You can use an FSA for past medical bills, but only if the care itself was provided during your current plan year or a grace period your employer offers. The date on the bill and the date you actually pay it don’t matter for FSA purposes. What matters is when the doctor, hospital, or clinic delivered the service.
The Date-of-Service Rule
An FSA expense is “incurred” the moment you receive care, not when the provider mails a statement and not when you swipe a card to pay it. Everything about using FSA money on an older bill flows from that one principle.
Say you had knee surgery on October 15, 2025, and the hospital’s billing department didn’t send the statement until February 2026. That expense belongs to your 2025 plan year because the surgery happened in 2025. Your 2026 FSA balance cannot touch it. Flip the scenario: you saw a specialist on March 3, 2026, but the claim sat in insurance limbo until July. You can absolutely use your 2026 FSA to pay that July bill. The five-month delay is irrelevant because the service occurred inside the active plan year.
This trips people up because it works differently from the medical expense deduction on your tax return. IRS Publication 502 spells it out: the pay-date rule that governs the tax deduction is “not the rule for determining whether an expense can be reimbursed by a flexible spending arrangement.”1Internal Revenue Service. Publication 502 (2025), Medical and Dental Expenses For FSAs, the service date is what counts.
Bills From Before Your Plan Year Are Never Eligible
Every FSA runs on a plan year, typically 12 months aligned with either the calendar year or your employer’s fiscal year. Care received before that window opened is permanently out of reach, no matter how much money is sitting in your account.
Someone who enrolls in a new FSA on January 1, 2026 cannot use those funds to reimburse a hospital bill from the previous summer. The money is earmarked for care received during the 2026 plan year and any applicable grace period, nothing earlier. If you’re staring at an old bill and wondering whether a fresh FSA election can cover it, the answer is no.
At the other end of the plan year, the use-it-or-lose-it rule applies: any unused balance is forfeited when the year closes.2Internal Revenue Service. Modification of Use-or-Lose Rule For Health Flexible Spending Arrangements Notice 2013-71 That’s why the extensions in the next section matter so much when you have late-arriving bills.
Grace Periods, Run-Out Periods, and Carryovers
Employers can soften the strict plan-year cutoff in three different ways. Each one affects how you handle a past bill differently, so it’s worth knowing which your plan uses before you assume anything.
Grace Period
A grace period gives you an extra two and a half months after the plan year ends to incur new expenses and pay them with last year’s leftover funds. For a calendar-year plan ending December 31, the grace period runs through March 15. IRS Notice 2005-42 established this option, which employers may or may not adopt.3Internal Revenue Service. Modification of Application of Rule Prohibiting Deferred Compensation Under Section 125 Cafeteria Plans, Notice 2005-42 A doctor visit on February 20 can pull from your leftover 2025 balance if your plan includes this provision.
Run-Out Period
A run-out period is a different animal. It gives you extra time to file paperwork for services that already happened during the plan year. No new care qualifies. The typical window is 90 days after the plan year ends, though some plans set it shorter. If you had a December procedure but didn’t gather the documentation until January, the run-out period is what lets you still submit that claim. It’s a filing extension, not a spending extension.
This is the provision that most often rescues a genuinely past bill. If the statement finally arrives in February for care you received in November, you’re relying on the run-out period to get the claim submitted before the plan closes for good.
Carryover
Instead of a grace period, some employers offer a carryover that rolls unused funds into the next plan year. For the 2026 plan year, participants can carry over up to $680 into 2027.4FSAFEDS. New 2026 Maximum Limit Updates Anything above that threshold is forfeited. Rolled-over money behaves like regular new-year funds, so you can use it on any qualifying expense in the new plan year without worrying about a narrow grace period window.
An employer can offer either a grace period or a carryover, not both. If your plan documents don’t mention either option, your employer hasn’t adopted one, and the strict year-end deadline applies. Check with your benefits administrator before assuming you have extra time.
Your Full Election Is Available on Day One
If you’re sitting on a large bill from early in the plan year, one detail works in your favor: the uniform coverage rule. Elect $3,400 for 2026, and the entire $3,400 is available for reimbursement on January 1, even though only a fraction has been deducted from your paycheck.
Many participants sit on early-year bills assuming they need to wait for the balance to build up. That’s not how health FSAs work. Submit the claim immediately. The reimbursement isn’t capped at what you’ve contributed to date.
Documentation for a Late-Arriving Bill
When a bill shows up weeks or months after the visit, the documentation burden shifts onto you. Most FSA administrators require an itemized receipt or Explanation of Benefits proving the service happened during the eligible period. Five details need to appear on whatever you submit:
- Patient name
- Provider name and address
- Date of service
- Description of the service (not just a billing code)
- Amount charged after insurance adjustments
Credit card receipts and canceled checks won’t cut it. They prove payment but not what the payment was for or when the service happened. An itemized statement from the provider or an Explanation of Benefits from your insurer covers all the bases. For a bill that arrived months late, request an itemized statement directly from the provider’s billing department. That date-of-service line is the detail your administrator will scrutinize most closely.
The Orthodontia Exception
Orthodontic treatment is the major exception to the date-of-service rule. Because braces and aligners involve ongoing monthly payments spread over years, the IRS allows FSA reimbursement based on when you make each payment rather than when the orthodontist fitted the brackets. A January installment on a treatment that started the previous year is eligible for current-year funds, as long as the payment itself falls within the active benefit period.5FSAFEDS. Orthodontia Quick Reference Guide
Full prepayment works too. If the orthodontist requires the entire fee upfront before starting treatment, that lump sum is eligible for reimbursement in the year you pay it, regardless of whether the actual orthodontic work stretches into the next plan year.5FSAFEDS. Orthodontia Quick Reference Guide You’ll need an itemized bill showing the total amount, proof of payment, and typically a treatment plan. If dental insurance covers part of the cost, the FSA reimbursement is reduced by whatever insurance pays.
So if your “past bill” is an orthodontia invoice, don’t apply the standard date-of-service rule. Look at when the payment is due or was made, and match that against your current plan year.