Mirtazapine (Romeron) is an atypical antidepressant that increases levels of norepinephrine and serotonin at specific receptors, which is why it's called a noradrenergic and specific serotonergic antidepressant (NaSSA).
Mechanism of action: mirtazapine inhibits central presynaptic α2-receptors, which increases norepinephrine release. It also inhibits serotonin (5-HT) receptors: 2A, 2C, and 3, thus leaving the free 5-HT1 receptors to interact with the remaining serotonin. In other words, the amount of serotonin remains the same, but the only receptor they can activate is 5-HT1, thus effectively increasing serotonin levels for this receptor only, producing antidepressant effect.
It paradoxically causes more sedation at lower doses (≤15 mg) than at higher ones. This is because the drug is a potent antagonist of central histamine-1 (H1) receptors (producing a sedating), but as the dose gets higher, it increases norepinephrine release which offsets the anti-histamine sedative effects.
Like many other antidepressants, it can paradoxically cause suicidal ideation and worsen depression in some patients.
