What Is a Letter of Medical Necessity?
Your HSA or FSA can only pay for medical care, and the IRS draws that line in Publication 502: medical expenses are "the costs of diagnosis, cure, mitigation, treatment, or prevention of disease." A sauna, a gym membership, or an air purifier can sit on either side of that line, ordinary comfort on one side and a documented medical purpose on the other. The Letter of Medical Necessity is the document that places your specific purchase on the medical side.
Benefits administrators call these dual-purpose items. The term is theirs rather than the IRS's, but the logic tracks Publication 502 exactly: an item useful to the general population is not reimbursable on its own, and the same item becomes reimbursable when it serves a documented medical purpose for you. The letter is that documentation. A licensed provider names your specific health condition, states the item or service, and explains how it helps in preventing or managing that condition. Signed and dated, it is what your administrator reads when your claim comes in.
The letter works identically for an HSA and an FSA. The account type changes how the money gets there, not what counts as a medical expense, so one compliant letter covers either account. If you are still choosing between them, our HSA vs FSA guide breaks down the differences.
There is no master list of qualifying conditions. The standard is the pairing: a dual-use item, a letter, and a specific named condition the item helps prevent or manage. Whether a given pairing holds is a clinical judgment: the medical community decides what is appropriate, not a benefits catalog and not marketing copy. Asthma and an air purifier, or chronic back pain and a supportive mattress, are pairings providers recognize and document every day.
A Letter of Medical Necessity Is Not a Prescription
A prescription tells a pharmacy to dispense something. A Letter of Medical Necessity dispenses nothing: it is documentation, written for whoever reviews your claim, that a purchase you make yourself serves your condition. No pharmacy is involved at any point. A licensed provider issues the letter, you buy the item at any retailer you like, and the letter travels with your receipt when you file for reimbursement. In practice, "getting a prescription for a sauna" is the wrong mental model. Providers do not write scripts for saunas; they issue letters documenting why one is appropriate for your condition.
The Letter of Medical Necessity Template
A complete letter typically runs one to a few pages, up to about four. Many administrators publish their own Letter of Medical Necessity forms, and a form or a freestanding letter works equally well: the ten field groups below carry over regardless of format. Each field includes the reason a reviewer looks for it. And if you came here planning to write it yourself: a licensed provider writes and signs the letter; your job is bringing them the raw material this list organizes.
1. Provider letterhead and contact details
Practice name and address, phone or fax, and the provider's full name with credentials, state license number, and NPI. Why the reviewer looks for this: the first check is whether a verifiable licensed provider stands behind the letter, and a license number plus NPI makes that check take seconds; who counts as a licensed provider is covered in the legitimacy section below.
2. Date of issue and recipient line
The date the provider signs the letter, plus a simple recipient line naming your administrator; "To Whom It May Concern" is acceptable. Why the reviewer looks for this: the date gets compared against your receipt; the letter must exist on or before the purchase, so this line decides whether the claim's timeline holds.
3. Patient information
Your full legal name and date of birth. Why the reviewer looks for this: these two details let the reviewer match the letter to the account holder and the claim. A name that does not match the account stalls processing.
4. The specific health condition
The named condition the purchase helps in preventing or managing, with an ICD-10 code when the provider includes one. Why the reviewer looks for this: this is the field that turns a dual-purpose purchase into a medical expense. A generic wellness phrase in this field is the classic rejection; a specific named condition is what the letter exists to record.
5. The recommended item or service
The exact product or service, with frequency and duration of use. Why the reviewer looks for this: the reviewer matches this line against the itemized receipt. "HEPA air purifier for daily home use" connects a purchase to a condition; "equipment" connects nothing to nothing.
6. The clinical rationale
The letter's center of gravity and usually its longest part: a few short paragraphs, not a sentence or two, moving in three steps. How the condition works and shows up for this patient; the evidence that this type of item helps prevent, manage, or reverse the condition, sources woven inline; and why this specific item fits. Why the reviewer looks for this: this is the judgment the signing provider is professionally vouching for. Specific beats long, and borrowed boilerplate reads exactly like what it is.
7. The exclusion statement
A plain statement that the item is solely for the documented medical purpose, not for cosmetic, aesthetic, or general-wellness use. Why the reviewer looks for this: Publication 502 excludes expenses "merely beneficial to general health," and this sentence draws that line inside the letter itself.
8. Duration of need
A validity statement; 12 months is the standard. Why the reviewer looks for this: administrators expect letters to be time-limited, and a stated window tells the reviewer precisely how long the letter covers the recommendation. A letter without one reads incomplete.
9. The closing medical-necessity statement
A dense closing paragraph anchoring the purchase to the IRS definition: the letter cites Publication 502 and restates that the item serves the diagnosis, mitigation, or prevention of the named condition, not general health. Why the reviewer looks for this: it ties the letter back to the standard the claim is judged against, in one paragraph the reviewer can quote.
10. Signature, printed name, and credentials
The provider's signature and signing date, with printed name and credentials beneath. Why the reviewer looks for this: an unsigned template is stationery. The signature and date are what convert the page into clinical documentation.
