Mirtazapine Overview
Mirtazapine is FDA-approved for major depressive disorder, with typical dosing starting at 15 mg nightly and titrating to 30-45 mg daily, and is particularly valuable in older adults and patients with comorbid anxiety or insomnia. 1
Primary Indication
Mirtazapine is indicated for the treatment of major depressive disorder (MDD) 1. It functions as a noradrenergic and specific serotonergic antidepressant (NaSSA), blocking presynaptic α2-adrenergic receptors and postsynaptic serotonin 5-HT2 and 5-HT3 receptors 2.
Dosing Regimen
The recommended starting dose is 15 mg once daily at bedtime 3, 1. After initial tolerability assessment:
- Increase to 30 mg daily after 4 days if tolerated
- May further increase to 45 mg daily if insufficient response after 10 days at 30 mg
- Maximum dose: 45 mg daily
- Administer as a single evening dose to leverage sedative effects
For older adults, start at approximately 50% of the standard adult dose due to increased risk of adverse reactions 3. Mirtazapine is specifically recommended as a preferred agent in older patients alongside citalopram, escitalopram, sertraline, venlafaxine, and bupropion 3.
Dose adjustments are required in hepatic or renal impairment - careful titration with close monitoring is essential 3, 1.
Absolute Contraindications
Mirtazapine is absolutely contraindicated in 1:
- Concurrent use with MAOIs or within 14 days of MAOI discontinuation (including linezolid and IV methylene blue) due to serotonin syndrome risk
- Known hypersensitivity to mirtazapine or any excipients - severe skin reactions including DRESS, Stevens-Johnson syndrome, bullous dermatitis, erythema multiforme, and toxic epidermal necrolysis have been reported
Common Adverse Effects
The most frequent adverse effects (≥5% and at least twice placebo rates) are 1:
Very Common:
- Somnolence: 54% (vs 18% placebo) - most common adverse effect
- Dry mouth: 25% (vs 15% placebo)
- Increased appetite: 17% (vs 2% placebo)
- Weight gain: 12% (vs 2% placebo)
- Constipation: 13% (vs 7% placebo)
- Dizziness: 7% (vs 3% placebo)
Important clinical note: Sedation is paradoxically more pronounced at lower doses (below 15 mg) and decreases at therapeutic doses ≥15 mg 4. This is why starting at 15 mg as a single evening dose is preferred over lower initial dosing.
Serious Adverse Effects and Warnings
Critical warnings include 1:
- Agranulocytosis - rare (approximately 1 in 1,000) but potentially serious; usually reversible with discontinuation 5
- Suicidal thoughts and behaviors - particularly in young adults
- Serotonin syndrome - especially with concurrent serotonergic agents
- QT prolongation and torsades de pointes
- Angle-closure glaucoma
- DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms)
- Activation of mania/hypomania
- Seizures (though mirtazapine has very low seizure potential compared to other antidepressants) 6
- Elevated cholesterol and triglycerides
- Hyponatremia
- Transaminase elevations
Discontinuation rates: Approximately 16% of patients discontinued mirtazapine in clinical trials due to adverse reactions (vs 7% for placebo), with somnolence (10.4%) and nausea (1.5%) being the most common reasons 1.
Clinical Advantages
Mirtazapine offers specific advantages over SSRIs 3, 7:
- Minimal sexual dysfunction - useful when patients experience sexual side effects from other antidepressants 5
- Less nausea and gastrointestinal adverse effects compared to SSRIs
- Beneficial for comorbid anxiety and insomnia - may reduce need for additional anxiolytic or hypnotic medications 4, 8
- Fewer anticholinergic effects than tricyclic antidepressants 9
- Relatively safe in overdose 5
Treatment Duration
First episode of major depression: minimum 4 months of treatment 3. Patients with recurrent depression may require prolonged or indefinite treatment, as recurrence risk increases to 70% after two episodes and 90% after three episodes 3.
Special Populations
Older adults: Mirtazapine is a preferred agent due to favorable tolerability profile, though lower starting doses are essential 3.
Hepatic/renal impairment: Requires dose reduction and careful monitoring as mirtazapine undergoes extensive hepatic metabolism and renal excretion 4.
Comparative Efficacy
In treatment-resistant depression (second-step treatment), switching to mirtazapine or augmenting existing SSRI therapy with mirtazapine showed similar efficacy - no significant difference between switch versus augmentation strategies 9. Mirtazapine demonstrated comparable efficacy to tricyclic antidepressants and SSRIs in multiple trials 9, 6.