Letter of Medical Necessity: The Complete 2026 Guide (What It Is, What Plans Require, How to Get One)

Adam Z. Kawalek, MD

June 5, 2026 · 12 min read

A gym membership, a stack of supplements, an ergonomic chair — on their own, none of these are things your HSA or FSA will pay for. But with the right documentation, some of them can be. That document is a Letter of Medical Necessity. I have written many of them, and I have turned many people down. Here is a plain explanation of what an LMN is, who actually qualifies, what a valid one must contain, and how reimbursement really works. No hype — just the rules as the IRS actually writes them.

What Is a Letter of Medical Necessity?

A Letter of Medical Necessity (LMN) is a signed statement from a licensed healthcare provider explaining that a specific product or service is needed to treat, manage, or prevent a diagnosed medical condition. It is not a prescription, and it is not a note for missing work. It is a bridge document: it connects a purchase your plan would normally call personal to a specific medical reason that makes it eligible.

The key word is specific. A good letter does not say "exercise is healthy." It says: this patient has this diagnosed condition, and I am recommending this treatment to help manage it.

Why Some Expenses Need a Letter: the IRS Rule

HSAs and FSAs follow the IRS definition of "medical care." Under Internal Revenue Code §213(d), medical care means amounts paid "for the diagnosis, cure, mitigation, treatment, or prevention of disease, or for the purpose of affecting any structure or function of the body."

The IRS draws a hard line between treating a condition and simply being good for you. IRS Publication 502 states it directly: you "can't include in medical expenses health club dues or amounts paid to improve one's general health or to relieve physical or mental discomfort not related to a particular medical condition." A gym membership to stay in shape is, in the IRS's eyes, a personal expense.

But the same publication carves out the exception that makes an LMN worth having. Publication 502 lets you include the cost of a weight-loss program when it is "a treatment for a specific disease diagnosed by a physician (such as obesity, hypertension, or heart disease)." That is the principle in one sentence: when an expense is tied to a specific diagnosis and treatment, it can cross from personal to medical. The letter is what documents that connection.

Who Qualifies for a Letter of Medical Necessity?

You need a genuine medical condition that the expense actually helps treat or manage. In my practice, the conditions that most often support a letter for a structured exercise program include:

• Obesity, or overweight with related health risks
• High blood pressure (hypertension)
• Type 2 diabetes or prediabetes
• High cholesterol
• Heart disease or cardiac rehabilitation
• Chronic back, joint, or muscle pain
• Anxiety or depression
• Osteoporosis or low bone density

Two honest caveats. First, this list is not exhaustive, and having a condition on it does not automatically qualify you — a provider still has to review your situation and judge that the treatment is appropriate for you specifically. Second, and I will be blunt: if you are healthy and just want a tax break, you do not qualify. A provider who signs a letter with no real medical basis is risking their license, and you are exposing yourself in an audit. I do not write those, and you should be skeptical of any service that claims everyone qualifies.

What a Valid Letter of Medical Necessity Must Include

Administrators reject letters that are vague. A solid LMN should contain:

• Your full name and date of birth
• The specific diagnosed condition being treated (often with its ICD-10 code)
• The recommended item or service, named specifically (for example, a structured exercise program through a gym membership)
• How that item treats or manages the condition
• The recommended duration (commonly 12 months)
• The provider's name, credentials, license information, signature, and the date

Short, specific, and signed holds up. Vague and generic does not.

What an LMN Can Cover: Expense by Expense

The same principle applies to every dual-purpose expense, but the details differ. We keep a dedicated guide for each of the common ones:

• Gym memberships — eligible with an LMN tied to a qualifying diagnosis; the most common use
• Home exercise equipment — treadmills, bikes, and home gyms; the letter should come before the purchase
• Treadmills and walking pads specifically
• Massage therapy — for diagnosed musculoskeletal conditions, not relaxation
• Personal training — supervised exercise as treatment
• The full what-qualifies catalog for everything else

The Three Rules Most People Learn Too Late

1. Get the letter before you pay for the expense. Plans reimburse from the letter's date forward. Months of gym fees you paid before the letter existed are not reimbursable, and several administrators, Fidelity among them, apply a stricter test when a membership predates the recommendation — the logic being that you were paying for it anyway. Letter first, purchase second.

2. Your HSA/FSA card will usually decline at the gym. Gyms are coded as non-healthcare merchants, so benefits cards are typically blocked at the terminal no matter what documentation you hold. That is normal, not a rejection of your letter. Pay out of pocket, keep the itemized receipt, and submit the claim with your letter.

3. The letter expires — plan around 12 months. Most administrators treat an LMN as valid for about a year, and HealthEquity caps it at 12 months outright. If you keep claiming the expense, renew the letter annually. FSA holders have a second clock: most FSA dollars expire at plan year-end, which is why claims pile up every November and December.

