# Mirtazapine

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 2022, it was the 105th most commonly prescribed medication in the United States, with more than 6 million prescriptions.

## Medical uses

Mirtazapine is approved by the United States Food and Drug Administration for the treatment of major depressive disorder in adults.

### Depression

Mirtazapine is primarily used for major depressive disorder and other mood disorders. Onset of action appears faster than some selective serotonin reuptake inhibitors and similar to tricyclic antidepressants.
In 2010, the National Institute for Health and Care Excellence recommended generic selective serotonin reuptake inhibitors as first-line choices, as they are "equally effective as other antidepressants and have a favourable risk–benefit ratio." For mirtazapine, it found "no difference between mirtazapine and other antidepressants on any efficacy measure, although in terms of achieving remission mirtazapine appears to have a statistical though not clinical advantage. In addition, mirtazapine has a statistical advantage over selective serotonin reuptake inhibitors in terms of reducing symptoms of depression, but the difference is not clinically significant. However, there is strong evidence that patients taking mirtazapine are less likely to leave treatment early because of side effects, although this is not the case for patients reporting side effects or leaving treatment early for any reason."
A 2011 Cochrane review comparing mirtazapine to other antidepressants found that while it appeared to have a faster onset in people for whom it worked (measured at two weeks), its efficacy was about the same as other antidepressants after six weeks' use.
A 2012 review focused on antidepressants and sleep found that mirtazapine reduced the time it took to fall asleep and improved the quality of sleep in many people with sleep disorders caused by depression, but that it could also disturb sleep in many people, especially at higher doses, causing restless leg syndrome in 8 to 28% of people and in rare cases causes REM sleep behavior disorder. This seemingly paradoxical dose–response curve of mirtazapine with respect to somnolence is owed to the exceptionally high affinity of the drug for the histamine H$_1$, 5-HT$_{2A}$, and 5-HT$_{2C}$ receptors; exhibiting near exclusive occupation of these receptors at doses ≤15 mg. However, at higher doses, inverse agonism and constitutive activation of the α$_{2A}$-, α$_{2B}$-, and α$_{2C}$-adrenergic receptors begins to offset activity at H$_1$ receptors leading to decreased somnolence and even a subjective sensation of "activation" in treated patients.
A 2018 analysis of 21 antidepressants found them to be fairly similar overall. It found tentative evidence for mirtazapine being in the more effective group and middle in tolerability.
After one week of usage, mirtazapine was found to have an earlier onset of action compared to selective serotonin reuptake inhibitors.

### Other

There is also some evidence supporting its use in treating the following conditions, for which it is sometimes prescribed off-label:
* Generalized anxiety disorder
* Social anxiety disorder
* Obsessive–compulsive disorder
* Panic disorder
* Post-traumatic stress disorder
* Low appetite/underweight
* Insomnia
* Nausea and vomiting
* Itching
* Headaches and migraine

## Side or adverse effects

A 2011 Cochrane review found that, compared with other antidepressants, it is more likely to cause weight gain and sleepiness, but it is less likely to cause tremors than tricyclic antidepressants, and less likely to cause nausea and sexual dysfunction than selective serotonin reuptake inhibitors.
Very common (≥10% incidence) adverse effects include constipation, dry mouth, sleepiness, increased appetite (17%) and weight gain (>7% increase in <50% of children).
Common (1–10% incidence) adverse effects include weakness, confusion, dizziness, fasciculations (muscle twitches), peripheral edema (swelling, usually of the lower limbs), and negative lab results like elevated transaminases, elevated serum triglycerides, and elevated total cholesterol.
Mirtazapine is not considered to have a risk of many of the side effects often associated with other antidepressants like the selective serotonin reuptake inhibitors and may improve certain ones when taken in conjunction with them. (Those adverse effects include decreased appetite, weight loss, insomnia, nausea and vomiting, diarrhea, urinary retention, increased body temperature, excessive sweating, pupil dilation and sexual dysfunction.)
In general, some antidepressants, especially selective serotonin reuptake inhibitors, can paradoxically exacerbate some peoples' depression or anxiety or cause suicidal ideation. Despite its sedating action, mirtazapine is also believed to be capable of this, so in the United States and certain other countries, it carries a black box label warning of these potential effects, especially for people under the age of 25.
Mirtazapine may induce arthralgia (non-inflammatory joint pain).
A case report published in 2000 noted an instance in which mirtazapine counteracted the action of clonidine, causing a dangerous rise in blood pressure.
In a study comparing 32 antidepressants of all pharmacological classes, mirtazapine was one of the antidepressants most likely to cause nightmare disorder, sleepwalking, restless legs syndrome, night terrors and sleep paralysis.
Mirtazapine has been associated with an increased risk of death compared to other antidepressants in several studies. However, it is more likely that the residual differences between people prescribed mirtazapine rather than a selective serotonin reuptake inhibitor account for the difference in risk of mortality.

