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Short answer: A letter of medical necessity (LMN) for an HSA or FSA is a statement from a licensed healthcare provider saying that a product or service is needed to treat a diagnosed condition. It is mainly a plan administrator’s tool, not an IRS form: the IRS sets the test for what counts as medical care, and the administrator decides when it wants an LMN as evidence. This page is general information, not medical, tax or legal advice.
Definition and scope
An LMN is a document written by a licensed healthcare provider verifying that a treatment, service or product is medically necessary for a patient. WEX, a benefits administrator, describes it as typically required when an expense is not automatically deemed eligible under IRS guidelines, and says to submit it to the HSA, FSA or HRA administrator when one is required (WEX blog, “What is a letter of medical necessity?”, checked 2026-11-01). The variants people search for, including “FSA letter of medical necessity”, “HSA letter of medical necessity” and “letter of medical necessity for HSA FSA”, all describe this one document.
What the IRS does and does not say
The rule behind the letter comes from the tax rules on medical care, not from the letter itself. The IRS publications cited below set the medical-care test but supply no LMN form.
- The test. Under 26 CFR 1.213-1(e)(1)(ii), medical care means expenses primarily for the prevention or alleviation of a physical or mental defect or illness; expenses that are merely beneficial to general health do not qualify. Publication 502 gives vitamins and a vacation as examples of the latter (IRS Publication 502, 26 CFR 1.213-1, checked 2026-11-01).
- HSA expenses. Qualified medical expenses are amounts paid for medical care under section 213(d) for the account holder, spouse and dependents, but only to the extent not compensated by insurance or otherwise (IRS Publication 969, checked 2026-11-01).
- Substantiation. Reg. 1.213-1(h) calls for the payee’s name and address, the amount and the date, and a statement or itemized invoice from the provider when requested.
HSA vs FSA: who checks the paperwork
Publication 969 states that a health FSA requires a written statement from an independent third party confirming the expense and its amount, plus a statement that the amount has not been reimbursed under other coverage. For an HSA, the publication ties eligibility to the section 213(d) definition of medical care. Because the administrator, not the IRS, runs the claims process, whether it asks for an LMN, and in what form, depends on the plan.
Which expenses raise the question
The checklist shows where the IRS itself ties eligibility to a medical purpose or a physician’s statement, which is where an administrator is most likely to ask for supporting paperwork.
| Type of expense | What the source says | Source |
|---|---|---|
| Items for general health (vitamins, a vacation) | Not medical care when merely beneficial to general health | Pub. 502; Reg. 1.213-1(e)(1)(ii) |
| Supplements | Excluded when taken to maintain ordinary good health and not for medical care | Pub. 502 |
| Weight-loss program | Includible when it is a treatment for a specific disease diagnosed by a physician | Pub. 502 |
| Health institute treatment | The physician issues a statement that the treatment is necessary to alleviate a physical or mental disability or illness | Pub. 502 |
| Special food | The need must be substantiated by a physician | Pub. 502 |
| Home accommodation for a disabled condition | Only reasonable costs to accommodate the home to the disabled condition are medical care | Pub. 502 |
| Capital expenditures (equipment) | Generally not deductible unless the primary purpose is medical care; eye glasses and a wheel chair are among the regulation’s examples | Reg. 1.213-1(e)(1)(iii) |
The table is a map of where the IRS test turns on a medical purpose. It does not say that any of these purchases needs an LMN, and it does not say any will be approved. That is the plan administrator’s call.
What the letter typically contains
According to WEX (an administrator’s practice, not an IRS requirement), an LMN includes the provider’s name, credentials and signature; the patient’s name and diagnosis; the recommended treatment or item; a statement of medical necessity; and the date. WEX also states that most LMNs are valid for one year, though some plans may require renewal sooner, and advises checking with the account provider before purchase. The provider, not the supplier or the patient, decides whether the statement is clinically appropriate.
Common misunderstandings
An LMN is not a CMN
A Certificate of Medical Necessity is a Medicare form for certain durable medical equipment. See what a CMN is, DME HCPCS codes and hospital bed Medicare requirements for the Medicare side. An LMN for an HSA or FSA is a separate document used with a benefits administrator.
An LMN is not an IRS form
The IRS publications cited on this page set the medical-care test and substantiation expectations but supply no LMN template.
An LMN does not guarantee reimbursement
The administrator applies the plan’s rules. A letter is evidence for that review, and it does not change what the tax rules treat as medical care. It also differs from insurer approval steps such as those compared in prior authorization vs precertification.
FAQ
Is a letter of medical necessity required for an HSA?
The IRS publications cited here do not prescribe an LMN. An HSA is tied to the section 213(d) definition of medical care, and an LMN is useful as supporting records for an item that is not obviously medical.
What is an FSA letter of medical necessity?
It is the same kind of provider statement, used when a health FSA administrator asks for support. Publication 969 separately requires FSA claims to carry a written statement from an independent third party and a not-reimbursed statement.
Who writes the letter?
A licensed healthcare provider, according to WEX.
How long is it valid?
WEX states most LMNs are valid for one year, though some plans require renewal sooner. Confirm with the plan.
Can the same letter go to an HSA, FSA or HRA administrator?
WEX says to submit the LMN to your HSA, FSA or HRA administrator when one is required; the accepting administrator makes the final decision.
Sources
- IRS Publication 502, Medical and Dental Expenses (2025), checked 2026-11-01.
- IRS Publication 969, Health Savings Accounts and Other Tax-Favored Health Plans (2025), checked 2026-11-01.
- 26 CFR 1.213-1, Allowance of deduction for medical, dental, etc., expenses, checked 2026-11-01.
- WEX, What is a letter of medical necessity?, checked 2026-11-01 (administrator practice).
