly — you can complain to your insurance company first. If they do not resolve it, you can file a complaint with your state's insurance commissioner or with CMS. CMS has the authority to fine insurance companies or remove them from the Medicare program if they break the rules.

Understanding this distinction matters because it tells you who to contact. If you disagree with a coverage decision, your insurance company made it — but CMS wrote the rule the company was supposed to follow. If the insurance company is not following CMS rules, that is a complaint for CMS, not just the insurance company.

Frequently Asked Questions

Can CMS force my doctor to accept Medicare?

No. Doctors can choose whether to participate in Medicare. However, if a doctor participates, CMS sets the amount Medicare will pay for each service. Some doctors do not participate because they believe the Medicare payment is too low. You can search Medicare.gov to find doctors who accept Medicare in your area.

If CMS denies coverage for a drug my doctor prescribed, can I appeal?

Yes. You have the right to appeal any coverage denial. The process depends on whether the denial came from your insurance company or from CMS directly. Your denial notice will explain how to appeal and what important date you have. You can ask your doctor to provide medical evidence supporting why you need the drug.

Does CMS make decisions about individual patients, or only about what services are covered in general?

CMS makes coverage decisions about services and drugs in general. Your insurance company or Medicare contractor makes decisions about whether you personally meet the coverage rules for a specific service. For example, CMS might cover a certain test, but your insurance company might deny it if you do not meet the medical criteria.

How often does CMS change Medicare rules?

CMS updates Medicare rules throughout the year, but the biggest changes happen annually. In the fall, CMS announces changes to premiums, deductibles, and coverage that take effect January 1. Smaller changes to coverage decisions or payment rates can happen anytime. Your insurance company is required to notify you of changes that affect your plan.

What should I do if I think CMS made a mistake in a coverage decision?

You can file a formal appeal. Start by contacting your insurance company or the Medicare contractor who made the decision. If you disagree with their response, you can request a reconsideration and eventually a hearing before an administrative law judge. The appeal notice you receive will explain each step and the time limits for each one.