Letter of medical necessity
Also called: LMN
Clinical reviewer: Elena Gizzi, MSN, RN · Last checked September 16, 2026
What is a letter of medical necessity?
Letter of medical necessity is a statement from your prescriber explaining why a specific drug is medically necessary for you. It supports a prior authorization, a drug exceptions request, or an internal appeal. HealthCare.gov lists a letter from your doctor among the documents you can submit with an internal appeal.
What is "medically necessary," and where does the letter fit?
Health care is medically necessary when it's needed to diagnose or treat your condition and meets accepted standards of medicine [1]. A letter of medical necessity is how your prescriber puts that judgment in writing (or states it orally) for your insurer, most often to support a prior authorization or a request for a drug your plan doesn't normally cover.
What does the letter need to show?
Through your plan's drug exceptions process, your prescriber confirms that the requested drug is appropriate for your condition [2]. Common grounds include that covered alternatives haven't been, or wouldn't be, as effective, or that they've caused or are likely to cause harmful side effects [2]. If you're also filing an internal appeal, HealthCare.gov lists a letter from your doctor among the documentation you can submit for the insurer to consider [3], and if a denial is based on medical necessity, the notice must explain the clinical reasoning or say that explanation is available free on request [4].
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This entry is for education and is not medical advice. Talk with a licensed clinician about your own situation before starting, stopping, or changing any treatment.
