What Is a Letter of Medical Necessity — and How It Makes Your Gym FSA/HSA Eligible

Adam Z. Kawalek, MD

June 5, 2026 · 5 min read

“Letter of Medical Necessity” sounds like bureaucratic jargon, and in a sense it is — but it’s also a genuinely useful document. It’s what turns an expense your plan would normally reject into one it will reimburse with pre-tax dollars. Here’s what it is, in plain English.

The definition

A Letter of Medical Necessity (LMN) is a signed statement from a licensed physician explaining that a particular product or service is needed to treat, mitigate, or prevent a diagnosed medical condition. HSA and FSA administrators use it as proof that an otherwise-personal expense qualifies as “medical care” under the tax code.

The rule it satisfies

Everything an LMN does traces back to Section 213(d) of the Internal Revenue Code and IRS Publication 502. Together they define medical care as money spent on the “diagnosis, cure, mitigation, treatment, or prevention of disease.” The catch is that many things that improve health — a gym, supplements, a massage — only meet that definition when a physician ties them to a specific condition. The LMN is how that tie is made and documented.

What a valid LMN has to contain

Administrators look for a few specific elements. A properly written LMN includes:

• The patient’s diagnosed condition (often with its ICD-10 code)
• A statement that the expense treats, mitigates, or prevents that condition
• The recommended duration of treatment (commonly 12 months)
• The prescribing physician’s name, signature, NPI number, and state license

A vague note that just says “exercise is good for this patient” won’t hold up. The letter has to connect a named condition to the specific expense — which is exactly why it takes a physician who has reviewed your case to write one.

Who can write one

An LMN must come from a licensed medical provider — typically a physician (MD or DO). It cannot be self-certified, and a personal trainer or gym can’t issue it. The physician is attesting, under their license, that the medical necessity is real, which is what gives the letter its weight with your plan administrator and the IRS.

How long it lasts

Most LMNs are valid for 12 months from the date of issue, after which they’re renewed if the condition and treatment still apply. That annual cadence lines up neatly with how HSA and FSA plan years work, so one letter typically covers a full year of membership.

Why it’s worth the small amount of effort

An LMN is a one-time, low-effort document that can convert a recurring after-tax expense into a pre-tax one — year after year. For something you’re already paying for, that’s among the highest-return pieces of paperwork in personal finance.

If a gym membership is your case, MedSlip connects you with a board-certified physician who reviews your health and, when appropriate, issues a signed Letter of Medical Necessity you can submit straight to your HSA or FSA — no office visit required.

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.