What a Letter of Medical Necessity Actually Requires

A doctor signing a letter of medical necessity at a clinic desk

An LMN is what unlocks HSA/FSA spending on a "dual-purpose" item — something that could be either general wellness or actual medical treatment, depending on the person. Here's what has to be true for one to be valid, and why that means no page, including this one, can promise you in advance that yours will be approved.

On this page

What it has to establish Who can issue one Why "tell me what to bring my doctor" doesn't work What's actually useful to know beforehand

Key takeaways

A valid LMN documents a real diagnosed condition, how the item treats it, and usually a duration — from an actual clinical evaluation.
No script or pre-written justification can substitute for that evaluation; it inverts the order the IRS requires.
An LMN supports a reimbursement claim — your plan administrator still makes the final call.

What it has to establish

A valid LMN isn't a form your doctor rubber-stamps. It has to document three specific things: a diagnosed medical condition you actually have, how the specific item or service treats, mitigates, or prevents that condition, and — usually — how long the treatment is expected to be needed. All three have to come from a licensed provider's independent clinical judgment, not from a script the patient brings in.

Who can issue one

Any licensed healthcare provider evaluating you can write one — your own doctor, or a provider through a telehealth intake service that specializes in this (several exist specifically for HSA/FSA purposes). What makes it valid isn't who writes it, it's whether the clinical relationship and evaluation are real.

Why "tell me what my doctor needs to write" doesn't work

The IRS's standard here is that the letter reflects an actual clinical determination made about a specific person. A wizard or checklist that hands you pre-written justification language to bring to your provider inverts that order — it's producing the conclusion before the evaluation happens. If the item turns out not to be a legitimate treatment for that person's actual condition, the LMN doesn't hold up, and neither does the tax-advantaged claim built on it. This site won't generate that kind of script for the same reason a pharmacy won't pre-fill your symptoms on a prescription pad.

What's actually useful to know beforehand

Whether your general category of condition is one providers commonly document for this type of item (see the category-specific guides, like gym memberships)
That the letter is typically valid for about a year before it needs renewing
That your HSA/FSA administrator makes the final call on reimbursement — an LMN supports a claim, it doesn't guarantee it

Source: IRS Publication 502, "Letter of Medical Necessity" guidance.