Mirtazapine, originally known as ORG 3770, was first synthesized by the Department of Medicinal Chemistry of NV Organon in the Netherlands (Kaspersen et al. 1989). First approved for use in major depression in the Netherlands in 1994, mirtazapine was introduced in the United States in 1996. The antidepressant mirtazapine has a dual mode of action. It is a noradrenergic and specific serotonergic antidepressant (NaSSA) that acts by antagonizing the adrenergic alpha2-autoreceptors and alpha2-heteroreceptors as well as by blocking 5-HT2 and 5-HT3 receptors. It enhances, therefore, the release of norepinephrine and 5-HT1A-mediated serotonergic transmission. This dual mode of action may conceivably be responsible for mirtazapine's rapid onset of action.

Esmirtazapine (S-(+)mirtazapine or ORG-50081) is an enantiomer of mirtazapine (REMERON®), a high-affinity antagonist at 5-HT2/5-HT3 and H1 receptors, used in the treatment of depression. Esmirtazapine has a shorter plasma half-life than the R(−) enantiomer. Esmirtazapine is preferentially metabolized into an 8-hydroxy glucuronide. Organon was developing esmirtazapine for the treatment of hot flushes (vasomotor symptoms) associated with the menopause and insomnia.