How to Get a Letter of Medical Necessity

There are two routes. The first is your own doctor. If you have a primary care physician who knows your history, ask at your next visit — especially if they have already advised you to exercise or lose weight. The cost is often just your normal visit copay. The downside is timing: if your next appointment is months away, or you would book a visit solely for paperwork, it is slow.

The second is an online physician service. Services like MedSlip let you complete a health questionnaire, have a physician review it, and receive a signed letter — usually within hours — when your situation supports one, for $69. The honest trade-off: it is fast and inexpensive, but a legitimate service still involves real physician review, not an automatic approval. If a site guarantees you a letter before anyone has reviewed your information, walk away.

How Reimbursement Actually Works

Once you have your letter, the process is straightforward: pay for the item yourself and keep the receipt; submit a claim to your HSA or FSA administrator, usually online, attaching the receipt and the letter; wait for their review, typically a few days to a couple of weeks; then get reimbursed from your pre-tax funds, or have the expense approved if you paid with an HSA or FSA card.

Keep the letter and receipts on file. One practical difference between the accounts, per IRS Publication 969: HSA funds carry over year to year and are yours to keep, while FSAs are generally use-it-or-lose-it, with only a limited carryover or short grace period if your employer offers one. Plan to renew your letter about every 12 months if you keep claiming the expense.

The review itself also works differently between the two accounts, which is worth understanding — our HSA vs FSA guide covers it in depth. An FSA administrator substantiates every claim before paying it, so a weak letter fails fast and visibly. An HSA usually has no upfront review at all: you reimburse yourself, and the documentation only gets tested if the IRS ever looks. An approved HSA withdrawal is not a ruling that the expense qualified — your letter and receipts are what stand between you and a problem in that review. That is exactly why the letter needs to be genuinely solid, not just to exist.

What the Major Administrators Actually Say

Reimbursement rules are set by your plan administrator, and their public documentation differs more than most people expect. As of this writing:

• HealthEquity — documents accepting LMNs, valid up to 12 months
• FSAFEDS (the federal employee FSA) — accepts gym claims but requires an approved LMN on file before you claim, plus an individual membership contract
• Fidelity — allows it with an LMN, with the predate caution above
• WEX — eligible "only if prescribed for a medical reason", documented by an LMN
• Lively — lists fitness programs as eligible with an LMN
• Optum — its standard eligible-expense list treats gym memberships as not qualified and steers wellness spending toward employer Lifestyle Spending Accounts; expect the most friction here

Two cautions on top of that list. Administrator policies change, so treat their current help pages as the authority. And an employer can design its FSA plan to exclude gym reimbursement entirely — a letter cannot override the plan document. When in doubt, ask your administrator before you buy.

What a Letter of Medical Necessity Cannot Do

This is the part some companies will not tell you. A letter supports a claim; it does not guarantee one. Your administrator makes the final decision, and policies vary.

More importantly, a letter cannot turn a genuinely personal expense into a medical one. In 2024 the IRS specifically cautioned that a note from a doctor based on self-reported health information "cannot convert" nutrition, wellness, or general-health purchases into reimbursable medical expenses — the expense has to be tied to a real, targeted diagnosis and treatment. That is not a loophole to game; it is the line you have to stay on the right side of. A letter grounded in a genuine diagnosis does its job. One that is not will not protect you if a claim is reviewed.

How to Tell a Real Letter From a Letter Mill

The IRS's 2024 warning was aimed at a specific business model: companies selling doctor's notes, for a fee, that rubber-stamp whatever the customer typed in. Those letters are the ones administrators reject and audits dismantle. Four markers separate a letter that holds up from one that does not: a named, licensed physician on the letter itself, with NPI and state license number, not an anonymous "medical team"; a real review that can say no — if everyone qualifies, nobody does, and a legitimate service declines unqualified applicants and refunds them; a specific diagnosis with its ICD-10 code tied to the specific expense; and a way for your administrator to verify the letter is genuine. That standard is what we built MedSlip to meet — every letter is reviewed and signed by a named physician, carries the NPI, license, and diagnosis code, and can be verified by your plan at medslip.co/verify.

This article is for general information and is not medical, tax, or legal advice. IRS rules and plan policies change and vary by administrator. Confirm current rules with your HSA or FSA administrator or a tax professional.

Adam Z. Kawalek, MD

Board-Certified Physician · Founder, MedSlip · Cedars-Sinai · Johns Hopkins

Dr. Kawalek is a board-certified internal medicine physician with 15+ years of clinical experience. He founded MedSlip to give patients fast, affordable access to the Letters of Medical Necessity that make fitness and wellness spending HSA/FSA-eligible.