April 3, 2026
·7 min read
·Hammock Team
What Is a Letter of Medical Necessity? Everything You Need to Know
Learn what a Letter of Medical Necessity (LMN) is, how it works, who can write one, and how it makes wellness expenses HSA and FSA-eligible.
If you've ever wondered how people pay for wellness expenses with pre-tax dollars, the LMN is the answer.
Why LMNs Exist
The IRS sets strict rules about what qualifies as a medical expense for HSA and FSA purposes. Under IRS Publication 502, eligible expenses must be primarily for "the diagnosis, cure, mitigation, treatment, or prevention of disease, or for the purpose of affecting any structure or function of the body."
Many wellness expenses clearly have medical benefits but aren't automatically classified as medical expenses. A gym membership helps manage diabetes. Massage therapy treats chronic pain. Supplements address nutritional deficiencies. The LMN is the bridge between "this is good for your health" and "this is a qualified medical expense."
Without an LMN, these expenses are personal. With one, they're tax-free.
Who Can Write a Letter of Medical Necessity?
An LMN must come from a licensed healthcare provider. Qualifying providers include:
- Medical doctors (MD) and doctors of osteopathy (DO)
- Nurse practitioners (NP)
- Physician assistants (PA)
- Licensed clinical psychologists
- Licensed clinical social workers
- Chiropractors (for conditions within their scope)
- Physical therapists (for conditions within their scope)
- Naturopathic doctors (in states where they're licensed)
- Psychiatrists
The provider must be licensed to practice in your state and the condition must fall within their scope of practice. Your primary care doctor is usually the most straightforward choice, but specialists can write LMNs for conditions they treat.
What's Included in an LMN?
A proper LMN should contain:
1. Patient Information
Your full name, date of birth, and any relevant identification (like a patient ID).
2. Provider Information
The provider's name, credentials, license number, practice name, and contact information.
3. Diagnosis
The specific medical condition being treated. Including the ICD-10 diagnostic code strengthens the letter, though it's not always required.
4. Recommended Treatment or Product
A clear description of what's being prescribed — "gym membership," "massage therapy twice monthly," "magnesium supplement 400mg daily," etc.
5. Medical Rationale
An explanation of why the treatment is medically necessary for the patient's condition. This is the core of the LMN — it connects the expense to the diagnosis.
6. Duration and Frequency
How often the treatment should be used and for how long. For ongoing expenses, most LMNs are written for 6–12 months before requiring renewal.
7. Provider Signature and Date
The provider's signature (or electronic signature) and the date the letter was written.
What Expenses Can an LMN Cover?
An LMN can make a wide range of wellness expenses HSA and FSA-eligible:
Fitness:- Gym memberships
- Personal training
- Yoga and Pilates classes
- CrossFit and group fitness
- Home exercise equipment
- Online fitness subscriptions
- Massage therapy
- Sauna and infrared sauna sessions
- Cold plunge and cryotherapy
- Float tanks
- Compression therapy devices
- Protein powder
- Vitamins and minerals (D, magnesium, B-complex, etc.)
- Omega-3 / fish oil
- Probiotics
- Adaptogens (ashwagandha, etc.)
- Collagen, creatine, turmeric
- Meditation apps
- Light therapy lamps
- Weighted blankets
- White noise machines
- Biofeedback devices
- Air purifiers
- Ergonomic equipment
- Standing desks
- Water filtration systems
One LMN can cover multiple items if they're all part of the same treatment plan. For example, a single letter could prescribe a gym membership, protein powder, and massage therapy for managing chronic pain.
How to Get an LMN: Three Paths
Path 1: Your Existing Healthcare Provider
Schedule an appointment (in-person or telehealth) with your doctor and discuss the expenses you'd like to cover. Explain that you're looking for an LMN to use your HSA/FSA funds. Some providers are very familiar with LMNs; others may need a brief explanation.
Pros: Uses your existing provider relationship; no extra cost beyond your copay Cons: Some providers aren't familiar with LMNs for wellness; may require an office visitPath 2: A Telehealth LMN Service
Several companies offer telehealth consultations specifically for LMN evaluations. You meet with a licensed provider who assesses your health conditions and writes LMNs for qualifying expenses.
Pros: Convenient and focused on the LMN process Cons: May involve separate fees per letterPath 3: Hammock
Hammock includes unlimited LMNs as part of its membership. Licensed providers evaluate your health needs and issue LMNs for all qualifying wellness expenses — gym, supplements, massage, and more.
Pros: Unlimited LMNs, integrated HSA debit card, no per-letter fees Cons: Annual membership cost (which typically pays for itself with tax savings)Common Misconceptions About LMNs
"LMNs are just a loophole"
No. LMNs are the IRS-recognized mechanism for documenting medical necessity. They exist because the line between "medical" and "wellness" isn't always clear, and providers are the right people to make that determination. Using an LMN is following the rules, not bending them.
"Any doctor will write one if you ask"
Not necessarily. A responsible provider will only write an LMN if there's a genuine medical basis. You need a diagnosed condition, and the recommended treatment must be clinically appropriate. Providers risk their license by writing unjustified LMNs.
"You only need one LMN and you're set forever"
LMNs typically need annual renewal. Your medical condition and treatment needs can change, and HSA administrators may require current documentation. Plan to renew your LMN each year.
"LMNs are only for HSAs"
LMNs work for FSAs, HRAs, and HSAs — any tax-advantaged health account. The IRS rules are the same across account types.
FAQ
How long does it take to get an LMN?
It depends on the path. Through your existing doctor, it might take a few days to a couple of weeks (depending on appointment availability). Through telehealth services or Hammock, you can often get an LMN within 24–48 hours.
Can an LMN be retroactive?
This is a gray area. Some providers will write an LMN that acknowledges a pre-existing treatment plan, but it's always better to get the LMN before incurring expenses. The IRS expects documentation to be in place at the time of the expense.
What if my HSA administrator rejects my LMN?
This is rare if the LMN is properly written. If it happens, ask the administrator what's missing and have your provider update the letter. Most rejections are due to incomplete documentation, not invalid medical claims.
Do I need a separate LMN for each expense?
No. A single LMN can cover multiple expenses if they're all part of the same treatment plan. For example, one letter can prescribe a gym membership, supplements, and massage therapy for managing a chronic condition.
How much does an LMN cost?
Through your doctor, it's typically covered by your copay. Standalone LMN services charge $50–$150 per letter. Hammock includes unlimited LMNs — often the most cost-effective option if you have multiple wellness expenses.
Get Unlimited LMNs with Hammock
The LMN is the most powerful — and underused — tool in the HSA world. It turns hundreds or thousands of dollars in wellness spending into tax-free medical expenses. But the process of getting one has traditionally been confusing and inconvenient.
Hammock fixes that. You get unlimited Letters of Medical Necessity from licensed providers, plus an HSA debit card that makes paying for wellness expenses seamless. Your gym, supplements, massage, sauna, yoga — all tax-free, all documented, all easy.
Ready to start using your HSA for wellness? Hammock includes unlimited Letters of Medical Necessity — so your gym, supplements, and massage are all tax-free.