Medicare Part C, also called Medicare Advantage, is not mandatory. It is an optional alternative to Original Medicare (Parts A and B) offered through private insurance companies. No one is automatically enrolled in a Part C plan, and you will never be required to join one. When you become eligible for Medicare, you are enrolled in Original Medicare by default, and choosing a Medicare Advantage plan instead is always a voluntary decision.
How Part C Differs From Original Medicare
Original Medicare is the federal program that covers hospital stays (Part A) and outpatient medical services (Part B). With Original Medicare, you can see any doctor or visit any hospital in the country that accepts Medicare, with no referrals needed for specialists.
Medicare Advantage (Part C) bundles those same benefits into a plan run by a private insurer. Most plans also include prescription drug coverage and extras like dental, vision, or hearing benefits that Original Medicare does not cover. In exchange for those added benefits, Part C plans typically restrict you to a network of providers and may require referrals to see specialists.
The tradeoff comes down to flexibility versus extras. Original Medicare gives you broad provider access but leaves gaps you may want to fill with a separate supplement (Medigap) policy and a standalone drug plan. Medicare Advantage gives you more bundled coverage but with tighter rules about where and how you get care.
How Enrollment Works
When you first qualify for Medicare at 65 or through disability, you are placed into Original Medicare. Joining a Medicare Advantage plan requires you to actively choose one during a designated enrollment period. You must already have both Part A and Part B to enroll in Part C.
The main window to sign up is the Annual Enrollment Period, which runs from October 15 through December 7 each year. If you’re turning 65, you can also enroll during your Initial Enrollment Period, a seven-month window surrounding your 65th birthday. There is no scenario where Medicare or Social Security automatically places you into a Part C plan.
What Happens if You Change Your Mind
Choosing Medicare Advantage is not a permanent decision. If you join a plan and decide it’s not right for you, there are several ways to switch back to Original Medicare.
If you enrolled in Medicare Advantage when you first became eligible at 65, you have a 12-month trial period. During that window, you can drop the plan, return to Original Medicare, and you’ll have guaranteed rights to buy a Medigap policy regardless of your health status. The same trial right applies if you dropped a Medigap policy to join Medicare Advantage for the first time.
Outside of that trial period, you can switch back to Original Medicare during the Annual Enrollment Period or during the Medicare Advantage Open Enrollment Period (January 1 through March 31). However, returning to Original Medicare after the trial period does not guarantee you can buy a Medigap policy at standard rates. In most states, Medigap insurers can charge higher premiums or deny coverage based on pre-existing conditions once your initial Medigap open enrollment window has passed.
One Important Rule: No Medigap With Part C
You cannot hold both a Medicare Advantage plan and a Medigap policy at the same time. Medigap is designed to supplement Original Medicare by covering copays, deductibles, and coinsurance. It does not work with Medicare Advantage. If you’re enrolled in Part C, a Medigap policy won’t pay toward your plan’s copayments, deductibles, or premiums. You must be on Original Medicare to use Medigap.
Network Rules Vary by Plan Type
Not all Medicare Advantage plans work the same way. The type of plan you choose determines how much freedom you have in picking doctors and hospitals.
- HMO plans generally require you to use in-network providers for all non-emergency care. You’ll typically need a referral from your primary care doctor to see a specialist.
- PPO plans let you see out-of-network providers, but you’ll pay more for doing so. Referrals are usually not required.
- Private Fee-for-Service (PFFS) plans allow you to visit any Medicare-approved provider who accepts the plan’s payment terms, with no network restrictions in many cases.
- Special Needs Plans (SNPs) serve people with specific chronic conditions, institutional care needs, or dual eligibility for Medicare and Medicaid. Network rules depend on whether the SNP is structured as an HMO or PPO.
- Medical Savings Account (MSA) plans generally have no network at all. You can see any Medicare-approved provider.
Financial Protections in Part C
One feature that draws people to Medicare Advantage is the annual cap on out-of-pocket spending. Original Medicare has no built-in maximum, meaning your costs can climb without limit in a year with heavy medical use. Medicare Advantage plans are required to set a ceiling. In 2025, that cap cannot exceed $9,350 for in-network services or $14,000 for in-network and out-of-network services combined.
Many Part C plans also charge $0 in monthly premiums beyond what you already pay for Part B, though lower-premium plans often come with higher copays or narrower networks.
How Many People Choose Part C
Despite being entirely optional, Medicare Advantage has become the more popular choice. In 2025, about 34.1 million people, representing 54% of all eligible Medicare beneficiaries, are enrolled in a Medicare Advantage plan. That share has grown steadily over the past decade, driven largely by the appeal of bundled benefits, out-of-pocket caps, and $0-premium plan options. Still, nearly half of Medicare beneficiaries remain on Original Medicare, often pairing it with a Medigap policy and a standalone Part D drug plan.