Letter of medical necessity

A letter of medical necessity (LMN) is written documentation from a clinician stating that an item or service is being used to diagnose or treat a specific medical condition, rather than for general health. "Medically necessary" has a standard meaning in US health coverage: services or supplies needed to diagnose or treat an illness, injury, condition, disease or its symptoms, that meet accepted standards of medicine.

Letters of medical necessity come up most often with health savings accounts (HSAs) and flexible spending accounts (FSAs). IRS rules allow these accounts to pay for medical care, defined as care primarily to alleviate or prevent a physical or mental illness, and exclude expenses that are merely beneficial to general health. Where a purchase could read either way, a plan administrator may ask for a letter before reimbursing it. Whether one is needed, and for what, is the administrator's decision, not the seller's.