A Filled-In Example
Here is the template completed for a common pairing: a HEPA air purifier documented for asthma. This is a fictional example for illustration; every name and detail is invented. The structure is what to copy: all ten field groups appear, and the rationale runs a real letter's three steps, compressed.
Field 1: Provider letterhead and contact details
Lakeside Family Medicine | Portland, Oregon | (503) 555-0142
Avery Chen, MD | Oregon medical license MD 84512 | NPI 1234567890
Field 2: Date of issue and recipient line
Date: March 3, 2026
To: HSA/FSA Plan Administrator
Field 3: Patient information
Re: Jordan Sample, DOB 05/14/1991
Field 4: The specific health condition
Jordan Sample is under my care for mild intermittent asthma (ICD-10 J45.20). Asthma is a chronic inflammatory airway condition in which triggers such as dust mite allergens, pet dander, and mold spores produce airway narrowing, coughing, and shortness of breath. This patient's symptoms flare at home, where trigger contact is highest.
Fields 5 and 6: The recommended item and the clinical rationale
HEPA filtration is a well-established way to reduce indoor concentrations of exactly these particles, and lowering day-to-day trigger load is a recognized part of preventing and managing asthma symptoms. I recommend a HEPA air purifier for daily use in the patient's bedroom and main living space, where these allergens concentrate. The recommended duration of use is 12 months.
Field 7: The exclusion statement
This device is recommended solely for the management of this patient's asthma and is not intended for cosmetic, aesthetic, or general-wellness use.
Fields 8 and 9: Duration of need and the closing medical-necessity statement
Under IRS Publication 502, medical expenses include amounts paid for the diagnosis, mitigation, and prevention of disease. This device is a necessary component of preventing and managing this patient's asthma, not a general-health purchase. This letter is valid for 12 months from the date above.
Field 10: Signature, printed name, and credentials
Signature: Avery Chen, MD Date signed: March 3, 2026
That letter clears review because it can be verified (license and NPI), matched (patient, item, receipt), and defended (named condition, stepwise rationale, exclusion, duration). If an air purifier is the purchase you are documenting, our air purifier eligibility guide covers the qualifying conditions and device specifics.
What Makes a Letter of Medical Necessity Legitimate
A legitimate letter is one a licensed provider reviewed and signed. Physicians (MD or DO), nurse practitioners, and physician assistants are the signatures administrators see most, and other licensed clinicians, such as physical therapists or psychologists, can issue letters within their scope of practice. The license matters because the letter documents a clinical judgment, and only someone licensed to make that judgment can put their name on it.
The online path exists because telehealth law caught up with the technology. States modernized their telehealth rules, especially from 2020 onward, so licensed clinicians can establish a patient relationship and conduct an evaluation remotely, including asynchronous (store-and-forward) review of submitted health information where state law permits it. None of that loosened who reviews: licensure and scope-of-practice rules govern health care provided from a distance exactly as they govern an office visit.
The license requirement also answers whether you can write your own letter: no. You can assemble everything the provider needs, your condition, the exact item, and how you use it, but the letter becomes valid documentation only when a licensed provider reviews the facts and signs. The review is the substance; the signature records it. The same logic covers clinicians shopping for themselves: even a provider's own purchase is better documented through an independent review than a self-signed note.
Before signing, a real reviewer checks three things: that a specific, recognized health condition exists; that the item is something the medical community recognizes as appropriate for preventing or managing that condition; and that the requested use, frequency, and duration make sense for this patient. When the condition-item rationale holds, the provider issues the letter; when it does not, the provider declines, and legitimate reviewers do decline. A signature on an indefensible rationale puts the provider's license behind a claim they cannot support.
The line the review protects is the IRS's own. Publication 502 excludes expenses "merely beneficial to general health," and the IRS's nutrition and wellness FAQs draw the same boundary for dual-purpose purchases. This is why "I want better sleep" does not support a letter while the same benefit tied to a named condition does, and it is why administrators run a second review of their own: they verify the provider's credentials, match the letter to the receipt, and can question a rationale that reads generic.
So how hard is a letter to get? If you have a real condition and the item plausibly helps you prevent or manage it, it is straightforward: the review confirms what is already true, and with complete documentation, a decline is the exception. If there is no condition, a legitimate review will say no; that is the outcome the review exists to produce.
How to Get a Letter of Medical Necessity
There are exactly two paths, and both end in the same document: your own doctor, or an online licensed-provider review through Crates. The doctor route makes sense when a provider already manages your condition; the online route typically finishes in 24 to 48 hours.
Option 1: Through Your Doctor
If you already see a provider for your condition, they can write the letter:
- Book a visit and say you want to buy the item or service with HSA/FSA funds
- Ask them to document how it helps prevent, manage, or reverse your condition
- Make sure the letter is signed and dated
- Timeline: same day to a couple of weeks, and you may owe a copay
The ask itself is one sentence. Name the condition, name the purchase, and request the document: "I am managing my asthma and buying a HEPA air purifier to help with it. Could you issue a letter of medical necessity for my HSA claim, with the condition, the item, how it helps, and your signature and date?" Most providers know the format, and the template above covers anything they want to double-check.
