A Letter of Medical Necessity (LMN) is a formal document that a licensed healthcare provider writes to explain why a specific treatment, product, or service is essential for a patient's health. 

A health plan may request an LMN as part of a certification or utilization review process to determine if a requested medical expense is covered. Health insurance providers, health savings accounts (HSAs), or flexible spending accounts (FSAs) may also request an LMN before reimbursing an expense.

The IRS defines a medical expense as, the costs of diagnosis, cure, mitigation, treatment, or prevention of disease, and for the purpose of affecting any part or function of the body.