Mirtazapine is an antidepressant that increases the activity of serotonin and norepinephrine in the brain, which are chemicals that enhance mood. It works by blocking the central presynaptic alpha-2-adrenergic receptors, which causes the brain to release more of these chemicals.

Mirtazapine is a tetracyclic antidepressant (TeCA). It's used to treat major depressive disorder, but it can also be used off-label to treat other conditions like insomnia, panic disorder, and obsessive-compulsive disorder.