## Withdrawal

Stopping Mirtazapine and other antidepressants may cause withdrawal symptoms. A gradual and slow reduction in dose is recommended to minimize such symptoms. Effects of sudden cessation of treatment with mirtazapine may include depression, anxiety, tinnitus, panic attacks, vertigo, restlessness, irritability, decreased appetite, insomnia, diarrhea, nausea, vomiting, flu-like symptoms, allergy-like symptoms such as pruritus, headaches, and sometimes mania or hypomania.

## Overdose

Mirtazapine is considered to be relatively safe in the event of an overdose, although it is considered slightly more toxic in overdose than most of the selective serotonin reuptake inhibitors (except citalopram). Unlike the tricyclic antidepressants, mirtazapine showed no significant cardiovascular adverse effects at 7 to 22 times the maximum recommended dose.
Twelve reported fatalities have been attributed to mirtazapine overdose. The fatal toxicity index (deaths per million prescriptions) for mirtazapine is 3.1 (95% CI: 0.1 to 17.2). This is similar to that observed with selective serotonin reuptake inhibitors.

## Interactions

Concurrent use with inhibitors or inducers of the cytochrome P450 isoenzymes CYP1A2, CYP2D6, or CYP3A4 can result in altered concentrations of mirtazapine, as these are the main enzymes responsible for its metabolism. As examples, fluoxetine and paroxetine, inhibitors of these enzymes, are known to modestly increase mirtazapine levels, while carbamazepine, an inducer, considerably decreases them. Liver impairment and moderate chronic kidney disease have been reported to decrease the oral clearance of mirtazapine by about 30%; severe kidney disease decreases it by 50%.
Mirtazapine in combination with a selective serotonin reuptake inhibitor, serotonin–norepinephrine reuptake inhibitor, or tricyclic antidepressant as an augmentation strategy is considered to be relatively safe and is often employed therapeutically but caution should be given when combined with fluvoxamine. There is a combination of venlafaxine and mirtazapine, sometimes referred to as "California rocket fuel". Several case reports document serotonin syndrome induced by the combination of mirtazapine with other agents (olanzapine, quetiapine, tramadol and venlafaxine). Adding fluvoxamine to treatment with mirtazapine may cause a significant increase in mirtazapine concentration. This interaction may necessitate an adjustment of the mirtazapine dosage.
According to information from the manufacturers, mirtazapine should not be started within two weeks of any monoamine oxidase inhibitor usage; likewise, monoamine oxidase inhibitors should not be administered within two weeks of discontinuing mirtazapine.
The addition of mirtazapine to a monoamine oxidase inhibitor, while potentially having typical or idiosyncratic (unique to the individual) reactions not herein described, does not appear to cause serotonin syndrome. This is per the fact that the 5-HT$_{2A}$ receptor is the predominant serotonin receptor thought to be involved in the pathophysiology of serotonin syndrome (with the 5-HT$_{1A}$ receptor seeming to be protective). Mirtazapine is a potent 5-HT$_{2A}$ receptor antagonist, and cyproheptadine, a medication that shares this property, mediates recovery from serotonin syndrome and is an antidote against it.
There is a possible interaction that results in a hypertensive crisis when mirtazapine is given to a patient who has already been on steady doses of clonidine. This involves a subtle consideration, when patients have been on chronic therapy with clonidine and suddenly stop the dosing, a rapid hypertensive rebound sometimes (20%) occurs from increased sympathetic outflow. Clonidine's blood pressure lowering effects are due to stimulation of presynaptic α$_2$ autoreceptors in the CNS which suppress sympathetic outflow. Mirtazapine itself blocks these same α$_2$ autoreceptors, so the effect of adding mirtazapine to a patient stabilized on clonidine may precipitate withdrawal symptoms.
Mirtazapine has been used as a hallucinogen antidote to block the effects of serotonergic psychedelics like psilocybin and lysergic acid diethylamide (LSD).

