Mirtazapine came into medical use in the United States in 1996. The patent expired in 2004, and generic versions are available. In the United States the wholesale cost as of 2018 is about US$3 per month. In the United Kingdom a month supply costs less than £20 per month. In the United States about 5.5 million prescriptions were written for mirtazapine in 2016.
In 2010 NICE published a guideline for treating depression that included a review of antidepressants. It recommended generic SSRIs as first line choices, as they are "equally effective as other antidepressants and have a favourable risk–benefit ratio." With respect to mirtazapine, it found: "There is 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 SSRIs in terms of reducing symptoms of depression, but the difference is not clinically important. 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 that compared mirtazapine to other antidepressants found that while it appears to have a faster onset in people for whom it works (measured at 2 weeks), it is about the same as other antidepressants at 6 weeks.
A 2012 review focused on antidepressants and sleep found that in many people with sleep disorders caused by depression, mirtazapine reduces the time it takes to fall asleep and increases the quality of sleep, but that in some people it can disturb sleep, especially at higher doses, causing restless leg syndrome in 8 to 28% of people and in rare cases causes REM sleep behavior disorder.
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 SSRIs.
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 tremor than tricyclic antidepressants, and less likely to cause nausea and sexual dysfunction than SSRIs.
In general, some antidepressants, especially SSRIs, 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.
A case report published in 2000 noted an instance in which mirtazapine counteracted the action of clonidine, causing a dangerous rise in blood pressure.
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 SSRIs (except citalopram). Unlike the tricyclic antidepressants, mirtazapine showed no significant cardiovascularadverse effects at 7 to 22 times the maximum recommended dose. Case reports of overdose with as much as 30 to 50 times the standard dose described the drug as relatively nontoxic, compared to tricyclic antidepressants.
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 SSRIs.
According to information from the manufacturers, mirtazapine should not be started within two weeks of any monoamine oxidase inhibitor (MAOI) usage; likewise, MAOIs should not be administered within two weeks of discontinuing mirtazapine. Mirtazapine in combination with an SSRI, SNRI, or TCA as an augmentation strategy is considered to be relatively safe and is often employed therapeutically, with 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).
The (S)-(+) enantiomer of mirtazapine is responsible for antagonism of the serotonin 5-HT2A and 5-HT2C receptors, while the (R)-(–) enantiomer is responsible for antagonism of the 5-HT3 receptor. Both enantiomers are involved in antagonism of the H1 and α2-adrenergic receptors, although the (S)-(+) enantiomer is the stronger antihistamine.
Antagonism of the 5-HT2 subfamily of receptors and inverse agonism of the 5-HT2C 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 α2-adrenergic receptors and 5-HT2C receptors mirtazapine disinhibited dopamine and norepinephrine activity in these areas in rats. In addition, mirtazapine's 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 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 afflicted 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 SSRIs, SNRIs, MAOIs, and some TCAs increase the general activity of the 5-HT2A, 5-HT2C, and 5-HT3 receptors leading to a host of negative changes and side effects, the most prominent of which including anorexia, insomnia, sexual dysfunction (loss of libido and anorgasmia), nausea, and diarrhea, among others. 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 serotonergic activity and does not cause serotonergic side effects or serotonin syndrome. This is in accordance with the fact that it is not a serotonin reuptake inhibitor or MAOI, 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 SSRIs, although such reports are very rare, and do not necessarily implicate mirtazapine as causative. The addition of mirtazapine to an MAOI does not appear to cause serotonin syndrome. This is in accordance with the fact that the 5-HT2A receptor is the predominant serotonin receptor thought to be involved in the pathophysiology of serotonin syndrome (with the 5-HT1A receptor seeming to be protective). Mirtazapine is a potent 5-HT2A receptor antagonist, and cyproheptadine, a drug that shares this property, mediates recovery from serotonin syndrome and is an antidote against it.
Mirtazapine is a very strong H1 receptor inverse agonist 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 H1 receptor and to induce intense sleepiness. After a short period of chronic treatment, however, the H1 receptor tends to desensitize 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 H1 receptor may improve pre-existing allergies, pruritus, nausea, and insomnia in afflicted individuals. It may also contribute to weight gain, however. In contrast to the H1 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.
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
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-HT2A 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 D2 receptor, but sufficiently high doses to inversely agonize the 5-HT2A receptors.
Eight case reports have been reported in five papers on the use of mirtazapine in the treatment of hives as of 2017.
Mirtazapine also has some veterinary use in cats and dogs. Mirtazapine is sometimes prescribed as an appetite stimulant for cats or dogs experiencing anorexia due to medical conditions such as chronic kidney disease. It is especially useful for treating combined poor appetite and nausea in cats and dogs.
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