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Tetracyclic antidepressant

Mirtazapine

Mirtazapine is a tetracyclic antidepressant used primarily for major depressive disorder. Unlike SSRIs, it acts as an α2-adrenergic antagonist and blocks specific serotonin receptor subtypes. The most testable fact is that mirtazapine commonly causes sedation and weight gain, making it useful for depressed patients with insomnia or poor appetite.

Practice questionsHigh-yield facts
Drug classTetracyclic antidepressant (noradrenergic and specific serotonergic antidepressant)
Mechanismα2-adrenergic antagonist; blocks 5-HT2 and 5-HT3 receptors
Main usesMajor depressive disorder, insomnia associated with depression
Key adverse effectSedation, increased appetite, weight gain
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Mirtazapine is classified as a noradrenergic and specific serotonergic antidepressant (NaSSA). Its antidepressant effects arise from multiple receptor interactions:

  • α2-adrenergic antagonist: Blocks presynaptic α2-autoreceptors and α2-heteroreceptors on serotonergic neurons, resulting in increased release of norepinephrine and serotonin in the synaptic cleft.
  • 5-HT2 and 5-HT3 receptor antagonist: Blocks postsynaptic 5-HT2A, 5-HT2C, and 5-HT3 receptors. By blocking 5-HT2 and 5-HT3 receptors (which mediate anxiety, insomnia, and sexual dysfunction), mirtazapine avoids many typical SSRI side effects. The net effect is enhanced 5-HT1A receptor stimulation, which contributes to antidepressant and anxiolytic effects.
  • Histamine H1 receptor antagonist: Potent H1 blockade produces sedation and increased appetite.

The combination of increased noradrenergic and serotonergic neurotransmission without direct serotonin reuptake inhibition distinguishes mirtazapine from SSRIs and SNRIs. The unique receptor profile explains its sedating and appetite-stimulating properties, which can be therapeutically beneficial in select patients.

High-yield for exams
  • Mirtazapine is an α2-adrenergic antagonist that increases norepinephrine and serotonin release
  • Blocks 5-HT2 and 5-HT3 receptors, avoiding sexual dysfunction and nausea seen with SSRIs
  • Potent H1 antagonism causes sedation and increased appetite; useful in depression with insomnia or weight loss
  • Lower doses are more sedating than higher doses due to differential receptor affinity
  • Rare but serious adverse effect: agranulocytosis (monitor for fever, sore throat, infection)
  • Black box warning: increased suicidality in patients under 25 years
  • Contraindicated with MAOIs due to serotonin syndrome risk
  • Often combined with SSRIs/SNRIs in treatment-resistant depression (California rocket fuel)

Indications

Mirtazapine is FDA-approved for:

  • Major depressive disorder (MDD): First-line agent, particularly in patients with prominent insomnia, poor appetite, or weight loss.

Off-label uses include:

  • Insomnia: Low doses enhance sleep through H1 antagonism.
  • Post-traumatic stress disorder (PTSD): May reduce nightmares and improve sleep.
  • Generalized anxiety disorder
  • Nausea and vomiting: Particularly in palliative care and chemotherapy settings, due to 5-HT3 antagonism.
  • Appetite stimulation: In cancer cachexia, anorexia, and elderly patients with poor intake.

Mirtazapine is often selected when sedation and weight gain are desirable side effects rather than drawbacks. It may also be combined with SSRIs or SNRIs in treatment-resistant depression (California rocket fuel combination).

Pharmacokinetics

Absorption: Rapidly absorbed after oral administration, with peak plasma concentrations reached in approximately two hours. Bioavailability is around fifty percent due to first-pass metabolism.

Distribution: Highly protein-bound (approximately 85 percent). Volume of distribution suggests extensive tissue distribution.

Metabolism: Extensively metabolized in the liver via CYP1A2, CYP2D6, and CYP3A4. The major pathways include demethylation and hydroxylation, producing active metabolites that contribute minimally to clinical effects.