## Pharmacology
### Pharmacodynamics

Mirtazapine :
| | K$_i$ (nM) | Species | Ref |
| SERTTooltip Serotonin transporter | 10000+ | Human | |
| NETTooltip Norepinephrine transporter | 4600+ | Human | |
| DATTooltip Dopamine transporter | 10000+ | Human | |
| 5-HT$_{1A}$ | 3330–5010 | Human | |
| 5-HT$_{1B}$ | 3534–12600 | Human | |
| 5-HT$_{1D}$ | 794–5010 | Human | |
| 5-HT$_{1E}$ | 728 | Human | |
| 5-HT$_{1F}$ | 583 | Human | |
| 5-HT$_{2A}$ | 6.3–69 | Human | |
| 5-HT$_{2B}$ | 200 | Human | |
| 5-HT$_{2C}$ | 8.9–39 | Human | |
| 5-HT$_3$ | 8.1 | Human | |
| 5-HT$_{4L}$ | 10000+ | Human | |
| 5-HT$_{5A}$ | 670 | Human | |
| 5-HT$_6$ | ND | ND | ND |
| 5-HT$_7$ | 265 | Human | |
| α$_{1A}$ | 1815 | Human | |
| α$_{2A}$ | 20 | Human | |
| α$_{2B}$ | 88 | Human | |
| α$_{2C}$ | 18 | Human | |
| β | 10000+ | Human | |
| D$_1$ | 4167 | Rat | |
| D$_2$ | 5454+ | Human | |
| D$_3$ | 5723 | Human | |
| D$_4$ | 2518 | Human | |
| H$_1$ | 0.14–1.6 | Human | |
| H$_2$ | 10000+ | Rat | |
| H$_3$ | 83200 | Human | |
| H$_4$ | 100000+ | Human | |
| mAChTooltip Muscarinic acetylcholine receptor | 670 | Human | |
| VGSCTooltip Voltage-gated sodium channel | 6905 | Rat | |
| VDCCTooltip Voltage-dependent calcium channel | 10000+ | Rat | |
| Values are K$_i$ (nM). The smaller the value, the more strongly the drug binds to the site. ||||
Mirtazapine is sometimes described as a noradrenergic and specific serotonergic antidepressant (NaSSA), although the actual evidence in support of this label has been regarded as poor. It is a tetracyclic piperazine-azepine.
Mirtazapine has antihistamine, α$_2$-blocker, and antiserotonergic activity. It is specifically a potent antagonist or inverse agonist of the α$_{2A}$-, α$_{2B}$-, and α$_{2C}$-adrenergic receptors, the serotonin 5-HT$_{2A}$, 5-HT$_{2C}$, and the histamine H$_1$ receptor. Unlike many other antidepressants, it does not inhibit the reuptake of serotonin, norepinephrine, or dopamine, nor does it inhibit monoamine oxidase. Similarly, mirtazapine has weak or no activity as an anticholinergic or blocker of sodium or calcium channels, in contrast to most tricyclic antidepressants. In accordance, it has better tolerability and low toxicity in overdose. As an H$_1$ receptor antagonist, mirtazapine is extremely potent, and is in fact one of the most potent H$_1$ receptor inverse agonists among tricyclic and tetracyclic antidepressants and most antihistamines in general. Antagonism of the H$_1$ receptor is by far the strongest activity of mirtazapine, with the drug acting as a selective H$_1$ receptor antagonist at low concentrations.
The (S)-(+) enantiomer of mirtazapine is responsible for antagonism of the serotonin 5-HT$_{2A}$ and 5-HT$_{2C}$ receptors, while the (R)-(–) enantiomer is responsible for antagonism of the 5-HT$_3$ receptor. Both enantiomers are involved in antagonism of the H$_1$ and α$_2$-adrenergic receptors, although the (S)-(+) enantiomer is the stronger antihistamine.
Although not clinically relevant, mirtazapine has been found to act as a partial agonist of the κ-opioid receptor at high concentrations (EC$_{50}$ = 7.2 μM).

#### α$_2$-Adrenergic receptor

Antagonism of the α$_2$-adrenergic receptors, which function largely as inhibitory autoreceptors and heteroreceptors, enhances adrenergic and serotonergic neurotransmission, notably central 5-HT$_{1A}$ receptor mediated transmission in the dorsal raphe nucleus and hippocampus; hence, mirtazapine's classification as a NaSSA. Indirect α$_1$ adrenoceptor-mediated enhancement of serotonin cell firing and direct blockade of inhibitory α$_2$ 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-HT$_{1A}$ receptor. Increased activation of the central 5-HT$_{1A}$ receptor is thought to be a major mediator of efficacy of most antidepressant drugs.

