Introduction

If you’ve been told you need a “letter of medical necessity” to use your FSA or HSA funds on a health or wellness purchase, you’re not alone. Thousands of people every month search for what an LMN is, how it works, and where to get one. This guide breaks down everything you need to know.

What Is a Letter of Medical Necessity?

A Letter of Medical Necessity (LMN) is a document written or signed by a licensed healthcare provider that explains why a specific product, service, or treatment is medically necessary for a patient. It serves as the bridge between your health spending account (FSA, HSA, or HRA) and purchases that aren’t automatically considered eligible medical expenses by the IRS.

For example, a gym membership isn’t typically classified as an eligible medical expense. But if your doctor determines that regular exercise is medically necessary to treat a diagnosed condition—like obesity, cardiovascular disease, or clinical depression—an LMN documents that clinical rationale and unlocks your pre-tax health dollars for the purchase.

Why Do You Need an LMN?

The IRS defines eligible medical expenses under Section 213(d) of the tax code. Many health and wellness products fall into a gray area: they’re beneficial for health but aren’t automatically considered “medical” expenses. An LMN resolves this by establishing a direct medical purpose for the purchase.

Common scenarios where an LMN is required include gym and fitness memberships, home exercise equipment like Peloton or treadmills, wearable health devices like the Oura Ring or Whoop, saunas and cold plunge tubs, massage therapy and acupuncture sessions, standing desks and ergonomic equipment, supplements and vitamins, and air purifiers or mattresses.

What Does an LMN Include?

A valid Letter of Medical Necessity typically contains the patient’s name and date of birth, the provider’s name, credentials, and contact information, a specific medical diagnosis (ICD-10 code is ideal), a clear explanation of why the product or service is medically necessary to treat the diagnosis, the specific item or service being recommended, the provider’s signature and date, and the provider’s NPI number.

LMN vs. Doctor’s Note vs. Prescription

An LMN is more formal and specific than a generic doctor’s note. A doctor’s note might say “patient should exercise more.” An LMN says “patient has been diagnosed with Type 2 Diabetes (E11) and requires a structured exercise program at a fitness facility as part of their treatment plan.” Most FSA and HSA administrators require this level of specificity to approve reimbursement.

How to Get a Letter of Medical Necessity

Historically, getting an LMN meant scheduling an appointment with your primary care doctor, explaining what you needed, and hoping they were familiar with the process. Today, online telehealth platforms like Burst have streamlined this entirely. You can complete a health assessment online, get matched with a licensed provider, and receive your LMN digitally—often within 24 to 48 hours.

How Long Is an LMN Valid?

Most LMNs are valid for 12 months from the date of issuance, though this can vary by FSA/HSA administrator. Some administrators require annual renewal. Burst provides ongoing support to ensure your LMN stays current.

Frequently Asked Questions

Can I write my own LMN? No. An LMN must be written or signed by a licensed healthcare provider (MD, DO, NP, or PA).

Does my insurance company provide LMNs? No. Your insurance company administers claims, but the LMN must come from a treating healthcare provider.

What if my LMN gets rejected? Rejections typically happen because the letter lacks specificity. Working with a provider experienced in LMNs—like those on the Burst platform—significantly reduces rejection rates.