Excretion: Elimination half-life is 20 to 40 hours, allowing once-daily dosing (usually at bedtime). Metabolites are excreted in urine (75 percent) and feces (15 percent).

Special populations: Clearance is reduced in elderly patients, in hepatic impairment, and in renal impairment. Women have approximately higher plasma concentrations than men. Dosage adjustment is not typically required but clinical monitoring is advisable.

Adverse effects

The side effect profile of mirtazapine differs from SSRIs due to its unique receptor activity.

Common adverse effects

  • Sedation and somnolence: Very common, especially at lower doses (paradoxically, higher doses may be less sedating due to increased noradrenergic activity).
  • Increased appetite and weight gain: Occurs in a significant proportion of patients, mediated by H1 and 5-HT2C antagonism.
  • Dry mouth
  • Constipation
  • Dizziness

Serious adverse effects

  • Agranulocytosis and severe neutropenia: Rare but potentially life-threatening. Presents with fever, sore throat, and infection. Requires immediate discontinuation and hematologic evaluation. Most cases occur within the first few months of therapy.
  • Bone marrow suppression: Obtain baseline and periodic complete blood count if clinical signs of infection develop.
  • Serotonin syndrome: Risk increases when combined with other serotonergic agents (SSRIs, SNRIs, MAOIs, tramadol, linezolid). Presents with agitation, confusion, hyperthermia, hyperreflexia, tremor, and autonomic instability.
  • Hyponatremia: Syndrome of inappropriate antidiuretic hormone secretion (SIADH), particularly in elderly patients.
  • Activation of mania or hypomania: Risk in patients with bipolar disorder.
  • Suicidal ideation: Black box warning for increased suicidality in children, adolescents, and young adults (under 25 years).
  • QTc prolongation: Rare, but consider in patients with cardiac risk factors.
  • Hepatotoxicity: Rare cases of elevated liver enzymes and hepatitis.

Withdrawal symptoms

Abrupt discontinuation may cause dizziness, nausea, headache, and agitation. Taper gradually over several weeks when discontinuing.

Contraindications and warnings

Absolute contraindications:

  • Hypersensitivity to mirtazapine
  • Concurrent use with monoamine oxidase inhibitors (MAOIs) or within 14 days of MAOI discontinuation (risk of serotonin syndrome)
  • Use of MAOIs intended to treat psychiatric disorders within 14 days of stopping mirtazapine

Warnings and precautions:

  • Suicidality: Black box warning. Monitor closely for worsening depression, suicidal thoughts, or unusual behavior changes, especially during initial treatment and dose changes. Risk is highest in young adults.
  • Bipolar disorder: May precipitate manic or mixed episodes. Screen for personal or family history of bipolar disorder before initiating.
  • Bone marrow suppression: Educate patients to report signs of infection (fever, sore throat, malaise). Discontinue if absolute neutrophil count falls significantly.
  • Seizure disorder: Use with caution; mirtazapine may lower seizure threshold.
  • Hepatic or renal impairment: Reduced clearance necessitates careful monitoring and potential dose reduction.
  • Elderly patients: Increased risk of hyponatremia, sedation, and falls.
  • Angle-closure glaucoma: Anticholinergic effects may precipitate acute angle-closure glaucoma in patients with anatomically narrow angles.
  • Cardiovascular disease: Use caution in patients with recent myocardial infarction or unstable heart disease.
  • Pregnancy: Category C. Limited human data; use only if benefit outweighs risk. Neonatal complications have been reported with third-trimester exposure.
  • Lactation: Excreted in breast milk; consider risks and benefits.

