Letter of Medical Necessity: What It Is and How to Get One
A letter of medical necessity (LMN) is a signed statement from a licensed provider saying a specific product or service treats a diagnosed condition. It is the document that turns an item your HSA or FSA would normally reject into a qualified medical expense. This page covers what has to be in one, who can sign it, how long it lasts, and which products people most often claim.
Short answer: get a diagnosis on record, ask the provider treating you to write a letter naming the specific item and tying it to that diagnosis, confirm with your plan administrator what they require, buy the item and keep the itemised receipt. The letter must exist before or around the purchase, not be produced afterwards to justify one.
What a letter of medical necessity actually does
An HSA or FSA can only pay for qualified medical expenses. The IRS defines those in Publication 502 as costs of diagnosis, cure, mitigation, treatment or prevention of disease. Most consumer products fail that test on their own: a mattress, a treadmill, a sauna and an air purifier are all ordinary household purchases.
The letter changes the classification. It puts a licensed provider on record saying this specific item is part of treating this specific diagnosis, which moves the purchase from “personal expense” to “qualified medical expense” for the purposes of your plan.
Two things follow from that, and both catch people out:
- The item is not what qualifies. The link between the item and the diagnosis is. There is no list of HSA-eligible treadmills. There is a treadmill bought by someone with a documented cardiac rehabilitation plan.
- Your administrator, not the IRS, is who you have to satisfy first. The IRS sets the outer boundary. Your plan administrator applies it, and they can be stricter.
What the letter must contain
| Element | What it needs to say | Why it gets rejected without it |
|---|---|---|
| Patient identity | Your full name and date of birth | The administrator cannot tie the letter to your account |
| Diagnosis | The specific condition, ideally with the ICD-10 code | ”General wellness” is not a diagnosis and never qualifies |
| The item, named | The exact product or category being recommended | ”Exercise equipment” is too vague for most administrators |
| The clinical link | How this item treats, mitigates or prevents that condition | This is the sentence the whole letter exists for |
| Duration | How long the treatment is expected to continue | Determines how long the letter stays valid |
| Provider details | Name, credentials, licence number, signature, date | An unsigned letter is not evidence of anything |
Who can sign one
Any licensed provider treating you for the condition. Which provider makes sense depends on the diagnosis: a physician or nurse practitioner for most conditions, a chiropractor or physical therapist for musculoskeletal ones, a dermatologist for skin conditions, an optometrist for vision.
If you do not have a provider who knows the condition, there are online services where a licensed provider reviews a health questionnaire and issues the letter — Truemed is the one most retailers have integrated. That route is legitimate, but the same rules apply: a real diagnosis, a real clinical link, or the letter is worthless if the expense is ever reviewed.
The process, step by step
- Establish the diagnosis. The letter is downstream of a diagnosis on record. Without one there is nothing to write.
- Ask the provider treating you. Bring the specific item. Providers write these regularly and usually have a template.
- Check your plan rules first. Call the administrator and ask three things: do they require an LMN for this category, do they need a claim form alongside it, and do they reimburse the full price or only the difference over a standard equivalent.
- Buy the item and keep the itemised receipt. Product, price, date. A card statement is not enough.
- Pay directly or submit for reimbursement. Either use the HSA/FSA card at purchase, or submit the letter, receipt and claim form afterwards.
- Keep everything with your tax records. If the expense is ever questioned, the letter and receipt are what settle it.
What people actually claim
These are the categories where an LMN is most often the deciding factor, and where we have written the specifics out:
| Item | Typical qualifying conditions | Our guide |
|---|---|---|
| Mattress | Chronic lower back pain, arthritis, sciatica, spinal injury | LMN for a mattress |
| Air purifier | Asthma, chronic allergies, COPD, recurring respiratory infection | LMN for an air purifier |
| Humidifier | Chronic sinusitis, asthma, eczema, recurrent nosebleeds | LMN for a humidifier |
| Treadmill | Cardiac rehabilitation, obesity, hypertension, type 2 diabetes | LMN for a treadmill |
| Sauna | Chronic pain conditions, certain circulatory and rheumatic diagnoses | LMN for a sauna |
| Gym membership | Obesity, hypertension, cardiac rehab, physician-directed exercise | LMN for a gym membership |
The three mistakes that get claims denied
Writing the letter after the purchase. The letter is meant to document a treatment decision. One dated three months after the receipt reads as a justification, and administrators notice.
Vague wording. “The patient would benefit from better sleep” is not a clinical link. “Medium-firm orthopaedic support is indicated for diagnosed lumbar disc degeneration” is.
Assuming full reimbursement. Plenty of administrators cover only the difference between a standard version of the item and the medically necessary one. Ask before you spend.
Templates
We publish a copy-paste template your provider can adapt, plus versions tuned per product category, on the letter of medical necessity template page. For the plan-side rules — what HSAs allow that FSAs do not, deadlines, and the substantiation requirements — see using an LMN with an HSA or FSA.
Sources
- Internal Revenue Service, Publication 502: Medical and Dental Expenses — the IRS definition of a qualified medical expense, which is the standard every plan administrator applies.
- Internal Revenue Service, Publication 969: Health Savings Accounts and Other Tax-Favored Health Plans — the rules on HSA distributions, substantiation, and the additional tax on non-qualified distributions.
Frequently asked questions
What is a letter of medical necessity?
A letter of medical necessity is a signed statement from a licensed healthcare provider saying that a specific product or service is needed to treat, mitigate or prevent a diagnosed medical condition. It converts an item that would normally be a personal expense into a qualified medical expense your HSA or FSA can pay for.
Who can write a letter of medical necessity?
Any licensed provider who is treating you and can speak to the diagnosis. In practice that means a physician, nurse practitioner, physician assistant, chiropractor, physical therapist, dentist or optometrist, depending on the condition. The provider must be licensed, and most administrators want their credentials and signature on the letter.
How long is a letter of medical necessity valid?
Most administrators treat a letter as valid for twelve months from the date it is signed. Some accept a stated duration written into the letter itself. If your treatment continues past that window you need a new letter, and the letter should exist before or around the purchase rather than be written afterwards.
Does a letter of medical necessity guarantee reimbursement?
No. The letter establishes medical necessity, but your plan administrator applies their own rules on top of it. Some reimburse the full price, some reimburse only the difference between a standard item and the medically necessary one, and some exclude certain categories outright. Confirm with your administrator before you buy.
What happens if the IRS audits the expense?
You need to produce the letter and an itemised receipt showing the product, price and date. Keep both with your tax records. An HSA distribution that cannot be substantiated as a qualified medical expense becomes taxable income, and may carry an additional 20 percent penalty if you are under 65.