Most people become eligible for Medicare at age 65 if they or their spouse paid Medicare taxes for at least 10 years (40 quarters) while working. You can also qualify under 65 if you have received Social Security Disability Insurance (SSDI) for 24 months, or if you have ALS (Lou Gehrig's disease) or end-stage renal disease (kidney failure requiring dialysis or transplant).

Eligibility does not mean automatic enrollment. Unless you are already receiving Social Security benefits, you must actively sign up during your enrollment window — and missing it has permanent financial consequences.

Medicare pays for services and supplies that are medically necessary — meaning they are needed to diagnose or treat a medical condition, meet accepted standards of care, and are not primarily for the patient's convenience. That determination is made by Medicare, not by your doctor alone.

This distinction matters in practice. A doctor can order something and genuinely believe it is necessary. Medicare can still decide it does not meet their coverage criteria and decline to pay. This is why documentation of medical necessity in the physician's chart note is so important — it is what Medicare reviews when deciding whether to cover a service.

Original Medicare is the traditional federal program — Part A and Part B administered directly by the government. You can see any doctor or hospital that accepts Medicare, anywhere in the country. There is no network. There is no prior authorization for most services. You pay the government-set cost-sharing amounts.

Medicare Advantage (Part C) is an alternative delivered by a private insurance company that contracts with Medicare. The insurer receives a fixed payment from the government to cover your care. Advantage plans often have lower premiums, but they operate like private insurance — narrower provider networks, prior authorization requirements for many services, and an annual out-of-pocket maximum. The trade-off is lower monthly cost in exchange for less flexibility and more administrative process.

The decision that matters most: choosing between Original Medicare with a Medigap supplement and Medicare Advantage. Once you are in a Medicare Advantage plan and want to switch back to Original Medicare with a Medigap supplement, the Medigap insurer can medically underwrite you — meaning they can charge you more or decline you based on your health. The open enrollment protection you have at 65 does not repeat. Choose carefully.

Original Medicare covers 80% of most outpatient services after you meet the Part B deductible. You are responsible for the remaining 20% — with no annual cap on what that can total. A serious illness, a hospital stay, or a complex equipment order can add up to thousands of dollars in 20% shares even with Medicare.

A Medigap supplement is a private insurance policy that covers some or all of that remaining cost-sharing. Plan G is the most comprehensive option currently available to new enrollees — it covers the Part A deductible, the Part B 20% coinsurance, skilled nursing facility coinsurance for days 21 through 100, and emergency care in foreign countries. With Plan G, most Medicare-covered services cost you nothing out of pocket beyond the Plan G premium.

Medigap plans are standardized — a Plan G from one company covers exactly the same things as a Plan G from another. The only variable is the monthly premium, which varies by insurer and by your location. Shop on premium, not on the plan letter.

Your Initial Enrollment Period (IEP) runs for 7 months — beginning 3 months before the month you turn 65, including your birthday month, and ending 3 months after. This is the window when you can enroll in Parts A, B, and D without penalty and select a Medigap supplement without medical underwriting.

If you miss this window and enroll late in Part B, you pay a 10% premium surcharge for every 12-month period you were eligible but did not enroll — and that surcharge is added to your Part B premium permanently. Part D carries its own separate late enrollment penalty: 1% of the national base premium per month delayed, also permanent.

Exception: if you have qualifying employer-sponsored coverage at 65, you can delay Medicare enrollment without penalty. When that employer coverage ends, you have a Special Enrollment Period. Keep documentation of your continuous coverage — Medicare will ask for it.

Part A covers inpatient hospital care — but only when you are formally admitted as an inpatient. If you are placed under observation status instead of being admitted, you are technically an outpatient even if you spend days in a hospital bed. Observation days do not count toward the 3-day qualifying hospital stay required for Medicare to cover a skilled nursing facility stay after discharge. This distinction is invisible to most patients and has significant financial consequences.

Part A also covers stays in a skilled nursing facility (SNF) — but only after a qualifying 3-day inpatient hospital stay, and only for skilled care (nursing interventions, physical therapy, occupational therapy). Days 1 through 20 in the SNF are fully covered. Days 21 through 100 carry a daily coinsurance (~$209/day in 2025) that Medigap Plan G covers. After day 100, Medicare coverage ends entirely.

Part A covers hospice — all medications, equipment, and care related to the terminal diagnosis — with no time limit as long as eligibility continues. And Part A covers some home health, though most home health is billed under Part B.

Part B covers physician visits, outpatient procedures, durable medical equipment, lab work, preventive services, and most home health. After the annual deductible (~$257), Medicare pays 80% of the approved amount for most services. You pay 20%.

There is no annual out-of-pocket cap on that 20%. A $30,000 piece of equipment means a $6,000 bill. A complex surgery with multiple physician billings can easily produce tens of thousands of dollars in 20% shares. This uncapped exposure is why Medigap supplements exist — and why Plan G, which covers that 20%, is so widely used.

Preventive services are an exception — Medicare covers them at 100% with no cost-sharing when billed correctly. Annual wellness visits, mammograms, colonoscopies, flu and pneumonia vaccines, and diabetes and depression screenings are all 100% covered as preventive. The catch: if you raise a specific health complaint during a preventive visit, the billing can shift to a problem-focused visit and cost-sharing applies.

Part D prescription coverage is delivered through private insurance plans, each with its own formulary — a list of covered drugs organized into cost tiers. Tier 1 is typically generic drugs with low copays. Tier 5 is typically specialty drugs with high cost-sharing. The same drug can be on a different tier — or not covered at all — depending on which plan you choose.