Drug interactions

Pharmacodynamic interactions

  • MAOIs: Contraindicated. Risk of serotonin syndrome, hypertensive crisis, and death. Allow 14-day washout period.
  • Serotonergic agents (SSRIs, SNRIs, triptans, tramadol, St. John's wort, tryptophan, linezolid): Increased risk of serotonin syndrome.
  • CNS depressants (benzodiazepines, opioids, alcohol, sedating antihistamines): Additive sedation and respiratory depression.
  • Anticholinergic agents: Additive anticholinergic effects (dry mouth, constipation, urinary retention, confusion).

Pharmacokinetic interactions

  • CYP1A2 inhibitors (fluvoxamine, ciprofloxacin): Increase mirtazapine levels; consider dose reduction.
  • CYP3A4 inducers (carbamazepine, phenytoin, rifampin): Decrease mirtazapine levels; may require dose increase.
  • CYP3A4 inhibitors (ketoconazole, erythromycin, protease inhibitors): Increase mirtazapine exposure modestly.
  • Warfarin: Isolated reports of increased INR; monitor coagulation parameters closely.

Clinical considerations

Mirtazapine is sometimes combined with SSRIs or SNRIs in treatment-resistant depression, but this requires careful monitoring for serotonin syndrome. Avoid alcohol due to additive sedation.

Clinical pearls

  • Dosing paradox: Lower doses (7.5 to 15 mg) are often more sedating than higher doses (30 to 45 mg) because α2-antagonism (activating) becomes more prominent at higher doses, counteracting H1-mediated sedation. Give at bedtime.
  • Rapid onset: Some patients experience improvement in sleep and appetite within the first week, though full antidepressant effects take 2 to 4 weeks.
  • Weight gain utility: Mirtazapine is particularly useful in elderly depressed patients with poor appetite and unintentional weight loss, and in patients with comorbid cachexia or anorexia.
  • Antiemetic properties: 5-HT3 antagonism provides antiemetic effects comparable to ondansetron, useful in palliative care.
  • Augmentation strategy: Often combined with SSRIs or SNRIs in treatment-resistant depression (so-called California rocket fuel). Monitor for serotonin syndrome.
  • Minimal sexual dysfunction: Unlike SSRIs, mirtazapine does not typically cause sexual side effects due to lack of serotonin reuptake inhibition and 5-HT2/5-HT3 blockade.
  • Neutropenia monitoring: Educate patients about signs of infection and the need to seek immediate care if fever or sore throat develops.
  • Exam tip: Mirtazapine is the answer when a question stem describes a depressed patient who needs an antidepressant but has insomnia, poor appetite, or weight loss, or when SSRIs have caused intolerable sexual side effects.
Mnemonic

MIRTazapine: Makes Insomniacs Rest Tonight (sedation); also Makes Individuals Ravenously Thick (weight gain)

Practice questions

1. A 68-year-old woman with major depressive disorder is seen in clinic. She reports persistent low mood, anhedonia, early morning awakening, and a 12-pound weight loss over the past 3 months. Her past trials of sertraline and citalopram were discontinued due to intolerable nausea and diarrhea. Physical examination is unremarkable. Which of the following is the most appropriate pharmacotherapy?

Answer: C. Mirtazapine is the best choice for this patient with depression, insomnia, and significant weight loss. Its H1 antagonism increases appetite and promotes weight gain, while its sedating properties improve sleep. The 5-HT3 receptor blockade avoids the GI side effects (nausea, diarrhea) that caused her to discontinue SSRIs. Bupropion would be inappropriate given her insomnia and weight loss, as it is activating and can suppress appetite. MAOIs like tranylcypromine are reserved for treatment-resistant cases due to dietary restrictions and drug interactions.

2. A 45-year-old man treated with mirtazapine 30 mg nightly for depression presents to the emergency department with fever (39.2°C), sore throat, and malaise for 2 days. He started mirtazapine 6 weeks ago. Laboratory studies show WBC 1,800/μL (normal 4,000-11,000), absolute neutrophil count 400/μL, hemoglobin 13.5 g/dL, and platelets 210,000/μL. Which of the following is the most appropriate next step?

