Who Can Write a Letter of Medical Necessity

Published July 15, 2026

A letter of medical necessity has to be written by a licensed healthcare provider: a medical doctor, doctor of osteopathic medicine, or nurse practitioner who can evaluate your health information and make an independent clinical determination. No other type of note or documentation qualifies.

Why it matters who writes it

The IRS defines when an expense may qualify as a medical expense, but it does not determine which interventions are clinically appropriate for which conditions. That responsibility sits entirely with licensed medical professionals.

A wellness note from a health coach, a generic form letter, or anything that was not individually reviewed by a licensed clinician will not hold up to scrutiny from a plan administrator. What your HSA or FSA administrator needs to see is a document from a credentialed provider who actually evaluated your health information and made a clinical judgment specific to you.

Which providers can write one

Licensed providers who can issue a valid LMN include physicians (MD), doctors of osteopathic medicine (DO), and nurse practitioners (NP). The specific requirements can vary by state. Some states require an MD or DO signature rather than an NP, which is worth knowing if you are pursuing an LMN through a provider network.

What all of them have in common is that they can independently evaluate a patient's health history, diagnose conditions, and document clinical rationale. That is what makes the letter valid.

What makes the letter itself credible

The provider's credentials matter, but so does what they put in the letter. A strong LMN includes your diagnosed condition, the ICD-10 code tied to that condition, the specific product or service being recommended, and a clear clinical explanation of why that recommendation is appropriate for your situation.

Generic language does not cut it. The letter has to connect your condition to the specific intervention in a way that holds up to review. That level of specificity is what separates a valid LMN from a note that gets flagged or denied.

How Root Cause approaches it

Root Cause is a functional medicine practice. Our clinicians are licensed physicians and nurse practitioners who review each patient's health picture individually, including diagnosed conditions, medications, risk factors, and relevant history. There is no automation in that process and no predetermined outcome.

If an LMN is clinically appropriate based on what the clinician finds, they issue one. If it is not, they do not. That independence is what gives the documentation weight.