#### 5-HT$_2$ receptor

Antagonism of the 5-HT$_2$ subfamily of receptors and inverse agonism of the 5-HT$_{2C}$ receptor appears to be in part responsible for mirtazapine's efficacy in the treatment of depressive states. Mirtazapine increases dopamine release in the prefrontal cortex. Accordingly, it was shown that by blocking the α$_2$-adrenergic receptors and 5-HT$_{2C}$ receptors mirtazapine disinhibited dopamine and norepinephrine activity in these areas in rats. In addition, mirtazapine's antagonism of 5-HT$_{2A}$ 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. In contrast to mirtazapine, the selective serotonin reuptake inhibitors, serotonin–norepinephrine reuptake inhibitors, monoamine oxidase inhibitors, and some tricyclic antidepressants acutely increase the general activity of the 5-HT$_{2A}$, 5-HT$_{2C}$, and 5-HT$_3$ receptors, leading to a number of negative changes and side effects, the most prominent of which include anorexia, insomnia, nausea, and diarrhea, among others. However, most of these adverse effects are temporary, since down regulation of 5-HT$_{2A}$ receptors eventually occurs following chronic SSRI treatment, and desensitization of 5-HT$_3$ receptors often occurs within a week or less. This is precisely why SSRIs have a delayed antidepressant and anxiolytic effect, and occasionally, an acute anxiogenic effect before down regulation occurs. Mirtazapine, on the other hand, is an antagonist of the 5-HT$_{2A}$ receptor, and antagonists at this receptor typically induce reverse tolerance. Thus, the antidepressant and anxiolytic effects of mirtazapine occur more rapidly than with SSRIs. Furthermore, its reduced incidence of sexual dysfunction (such as loss of libido and anorgasmia) could be a product of negligible binding to the serotonin transporter and antagonism of the 5-HT$_{2A}$ receptors; however, Mirtazapine's high affinity towards and inverse agonism of the 5-HT$_{2C}$ receptors may greatly attenuate those pro-sexual factors (as evidenced by the pro-sexual effects of drugs like m-CPP and lorcaserin which agonize 5-HT$_{2C}$ receptors in a reasonably selective manner). As a result, it is often combined with these drugs to reduce their side-effect profile and to produce a stronger antidepressant effect.
Mirtazapine does not have pro-serotonergic activity and thus does not cause serotonin syndrome. This is in accordance with the fact that it is not a serotonin reuptake inhibitor or monoamine oxidase inhibitor, nor a serotonin receptor agonist. There are no reports of serotonin syndrome in association with mirtazapine alone, and mirtazapine has not been found to cause serotonin syndrome in overdose. However, there are a handful of case reports of serotonin syndrome occurring with mirtazapine in combination with serotonergic drugs like selective serotonin reuptake inhibitors, although such reports are very rare, and do not necessarily implicate mirtazapine as causative.

#### 5-HT$_3$ receptor

(R)-(–)-mirtazapine is a potent 5-HT$_3$ blocker. It may relieve chemotherapy-related and advanced cancer-related nausea.

#### H$_1$ receptor

Mirtazapine is a very strong H$_1$ receptor antagonist and, as a result, it can cause powerful sedative and hypnotic effects. A single 15 mg dose of mirtazapine to healthy volunteers has been found to result in over 80% occupancy of the H$_1$ receptor and to induce intense sleepiness. After a short period of chronic treatment, however, the H$_1$ receptor tends to sensitize and the antihistamine effects become more tolerable. Many patients may also dose at night to avoid the effects, and this appears to be an effective strategy for combating them. Blockade of the H$_1$ receptor may improve pre-existing allergies, pruritus, nausea, and insomnia in affected individuals. It may also contribute to weight gain, however. In contrast to the H$_1$ receptor, mirtazapine has only low affinity for the muscarinic acetylcholine receptors, although anticholinergic side effects like dry mouth, constipation, and mydriasis are still sometimes seen in clinical practice.

### Pharmacokinetics

The oral bioavailability of mirtazapine is about 50%. It is found mostly bound to plasma proteins, about 85%. It is metabolized primarily in the liver by N-demethylation and hydroxylation via cytochrome P450 enzymes, CYP1A2, CYP2D6, CYP3A4. The overall elimination half-life is 20–40 hours, and this is independent of dosage. It is conjugated in the kidney for excretion in the urine, where 75% of the drug is excreted, and about 15% is eliminated in feces.$^{:430}$ Desmethylmirtazapine is an active metabolite of mirtazapine which is believed to contribute about 3-10% to the drug's overall effects and has a half-life of about 25 hours.

## Chemistry

Mirtazapine is a tetracyclic piperazinoazepine; mianserin was developed by the same team of organic chemists and mirtazapine differs from it via the addition of a nitrogen atom in one of the rings.$^{:429}$ It is a racemic mixture of enantiomers. The (S)-(+)-enantiomer is known as esmirtazapine.
Analogues of mirtazapine include mianserin, setiptiline, and aptazapine.

