How to Get a Letter of Medical Necessity

Published July 15, 2026

To get a letter of medical necessity, you need a licensed clinician to review your health history and diagnosed condition. If an LMN is clinically appropriate for your situation, they issue one. The process is clinical, not automatic, and that's exactly what makes it valid. Here's what to know before you start.

What a letter of medical necessity is

A letter of medical necessity (LMN) is a formal document from a licensed clinician establishing that a specific product or service is medically necessary to treat, mitigate, or prevent a diagnosed condition. Under IRS Section 213(d), that documentation is what qualifies an expense for HSA or FSA reimbursement, shifting it from a general purchase to a qualified medical expense.

It's not a loophole. It's the documentation standard the IRS has recognized for decades. What it requires is a real clinical relationship and a real diagnosis.

Who can write one

An LMN has to come from a licensed healthcare provider: a physician, nurse practitioner, or other licensed clinician who can evaluate your health information and make a clinical judgment. A general wellness note or a pre-filled form won't hold up to scrutiny from a plan administrator.

What matters is specificity. A strong LMN connects your diagnosed condition to the exact product or service being recommended, with the clinical reasoning documented behind it.

What it can cover

When the clinical basis is there, an LMN can make a wide range of expenses HSA or FSA eligible. Common examples include gym memberships and fitness equipment for conditions like hypertension or metabolic disorders, red light therapy or sauna for chronic pain and inflammation, sleep technology for diagnosed sleep disorders, and dietary supplements for documented deficiencies or metabolic conditions.

A general health goal is not enough. What qualifies an expense is a diagnosed condition with documented clinical need.

How LMNs are issued through Root Cause

Root Cause is a functional medicine practice. Our clinicians review the full picture of your health, including your diagnosed conditions, risk factors, and relevant history, not just whether a product fits a checkbox.

The process works like this: you complete a clinical intake covering your health history, current conditions, and medications. A licensed Root Cause clinician reviews your information and evaluates whether an LMN is clinically appropriate for your individual situation. If it is, your letter is issued with ICD-10 coding, documented clinical rationale, and the duration of the recommendation's validity period.

Not everyone who submits an intake will qualify. That's intentional. An LMN carries weight because a clinician made a clinical recommendation, not because the system approved a form.

A few things worth knowing

LMNs are specific to the product and purpose reviewed. If you want to use HSA or FSA funds across different product categories, each needs its own clinical review.

Your plan administrator has final say on reimbursement. The LMN supports the claim, it doesn't override the review. Root Cause clinicians follow the same documentation standards TPAs and benefits administrators expect to see.