The honest answer
Do you need a Medicare supplement? You need a ceiling. Medigap is one of two.
Short answer: what you need is an annual limit on your medical costs, because Original Medicare alone doesn’t have one: its 20% coinsurance runs uncapped forever. There are exactly two ways to get that ceiling: a Medigap supplement, or a Medicare Advantage plan’s built-in out-of-pocket maximum. So the real question isn’t "do I need a supplement," it’s "which ceiling fits me," plus one warning about timing that can’t be unlearned later.
Key takeaways
- Original Medicare alone leaves 20% coinsurance with no annual cap: a bad outpatient year can cost tens of thousands.
- The two ceilings: Medigap (pay ~$120–$300/month, bills mostly vanish) or an Advantage plan’s out-of-pocket max (pay less monthly, cap your worst case).
- Skipping both is the one genuinely dangerous configuration, and people drift into it by inaction, not decision.
- Your 6-month Medigap window at Part B start is the only guaranteed-acceptance period; afterward most states allow underwriting.
- Legitimate exceptions to needing Medigap: TRICARE For Life, strong retiree wraparound coverage, Medicaid/dual eligibility, or a deliberate Advantage choice.
Why a ceiling is non-optional
Part B’s design is 80/20 with no maximum. Chemotherapy, dialysis, infused biologics, repeated imaging, outpatient surgeries: 20% of a $150,000 outpatient year is $30,000, and next year resets to zero and starts again. Employer plans trained everyone to assume an out-of-pocket max exists; Original Medicare simply doesn’t have one. Every configuration decision in Medicare is downstream of fixing that, which is why the "do I need a supplement" question deserves reframing: the supplement is optional; the ceiling is not.
The two ceilings, honestly compared
| Medigap (Plan G or N) | Advantage out-of-pocket max | |
|---|---|---|
| How the ceiling works | Plan pays the 20%; your exposure is roughly the Part B deductible ($283) | You pay copays as you go, capped at the plan’s annual max (commonly $4,000–$5,500+ in-network) |
| Monthly cost | ~$120–$300 premium + Part D plan | Often $0–$50 on top of Part B |
| Doctors | Any Medicare provider nationwide, no referrals | Plan network, often referrals and prior authorization |
| Bad-year cost | A few hundred dollars | The full out-of-pocket max, potentially yearly |
| Reversibility | Can drop anytime (returning later means underwriting) | Easy to enter, hard to leave for Medigap later |
Ranges are market-typical for 2026; specific plans vary by zip code. The reversibility asymmetry is the strategic point.
Both are legitimate. The choice tracks the same four facts as the broader path decision (doctors, health, temperament, travel), which we’ve mapped fully, and the 27-visit arithmetic between the two main Medigap letters is in the G-vs-N article.
Who can legitimately skip Medigap
- TRICARE For Life retirees: TFL already wraps Original Medicare more generously than any Medigap plan; buying one duplicates coverage.
- People with genuine retiree wraparound coverage from a former employer or union that pays Medicare’s cost-sharing; read the plan documents, because "retiree coverage" spans everything from gold-plated to decorative.
- Dual-eligibles: Medicaid wraps Medicare’s cost-sharing; a supplement adds nothing.
- Deliberate Advantage choosers: not skipping the ceiling, choosing the other one.
Who shouldn’t skip it: anyone on Original Medicare without one of those wraps who "just hasn’t gotten around to it." That’s not frugality; it’s an unhedged position with unlimited downside, held by default.
The timing warning that outranks everything else
Your 6-month Medigap open enrollment starts when Part B begins: every insurer must sell you any plan at standard rates, no health questions. It never repeats. Outside it, most states allow underwriting, and accumulated diagnoses can mean higher prices or declines. So even someone genuinely torn should decide during the window, because "Medigap later, maybe" silently converts into "Medigap if an underwriter approves." The full mechanics, including the states with friendlier rules, are in the switching article.
If the answer is yes: sixty seconds on which one
Plan G if you want maximum predictability; Plan N if you’ll trade small copays for a lower premium and your state bans excess charges; high-deductible G if you want catastrophic-only coverage at a small premium ($2,950 deductible in 2026). Benefits are federally standardized per letter, so shop carriers purely on price, rating method, and rate-increase history, three quotes minimum. Then add a Part D plan and the configuration is complete.
Questions people also ask
Is a Medicare supplement really necessary?
What happens if I have Original Medicare with no supplement?
Who doesn’t need a Medigap plan?
Can I buy a Medigap plan later if I change my mind?
Is Plan G worth the premium over no supplement?
Sources for this article
- Medicare.govBuying a Medigap policy: standardized benefits and how supplements work.
- Medicare.govWhen to buy: the 6-month open enrollment rules.
- CMS.gov2026 premiums and deductibles: the deductibles and cost-sharing behind the math.
Figures checked against these sources in August 2026. We update within weeks when CMS publishes new amounts.
Not sure which path fits you? Get a straight answer.
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