Option 2: Through Crates
Crates runs the same clinical review online, and it is legitimate for the same reason any letter is: a licensed provider does the reviewing, wherever the review happens.
- Complete a short health assessment, about 2 to 3 minutes
- A licensed provider reviews it, typically within 24 to 48 hours
- If you qualify, your Letter of Medical Necessity is issued for the item
- Buy it with your personal card at any retailer
- Submit with one-click reimbursement to process your HSA/FSA claim
- Your LMN stays valid for 12 months and renews each year
You do not need a pharmacy prescription; the letter itself is what establishes eligibility.
What a Letter of Medical Necessity Costs
Through your doctor, the letter is usually part of the visit, and the visit itself may carry a copay depending on your plan. The "free letter of medical necessity" results you see in search are mostly administrator forms, and a form is free paper: it becomes a letter only after a licensed provider completes and signs it, and that clinical review is the part you are actually after. Through Crates, the assessment and licensed-provider review are part of your Crates plan, with no separate per-letter charge.
Validity, Renewal & Timing Rules
A Letter of Medical Necessity is valid for 12 months from its date, renews annually, and must be dated on or before the purchase it supports. Those three clocks settle nearly every timing question administrators raise.
The 12-month window is the standard administrators apply, and renewal is a re-review rather than a rubber stamp: the provider confirms the condition and the item still pair. If your letter came through Crates, it renews automatically each year while your plan is active.
Your LMN must be dated before your purchase. A letter cannot reach backward; the reviewer needs the medical purpose established first and the spending second. If the timing already slipped, put a letter in place before the next purchase rather than trying to paper over the last one.
Keep the letter and your itemized receipts with your tax records. Publication 969 requires records "sufficient to show" that account distributions "were exclusively to pay or reimburse qualified medical expenses," and for a dual-purpose purchase, the letter plus the receipt is that showing. On how long to keep them: under the IRS's recordkeeping guidance (Topic No. 305), records supporting a return are kept until the assessment window closes, generally 3 years from filing, 6 years where income was substantially understated, and with no limit for a fraudulent or unfiled return. HSA owners often keep them longer, because an HSA distribution can happen years after the expense, and the records must support the return for the year of the distribution.
Common Mistakes & Why Letters Get Rejected
Letters get rejected for letter defects, not because the underlying item suddenly stopped qualifying. The list is short, and every item on it is preventable:
- Missing or unverifiable provider details. No license number, no NPI, or no signature means the reviewer cannot confirm a licensed provider stands behind the letter, and the claim fails at step one.
- A generic wellness rationale. "Improves general health" or "supports better sleep" names no condition, and a letter without a named, recognized condition gives the reviewer nothing to approve.
- A letter dated after the purchase. The document must exist on or before the purchase date it supports.
- An expired letter. Twelve months is the clock. A purchase claimed against a thirteen-month-old letter needs a renewal first.
- The wrong account type. A Limited-Purpose FSA covers dental and vision only, and a Dependent-Care FSA covers care for dependents; neither reimburses dual-purpose wellness items no matter how strong the letter.
- A receipt that does not match. The administrator matches the itemized receipt against the item named in the letter. A missing receipt, or one that buries the item in a bundled total, stalls the claim.
A compliant letter is documentation, not a guarantee: administrators make the final call, and standards vary at the margins from one to another. With proper documentation, denials are rare, and the letter is what helps you get reimbursed. When a denial happens, it is usually a correctable defect from the list above, so fix the flagged item and resubmit with the signed letter and the itemized receipt.
What Commonly Needs a Letter of Medical Necessity
Any dual-use purchase, meaning an item that serves everyday life and a medical purpose at once, needs a letter before an HSA or FSA reimburses it. That covers most of the wellness aisle, and we keep a dedicated eligibility guide for each category people actually buy.
Wearables are the classic case, since the same device tracks a workout and a health condition: see the guides for the Apple Watch, Oura Ring, WHOOP, Garmin watches, and Fitbit, or start with the fitness tracker guide for the category-level view.
Fitness spending follows the same pattern, with dedicated guides for gym memberships, workout equipment, and standing desks.
For recovery and home-environment purchases, see saunas, red light therapy, air purifiers, and humidifiers; for sleep, mattresses; and for daily supplementation, the vitamins and supplements guide.
Each guide carries its own verdict, including the conditions that commonly support a letter and any items that need no letter or do not qualify, so check the page for the thing you are actually buying.
Is a Letter of Medical Necessity for Insurance the Same Thing?
No. An insurance letter of medical necessity is a different document with a different reader: your provider writes to your insurer, usually inside a prior-authorization or appeal process, to justify coverage of care or a medication under your policy. Everything on this page is the HSA/FSA version, written for your plan administrator to document a purchase you make yourself. If an insurer asked you for documentation, that request runs through your insurer and your provider's office, not this template.
