Original Medicare does not pay for everything. According to research cited by Wikipedia's overview of the Medicare program, drawn from the annual Medicare Trustees reports and Congress' MedPAC group, Medicare covers only about half of the healthcare expenses of the people enrolled in it. The rest comes out of pocket, or out of a supplemental policy someone bought to fill the holes.
The gaps fall into a few familiar buckets: deductibles and copays, prescription drugs, and long-term custodial care, which Medicare does not cover at all. There are three main routes for closing them — a private Medigap policy, a Medicare Advantage plan, or a standalone Part D drug plan — and each works differently. This explainer walks through what is missing and what each route does about it. It is information, not medical or insurance advice.
Enrollment timing matters here too. As USAGov explains, the Initial Enrollment Period runs from three months before you turn 65 to three months after the month you turn 65 — seven months in total — and missing it can mean a penalty. That penalty is one more gap, and it is self-inflicted.
What exactly does Original Medicare leave out?
Original Medicare means Part A and Part B. Part A covers hospital stays, skilled nursing and hospice. Part B covers outpatient care, doctor visits and preventive services. Between them, they pay for a great deal — and they stop at predictable lines.
The first gap is cost-sharing. Beneficiaries typically owe deductibles on Parts A, B and D, plus copays under Parts B and C, according to the Wikipedia overview of the program. Those amounts arrive with every admission and every appointment, and Original Medicare puts no cap on them. A patient with a hard year can owe a great deal in pieces rather than all at once.
The second gap is what the program never covers. The Wikipedia overview names long-term custodial care — help with daily living, whether at home or in a facility — as a cost not covered by Medicare. That is the gap families discover last and feel longest, because custodial care is exactly the kind of care many people eventually need.
The third gap is dental, vision and hearing. Original Medicare's statutory parts cover hospital and medical services, not routine dental work, eyeglasses or hearing aids. Some Medicare Advantage plans add those benefits; Original Medicare on its own does not.
Who needs to fill the gaps, and who already has?
Most people on Original Medicare need something extra, because the program covers roughly half of enrollees' healthcare expenses. Enrollees cover most of the remaining costs in one of three ways, per the Wikipedia overview: additional private insurance (Medigap), a Part D prescription drug plan, or a private Medicare Advantage plan.
Some people are already covered without buying anything. Retirees with employer or union coverage sometimes keep it alongside Medicare. People with low incomes may qualify for Medicaid, which can pick up costs Medicare leaves behind — the Wikipedia overview notes that about 20% of beneficiaries in 2022 were "dual eligible" for both programs, most of them eligible for full Medicaid benefits. If you think you might be in that group, ask your state Medicaid office this exact question: "Do I qualify for Medicare Savings Program help with my premiums and cost-sharing?" This connects to our earlier piece, CMS Launched an Electronic Prior Authorization Push on May 6.
What are the three routes for filling the gaps?
Each route bundles a different set of holes shut. Here is how they compare.
| Route | What it is | What it fills |
|---|---|---|
| Medigap | Private supplemental insurance bought alongside Original Medicare | Cost-sharing — the deductibles and copays Original Medicare leaves to you |
| Part D | Standalone private drug plan | Outpatient prescription drugs, which Original Medicare does not cover |
| Medicare Advantage (Part C) | Private plan that replaces Original Medicare and bundles A, B and usually D | Drugs and often extras like dental or vision, under the plan's own rules and networks |
Medigap and Advantage are alternatives, not add-ons. You cannot hold a Medigap policy and a Medicare Advantage plan at the same time in any useful way — Advantage becomes your coverage in place of Original Medicare. Medigap keeps you in Original Medicare and pays the share you would otherwise owe. Part D stacks on top of either path, unless your Advantage plan already includes drugs.
The trade is straightforward. Medigap trades a monthly premium for predictability: you keep broad access to any provider that takes Medicare, and the policy absorbs the cost-sharing. Advantage trades that access for bundling — one plan, often with extra benefits, but with the plan's own network rules and prior authorization requirements. For a sense of how heavily the market has tilted toward the private route, our MedPAC's June Report Lands With Medicare Advantage at 55 Percent covers the shift in enrollment. Readers following this should also see MedPAC's June Report Lands With Medicare Advantage at 55 Percent.
Does this apply if I'm still working past 65?
It may not — and that is worth checking before you enroll at all. USAGov notes that if you are still working at 65 and not receiving Social Security benefits, you can apply for Medicare only, or potentially wait until you retire and sign up during a special enrollment period. Waiting can be the right call when employer coverage is doing the gap-filling for you.
The penalty risk cuts the other way, though. USAGov states plainly that you may have to pay a penalty if you miss your Initial Enrollment Period. So the question is not "should I sign up at 65?" but "what covers me at 65, and does it let me delay?" Ask your employer's benefits office this exact question: "Does my current coverage let me delay Medicare Part B without a future penalty?" Get the answer in writing.
What this means: practical steps before you choose
The gaps are structural, so the fix is a plan, not a purchase made in a hurry. A short sequence helps.
- Map your own gaps first. List the care you actually use — prescriptions, dental, any help with daily living — and match it against what Parts A and B cover.
- Check enrollment windows on the calendar. USAGov's guidance on the seven-month Initial Enrollment Period and the special enrollment period for people still working is the timing backbone; missing a window can cost money for years.
- Compare the two routes honestly. Price the Medigap premium against the Advantage plan's copays and network limits for the doctors you actually see.
- Confirm drug coverage either way. Part D is the standard route for outpatient prescriptions under Original Medicare; some Advantage plans include it.
- Ask about dual eligibility if money is tight. Medicaid may cover costs Medicare leaves behind for those who qualify.
One caution belongs here. Plan benefits, premiums and networks change from year to year, and this article does not evaluate any specific plan. Verify current details for any plan you are considering with the plan itself or through Medicare's own help line — 800-633-4227, per Medicare.gov's help guidance — before deciding.
Where the gaps end and the unknowns begin
The evidence here is durable: Medicare covers about half of enrollees' expenses, leaves cost-sharing to beneficiaries, and does not cover custodial care or outpatient drugs on its own. The supplemental routes — Medigap, Part D and Medicare Advantage — are the established ways people close those holes, and enrollment timing determines whether filling them comes with a penalty attached.
What the sources here do not settle is which route costs a given person less. That depends on premiums in a given year, the plan's network, and the care a person actually uses. The general shape of the answer is above; the specific numbers belong to your own enrollment window, checked against the current documents before you commit.
