Medicare has four main parts, and each one covers different services
Part A covers hospital stays, skilled nursing care after a hospital stay, hospice, and some home health services. Part B covers doctor visits, outpatient care, medical equipment, and preventive services. Part C (also called Medicare Advantage) is an alternative way to get Parts A and B through a private insurance company, often bundled with prescription drug coverage. Part D covers prescription drugs through a private plan you choose.
Most people on Original Medicare use Parts A and B together, then add Part D for prescriptions. Some people choose Part C instead, which replaces Parts A and B but still requires Part D as a separate choice (though many Part C plans include it). Understanding what each part covers helps you know what costs you'll face and which gaps you might want to fill with supplemental insurance.
Key Takeaways
- Part A covers hospital and skilled nursing stays; Part B covers doctor visits and outpatient services; most people have both through Original Medicare.
- Part C (Medicare Advantage) is a private insurance alternative that replaces Parts A and B, often with lower premiums but higher out-of-pocket costs at the time of care.
- Part D is prescription drug coverage you choose separately, whether you're on Original Medicare or Part C.
- You can add supplemental insurance (Medigap) to Original Medicare to cover costs Part A and B don't pay, but you cannot use Medigap with Part C.
- Enrollment important date matter: missing the initial enrollment window can mean permanent premium penalties unless you have a may have access to life event.
Part A: Hospital and Skilled Nursing Coverage
Part A covers inpatient hospital stays, including room, meals, and standard nursing care. It also covers up to 100 days in a skilled nursing facility after a hospital stay of at least three days, as long as you're receiving skilled care (physical therapy, wound care, or similar). Hospice care and some home health services are covered under Part A as well.
Part A has a deductible you pay once per benefit period (the period starts when you enter the hospital and ends 60 days after you leave). After you meet the deductible, Medicare pays most costs, though you pay a daily coinsurance amount for days 61–90 of a hospital stay and for days 21–100 in a skilled nursing facility. Most people don't pay a monthly premium for Part A if they or their spouse paid Medicare taxes for at least 10 years while working.
Part B: Doctor Visits and Outpatient Services
Part B covers doctor office visits, preventive care (like annual wellness visits and cancer screenings), lab tests, X-rays, and outpatient surgery. It also covers durable medical equipment such as wheelchairs, walkers, and oxygen supplies, as well as mental health services and rehabilitation therapy.
Part B has a monthly premium (the amount changes each year based on your income) and an annual deductible. After you meet the deductible, you typically pay 20% of the cost for most services, and Medicare pays 80%. Some preventive services, like flu shots and colorectal cancer screening, have no cost-sharing if you see an in-network provider.
Part C (Medicare Advantage): A Private Insurance Alternative
Part C plans are offered by private insurance companies approved by Medicare. They must cover everything Part A and Part B cover, but they do it through their own networks and rules. Many Part C plans include prescription drug coverage (Part D) built in, and some add dental, vision, or hearing coverage that Original Medicare doesn't offer.
Part C premiums are often lower than the combined cost of Part B plus a separate Part D plan, which appeals to people on a tight budget. However, you typically pay more at the time of care — higher copays for doctor visits, higher coinsurance for hospital stays, and often an annual out-of-pocket maximum you must reach before the plan pays 100%. You also must use doctors and hospitals in the plan's network, except in emergencies. If you choose Part C, you cannot use Medigap supplemental insurance to cover those out-of-pocket costs.
Part C plans change their coverage and costs every year, so you can switch plans during the annual enrollment period (October 15 to December 7) without penalty. If you decide to leave Part C and return to Original Medicare, you may be able to buy Medigap coverage, though some plans have limited enrollment periods.
Part D: Prescription Drug Coverage
Part D is prescription drug coverage you purchase from a private insurance company. If you're on Original Medicare (Parts A and B), you choose a Part D plan during enrollment. If you're on Part C, many plans include Part D, but you should confirm the drugs you take are covered at a cost you can afford.
Part D plans vary widely in which drugs they cover and what you pay. Most plans have a monthly premium, an annual deductible, and a coverage gap (sometimes called the "donut hole") where you pay a higher percentage of drug costs after you and the plan have spent a certain amount. Once your out-of-pocket costs reach a yearly limit, the plan covers most of the cost for the rest of the year.
