Mirtazapine, sold under the brand name Remeron amongst others, is an atypical antidepressant, and as such is used primarily to treat depression. Its effects may take up to four weeks, but can also manifest as early as one to two weeks. It is often used in cases of depression complicated by anxiety or insomnia. The effectiveness of Mirtazapine is comparable to other commonly prescribed antidepressants. It is taken by mouth.Common side effects include sleepiness, dizziness, increased appetite and weight gain. Serious side effects may include mania,  low white blood cell count, and increased suicide among children. Withdrawal symptoms may occur with stopping. It is not recommended together with an MAO inhibitor, although evidence supporting the danger of this combination has been refuted. It is unclear if use during pregnancy is safe. How it works is not clear, but it may involve blocking certain adrenergic and serotonin receptors. Chemically, it is a tetracyclic antidepressant (TeCA), and is closely related to mianserin. It also has strong antihistaminergic effects.Mirtazapine came into medical use in the United States in 1996. The patent expired in 2004, and generic versions are available. In 2019, it was the 106th most commonly prescribed medication in the United States, with more than 6 million prescriptions. Medical uses
Depression
Mirtazapine is primarily used for major depressive disorder and other mood disorders.
Indirect α1 adrenoceptor-mediated enhancement of serotonin cell firing and direct blockade of inhibitory α2 heteroreceptors located on serotonin terminals are held responsible for the increase in extracellular serotonin. Because of this, mirtazapine has been said to be a functional "indirect agonist" of the 5-HT1A receptor. Increased activation of the central 5-HT1A receptor is thought to be a major mediator of efficacy of most antidepressant drugs. 5-HT2 receptor
Antagonism of the 5-HT2 subfamily of receptors and inverse agonism of the 5-HT2C receptor appears to be in part responsible for mirtazapines efficacy in the treatment of depressive states. Mirtazapine increases dopamine release in the prefrontal cortex. Accordingly, it was shown that by blocking the α2-adrenergic receptors and 5-HT2C receptors mirtazapine disinhibited dopamine and norepinephrine activity in these areas in rats. In addition, mirtazapines antagonism of 5-HT2A receptors has beneficial effects on anxiety, sleep and appetite, as well as sexual function regarding the latter receptor. Mirtazapine has been shown to lower drug seeking behaviour (more specifically to methamphetamine) in various human and animal studies. It is also being investigated in substance abuse disorders to reduce withdrawal effects and improve remission rates.Mirtazapine significantly improves pre-existing symptoms of nausea, vomiting, diarrhea, and irritable bowel syndrome in affected individuals. Mirtazapine may be used as an inexpensive antiemetic alternative to Ondansetron. In conjunction with substance abuse counseling, mirtazapine has been investigated for the purpose of reducing methamphetamine use in dependent individuals with success.