A letter of medical necessity is a document your prescriber writes to your insurer making the clinical case for a specific drug: the diagnosis, relevant history and lab values, what has already been tried and why it failed or wasn't tolerated, and why this medication is the appropriate next step. For GLP-1 medications it typically documents BMI, weight-related conditions such as type 2 diabetes or sleep apnea, and any step-therapy drugs already attempted.
The letter is most useful when it mirrors the plan's own coverage criteria point by point, so the reviewer can check each box. It is the backbone of a prior authorization request and of an appeal or external review after a denial; see how to write an effective appeal letter and understanding insurance coverage for GLP-1 medications.