“Medically necessary” refers to any therapy or medical treatment that is deemed necessary to the care and treatment for a diagnosed disorder. For example, it is relatively easy to get coverage for antidepressant medications if depression is diagnosed, and if there is an acute cardiac issue requiring a stent, the tests demonstrate to insurance that surgery is needed. In these cases, there is a report or testing that is done that says those interventions are medically necessary.In the case of addiction, it is not as simple. If you believe, for example, that you require inpatient drug rehab because you will relapse if you undergo outpatient treatment, an insurance company will want to know why you need more expensive treatment over something that will cost them less. Without that evidence, they may not approve the charge for the more intense level of treatment.Currently, insurance companies do not accept a higher risk of relapse during outpatient treatment, as compared to inpatient treatment, as proof that more expensive residential care is a necessity.