Answer: B. This patient has agranulocytosis, a rare but serious adverse effect of mirtazapine, presenting with fever, infection symptoms, and severe neutropenia. Immediate discontinuation of mirtazapine is mandatory. Broad-spectrum antibiotics are indicated given the neutropenia and signs of infection, as these patients are at high risk for life-threatening bacterial infections. Continuing or dose-reducing mirtazapine would be dangerous. Bone marrow biopsy is not the immediate priority; treating the infection and removing the offending agent takes precedence. Most cases of mirtazapine-induced agranulocytosis occur within the first few months of therapy.

3. A 32-year-old woman with treatment-resistant depression is taking fluoxetine 60 mg daily with partial response. Her psychiatrist considers augmentation therapy. She reports persistent insomnia and has lost interest in eating. She has no history of seizures or cardiac disease. Which augmentation strategy would provide additional therapeutic benefit while addressing her symptoms?

Answer: C. Adding mirtazapine to fluoxetine is an effective augmentation strategy for treatment-resistant depression (sometimes called California rocket fuel). Mirtazapine's sedating and appetite-stimulating properties directly address her insomnia and poor appetite. The combination enhances both noradrenergic and serotonergic neurotransmission through complementary mechanisms. While bupropion is also used for augmentation, it would worsen her insomnia. Lithium is effective for augmentation but does not address her specific symptoms. Switching to an MAOI like phenelzine requires a lengthy washout period and has significant dietary restrictions. Serotonin syndrome is a theoretical risk when combining mirtazapine with SSRIs, so monitoring is required.

Frequently asked questions

How does mirtazapine work?

Mirtazapine works by blocking α2-adrenergic receptors, which increases the release of norepinephrine and serotonin in the brain. It also blocks specific serotonin receptor subtypes (5-HT2 and 5-HT3) and histamine H1 receptors. This unique mechanism produces antidepressant effects while causing sedation and increased appetite, and avoids the sexual side effects common with SSRIs.

What are the side effects of mirtazapine?

The most common side effects are sedation, increased appetite, and weight gain. Other frequent effects include dry mouth, dizziness, and constipation. Serious but rare side effects include agranulocytosis (severe drop in white blood cells), serotonin syndrome when combined with other serotonergic drugs, and hyponatremia. Mirtazapine carries a black box warning for increased suicidal thoughts in young adults under 25.

Is mirtazapine a controlled substance?

No, mirtazapine is not a controlled substance. It is a prescription antidepressant with no significant abuse potential and is not regulated under the Controlled Substances Act. However, it does require a prescription and should only be used under medical supervision due to potential serious side effects and the need for monitoring.

Mirtazapine vs trazodone — what is the difference?

Both are sedating antidepressants often used for insomnia, but they have different mechanisms. Mirtazapine blocks α2-receptors and increases norepinephrine and serotonin release, while trazodone primarily blocks serotonin receptors and inhibits serotonin reuptake. Mirtazapine commonly causes weight gain, whereas trazodone is more associated with orthostatic hypotension and priapism. Mirtazapine may be more effective as a primary antidepressant, while trazodone is often used specifically for sleep at low doses.

Does mirtazapine help with anxiety?

Yes, mirtazapine can help with anxiety, particularly when it occurs with depression. Its blockade of 5-HT2 and 5-HT3 receptors produces anxiolytic effects, and the resulting enhancement of 5-HT1A receptor activity contributes to anxiety reduction. It is used off-label for generalized anxiety disorder and PTSD. The sedating properties can also help reduce anxiety-related insomnia, though this effect is more pronounced at lower doses.

Conditions it is used for

Major depressive disorderInsomniaGeneralized anxiety disorderPost-traumatic stress disorderNausea and vomitingCachexiaAnorexia
SertralineFluoxetineVenlafaxineBupropionTrazodoneAmitriptylineDuloxetineEscitalopram

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N06AX11

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.