If you don't sign up for Part D when you first become may be able to access and you don't have other creditable drug coverage, you may pay a permanent penalty added to your premium if you join later. The penalty is calculated based on how many months you went without coverage.
Original Medicare Plus Medigap: Filling the Gaps
Original Medicare (Parts A and B) leaves you responsible for deductibles, coinsurance, and copays. Medigap (supplemental insurance) is sold by private companies and helps pay those costs. There are ten standardized Medigap plans (labeled A through N), each covering a different combination of out-of-pocket expenses.
Medigap plans have their own monthly premiums, but they can save you money if you use a lot of healthcare services. You can buy Medigap during your initial enrollment period (the six months after you turn 65 and sign up for Part B), and insurance companies must sell it to you without medical underwriting during that window. Outside that window, companies can deny coverage or charge more based on your health history.
You cannot use Medigap with Part C — the two are mutually exclusive. If you're on Part C and want to switch to Original Medicare with Medigap, you may have limited time to buy Medigap without medical underwriting, depending on your state and plan.
Comparing Your Choices: Original Medicare Versus Part C
| Feature | Original Medicare (A + B) | Part C (Medicare Advantage) |
|---|---|---|
| Monthly premium | Part B premium only (Part A usually free) | Often lower than Part B + Part D combined |
| Out-of-pocket costs at time of care | Deductibles and coinsurance; can be high without Medigap | Copays and coinsurance; capped annual maximum |
| Doctor and hospital choice | Any provider that accepts Medicare | In-network providers only (except emergencies) |
| Prescription drugs | Must buy Part D separately | Often included; sometimes separate |
| Supplemental coverage | Can buy Medigap to cover gaps | Cannot use Medigap; out-of-pocket maximum applies |
| Annual changes | Stable year to year | Coverage and costs change every year |
Enrollment important date and Penalties
Your Initial Enrollment Period is the seven-month window centered on your 65th birthday — three months before, the month of, and three months after. If you sign up during this window, coverage begins the month you turn 65 (or the following month, depending on when you sign up). If you miss this window and don't have other creditable coverage, you pay a permanent penalty on your Part B and Part D premiums for as long as you have Medicare.
After your initial enrollment period, you can make changes during the Annual Enrollment Period (October 15 to December 7 each year). During this time, you can switch between Original Medicare and Part C, change Part C plans, or change Part D plans. Changes take effect January 1 of the following year.
If you have a may have access to life event — such as losing employer coverage, moving out of your plan's service area, or experiencing a major life change — you may be able to enroll or switch plans outside the regular enrollment windows. Contact Medicare directly to confirm whether your situation qualifies.
Frequently Asked Questions
Do I have to choose all four parts?
No. If you choose Original Medicare, you must have Part A and Part B, but Part D is optional (though skipping it can mean a penalty). Part C replaces Parts A and B, so you don't have both. Medigap is optional and only works with Original Medicare.
Can I switch from Part C back to Original Medicare?
Yes, during the Annual Enrollment Period. However, if you switch back to Original Medicare, you may have a limited window to buy Medigap without medical underwriting. Rules vary by state, so contact Medicare or your state health insurance counselor to understand your options before you switch.
What happens if I miss the enrollment important date?
If you miss your Initial Enrollment Period and don't have other creditable coverage, you pay a permanent penalty on Part B and Part D premiums. The penalty is 10% of the Part B premium for each year you were may be able to access but not enrolled, and a similar calculation applies to Part D. You can still enroll, but the penalty stays with you.
Are prescription drugs covered the same way in all Part D plans?
No. Each Part D plan has its own formulary (list of covered drugs), and costs vary by plan. Some plans cover a drug at a low copay; another plan might not cover it at all or charge much more. You should review your current medications against each plan's formulary before you choose.
Can I use my current doctor with Part C?
Only if your doctor is in the plan's network. Before you switch to Part C, check whether your doctors and preferred hospital are in-network. If they're not, you may have to change providers or pay out-of-network costs, which are usually much higher.