Mirtazapine, sold under the brand name Remeron among others, is an atypical tetracyclic 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 a monoamine oxidase inhibitor, although evidence supporting the danger of this combination has been challenged. 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, 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 2021, it was the 124th most commonly prescribed medication in the United States, with more than 4 million prescriptions.