Plans change their formularies every year. A drug that was tier 2 this year might be tier 4 next year, or removed from the formulary entirely. This is why reviewing your Part D plan during open enrollment (October 15 through December 7) every year matters — the plan that was cheapest last year may not be cheapest this year. Use Medicare's Plan Finder tool at medicare.gov to compare plans based on your actual current medication list.

In 2025, a significant change took effect: the annual out-of-pocket cap for Part D drugs is $2,000. Once you have spent $2,000 on covered Part D drugs in a calendar year, you pay nothing for the rest of the year. This change dramatically reduces catastrophic drug costs for people on expensive specialty medications.

Medicare Advantage plans (Part C) combine Parts A and B — and often D — through a private insurer. They typically offer lower or zero monthly premiums and may include supplemental benefits like dental, vision, and hearing that Original Medicare does not cover. In exchange, you agree to use the plan's provider network and accept prior authorization requirements for many services.

What you are trading: flexibility and access. Original Medicare lets you see any Medicare-accepting doctor in the country. Medicare Advantage plans have networks — out-of-network care is either not covered or very expensive. If you travel frequently, have specialists outside the plan's service area, or are in a rural area with a limited network, this matters significantly.

Medicare Advantage plans also have annual out-of-pocket maximums — up to $8,300 in-network in 2025 — which limits your exposure on large claims. This sounds like protection, but Original Medicare with a Medigap Plan G has near-zero out-of-pocket on covered services, which is actually stronger protection. The comparison is: Advantage saves you money on monthly premiums but exposes you to network restrictions and the out-of-pocket maximum. Medigap costs more monthly but protects you more completely when something serious happens.

Prior authorization (PA) — also called pre-authorization or pre-approval — is a requirement from your insurance plan that certain services, medications, or equipment be reviewed and approved before they are provided. The insurance company or its contractor reviews whether the requested service meets their coverage criteria based on the clinical documentation submitted.

Under Original Medicare, prior authorization is required for some services — most notably certain complex durable medical equipment including power wheelchairs, and some outpatient procedures. Under Medicare Advantage, prior authorization is significantly more common — most plans require it for specialist referrals, hospitalizations, certain imaging, many procedures, and most equipment.

The requirement exists because insurers review whether the specific service is medically necessary for the specific patient based on documented clinical criteria — not just because a doctor ordered it.

For durable medical equipment — wheelchairs, oxygen, hospital beds — the supplier submits the prior authorization request to Medicare or the insurance company along with the clinical documentation. The patient does not submit it. The physician's role is to provide complete and specific documentation; the supplier's role is to assemble the authorization packet and submit it correctly.

For procedures and hospital admissions under Medicare Advantage, it is typically the treating physician's office or the hospital that submits the prior authorization. The patient generally does not initiate this process — but the patient should know it is happening and should confirm it has been submitted before scheduling.

Where patients get into trouble: assuming that because the doctor ordered something, it is automatically approved. The order and the authorization are two separate steps. A physician can order a power wheelchair on Monday; the prior authorization may not be approved for six weeks. The equipment cannot be provided until authorization is in hand.

Medicare and most insurance companies require documentation that establishes:

• The diagnosis — the condition driving the need
• Functional limitations — specifically how the condition affects what the patient can and cannot do. "Has MS" is not sufficient. "Has MS with bilateral lower extremity weakness resulting in inability to walk more than 15 feet without falling" meets the standard
• Why the requested service is the appropriate solution — and why lesser alternatives are not sufficient
• Supporting clinical records — office notes, evaluations, test results that back up the physician's statements

The most common reason prior authorizations are delayed or denied is not that the patient does not need the service — it is that the documentation does not specifically establish the need in the language Medicare's coverage criteria require. Vague documentation creates review delays. Specific functional documentation moves through faster.

Medicare denials fall into several categories:

• Documentation insufficient — the clinical record does not specifically establish medical necessity in the language Medicare's coverage criteria require. This is the most common reason for denial and the most fixable.
• Service not covered — the service requested is not covered under Medicare for any patient (dental, hearing aids, custodial care). This is a structural limitation, not a denial of an individual claim.
• Coverage criteria not met — the patient does not meet the specific eligibility criteria for the requested service (e.g., a power wheelchair is denied because the documentation does not show the patient tried and failed with a manual chair first).
• Billing errors — incorrect codes, missing information, or administrative errors in how the claim was submitted.
• Not medically necessary — the reviewer determined the service does not meet Medicare's medical necessity standard based on the documentation provided.

The denial notice you receive must include the specific reason for denial and information about how to appeal. Read it carefully — the reason code tells you exactly what went wrong and what needs to be addressed.

When Medicare denies a claim, you have the legal right to appeal. The denial notice must include your appeal rights and the deadline for each level. The general deadlines are: 120 days to file a redetermination (Level 1), 180 days for subsequent levels. Do not miss these windows — once the deadline passes, you typically cannot appeal that denial.

You also have the right to request your complete case file — all the documentation Medicare reviewed in making its decision. Reviewing this file often reveals exactly what was missing or what the reviewer focused on, which informs what to add on appeal.

If your Medicare Advantage plan denies a service you need urgently, you have the right to request an expedited appeal — which must be decided within 72 hours. If the plan upholds its denial, an independent review organization reviews the case at no cost to you.