### Synthesis

A chemical synthesis of mirtazapine has been published. The first step of synthesis is a condensation reaction between the molecule 2-chloro 3-cyanopyridine and the molecule 1-methyl-3-phenylpiperazine.

## History

Mirtazapine was first synthesized at Organon and published in 1989, was first approved for use in major depressive disorder in the Netherlands in 1994, and was introduced in the United States in 1996 under the brand name Remeron.$^{:429}$

## Society and culture
### Generic names

Mirtazapine is the English and French generic name of the drug and its INNTooltip International Nonproprietary Name, USANTooltip United States Adopted Name, USPTooltip United States Pharmacopeia, BANTooltip British Approved Name, DCFTooltip Dénomination Commune Française, and JANTooltip Japanese Accepted Name. Its generic name in Spanish, Italian, and Portuguese is mirtazapina and in German, Turkish and Swedish is mirtazapin.

### Brand names

Mirtazapine is marketed under many brand names worldwide, including Adco-Mirteron, Afloyan, Amirel, Arintapin Smelt, Avanza, Axit, Azapin, Beron, Bilanz, Blumirtax, Calixta, Ciblex, Combar, Comenter, Depreram, Divaril, Esprital, Maz, Menelat, Mepirzapine, Merdaten, Meronin, Mi Er Ning, Milivin, Minelza, Minivane, Mirastad, Mirazep, Miro, Miron, Mirrador, Mirt, Mirta, Mirtabene, Mirtadepi, Mirtagamma, Mirtagen, Mirtalan, Mirtamor, Mirtamylan, Mirtan, Mirtaneo, Mirtanza, Mirtapax, Mirtapil, Mirtapine, Mirtaron, Mirtastad, Mirtax, Mirtaz, Mirtazap, Mirtazapin, Mirtazapina, Mirtazapine, Mirtazapinum, Mirtazelon, Mirtazon, Mirtazonal, Mirtel, Mirtimash, Mirtin, Mirtine, Mirtor, Mirzapine, Mirzaten, Mirzest, Mitaprex, Mitaxind, Mitocent, Mitrazin, Mizapin, Motofen, Mytra, Norset, Noxibel, Pharmataz, Promyrtil, Rapizapine, Ramure, Razapina, Redepra, Reflex, Remergil, Remergon, Remeron, Remirta, Rexer, Saxib, Sinmaron, Smilon, Tazepin, Tazimed, Tetrazic, Tifona, U-Mirtaron, U-zepine, Valdren, Vastat, Velorin, Yarocen, Zania, Zapex, Zestat, Zismirt, Zispin, Zuleptan, and Zulin.

## Research

The use of mirtazapine has been explored in several additional conditions:
* Found ineffective for Sleep apnea/hypopnea
* Secondary symptoms of autistic spectrum conditions and other pervasive developmental disorders
* Antipsychotic-induced akathisia.
* Drug withdrawal, dependence and detoxification
* Negative, depressive and cognitive symptoms of schizophrenia (as an adjunct)
* A case report has been published in which mirtazapine reduced visual hallucinations in a patient with Parkinson's disease psychosis (PDP). This is in alignment with recent findings that inverse agonists at the 5-HT$_{2A}$ receptors are efficacious in attenuating the symptoms of Parkinson's disease psychosis. As is supported by the common practice of prescribing low-dose quetiapine and clozapine for PDP at doses too low to antagonize the D$_2$ receptor, but sufficiently high doses to inversely agonize the 5-HT$_{2A}$ receptors.
* Eight case reports have been reported in five papers on the use of mirtazapine in the treatment of hives as of 2017.
* Mirtazapine to alleviate severe breathlessness in patients with COPD or interstitial lung diseases (BETTER-B). Found ineffective and potentially harmful.

## Veterinary use

Mirtazapine also has some veterinary use in cats and dogs. Mirtazapine is sometimes prescribed as an appetite stimulant for cats or dogs experiencing loss of appetite due to medical conditions such as chronic kidney disease. It is especially useful for treating combined poor appetite and nausea in cats and dogs.
Mirtazapine is indicated for bodyweight gain in cats experiencing poor appetite and weight loss resulting from chronic medical conditions.
There are two options for administration: tablets given orally, and an ointment applied topically to the inner surface of the ear.
The most common side effects include signs of local irritation or inflammation at the site where the ointment is applied and behavioural changes (increased meowing, hyperactivity, disoriented state or inability to coordinate muscle movements, lack of energy/weakness, attention-seeking, and aggression).