Enroll in Medicare at 65 and Part A Backdates Six Months. Every HSA Dollar Contributed in That Window Is Taxed, Plus 6% a Year Until You Pull It Out
A single line on a Medicare enrollment notice quietly transformed six months of tax-free HSA deposits into a penalty that compounds every year the money sits untouched, and most savers never see it coming until after the deadline to fix…
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Consider a 65-year-old single filer who retires at the end of August 2026 and signs up for Medicare in September. Her enrollment notice lists a Part A start date of March 1. That one line on the notice turns six months of health savings account deposits into a tax bill.
Medicare lets premium-free Part A take effect retroactively, a practice that began in 1983. The IRS treats every backdated month as a Medicare month, and HSA contributions are barred in any Medicare month. Anything deposited during that period counts as an excess contribution. It loses its tax break and gets a 6% excise tax for every year it stays in the account.
How Part A’s Six-Month Lookback Catches HSA Savers
Premium-free Part A costs nothing, so the system assumes you want it as early as possible. When you apply after 65, Part A starts up to 6 months before the month you apply, but never before the month you turned 65. You can’t opt out of the backdated months, according to a 2026 analysis from Alhambra Investments.
Social Security sets off the same trap. Filing for retirement benefits at 65 or older automatically enrolls you in Medicare Part A, and the retroactive coverage comes with it.
If you sign up in the month you turn 65, nothing gets backdated. The people at risk are the ones who keep working on a high-deductible plan past their birthday month and enroll later, even a few months later.
Worked Example: $2,700 in Excess Contributions in One Year
Our composite turned 65 in January 2026 and has self-only HDHP coverage through work. She aims for the 2026 maximum of $4,400 plus the $1,000 catch-up for savers 55 and older, which works out to $450 a month through payroll. She retires after August and applies for Medicare in September, so Part A reaches back to March.
| Months (2026) | Status | HSA contributions |
|---|---|---|
| January to February | HSA-eligible | $900 |
| March to August | Backdated Medicare | $2,700 |
| Total deposited | $3,600 |
Her annual limit shrinks to one-twelfth of the annual limit for each month she was eligible. Two eligible months allow $900. The remaining $2,700 is excess.
Income Tax on the Excess, Then 6% Every Year
Payroll deposits never showed up in her taxable wages, so the $2,700 gets added back. Assume all of it falls inside the 22% federal bracket. That costs her $594. State income tax may apply on top, depending on where she lives.
Then comes the excise tax under Section 4973 of the tax code: $162 for 2026, so her first-year damage totals $756. The 6% hits again every year the money stays in the account. Leave it for five years and she pays $810 in excise tax alone. It’s one of several quiet IRS rules that chip away at retirement balances, and we mapped out the rest in a free guide to the retiree tax traps.
The added income can also push up her Medicare premiums later, because 2026 income sets her 2028 premiums. Using 2026’s levels as a guide, a single filer whose income rises above $109,000 pays $284.10 a month for Part B instead of $202.90. The 2028 income thresholds will be indexed for inflation.
Fix It Before Your Tax Filing Deadline
If you withdraw the excess contributions and earnings before the tax due date, you skip the 6% tax. You still owe income tax on the earnings, which are reported as other income. Ask your HSA custodian for a “removal of excess contribution.” A regular withdrawal won’t fix it.
If the deposits came through payroll, you may need your employer to issue an amended W-2. If the deadline has already passed, the 6% gets reported on Form 5329 every year until the money comes out.
Three Moves That Keep the Six-Month Window Clean
- Stop deposits six months early. Halt your own contributions and your employer’s about 6 months before the month you apply for Medicare or Social Security.
- Front-load what you’re allowed. Figure your prorated limit from your eligible months and fund it before the deadline.
- Time your Social Security claim. If you’re putting off Medicare so you can keep funding the HSA, claiming benefits starts Part A anyway. Plan the two decisions together.
Keep the account once contributions stop. After 65, HSA money can pay Part B, Part D, Medicare Advantage premiums and IRMAA tax-free, though Medigap premiums don’t count. When retirement dates, Social Security claims and HSA deposits all overlap in one year, it’s worth having a CPA run the numbers before you file.
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Vilma Rios is a tax professional and tax content contributor with more than 15 years of experience in tax and accounting. She specializes in federal tax research, tax education, and translating complex tax rules into clear, practical information for individuals, families, and small-business owners.
Vilma is a Content Tax Contributor II with the National Association of Tax Professionals (NATP), where she contributes to tax education and professional content. She has also presented tax information through webinars, including Spanish-language tax education, and has appeared on Telemundo 47 discussing tax topics and helping viewers understand important tax-filing requirements.
Her experience also includes tax and accounting work, tax research, IRS-related matters, and public tax education. While in college, Vilma volunteered in an IRS-sponsored tax assistance program and was recognized for her community service by local and state officials.
Known as “Your Tax Geek,” Vilma is passionate about making taxes easier to understand and helping people navigate an increasingly complex tax system.
