Letter of Medical Necessity: What It Is, Who Qualifies, and How to Get One (2026)

Adam Z. Kawalek, MD

June 5, 2026 · 6 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.

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.

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.

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.