Mirtazapine (Definition). Mirtazepine (remeron) is a noradrenergic/specific serotonergic antidepressant [nassa]. It is used for major depressive d/o, various anxiety disorders, chronic pain, and others.et help from a doctor now ›. None: There is nothing in Mirtazapine pharmacologically that acts on the mu receptor which is where narcotics site of action is. It is a noradrenergic and specific serotonergic antidepressant (nassa).