Mirtazapine
Mirtazapine is an atypical tetracyclic antidepressant indicated for the treatment of major depressive disorder. It has a unique mechanism of action that combines antidepressant efficacy with beneficial effects on sleep and appetite, making it particularly valuable for patients with depression accompanied by insomnia, weight loss, or anxiety.
Clinical Overview
Primary Clinical Applications
Mirtazapine is indicated for major depressive disorder and is commonly used off-label for insomnia, anxiety disorders, PTSD, and appetite stimulation. It is particularly beneficial for elderly patients and those with comorbid sleep disturbances or weight loss.
Mechanism and Clinical Benefits
Mirtazapine blocks α2-adrenergic autoreceptors and heteroreceptors, increasing norepinephrine and serotonin release. It also blocks 5-HT2A, 5-HT2C, 5-HT3, and H1 receptors, contributing to its sedating effects and appetite stimulation while reducing sexual side effects and nausea.
Unique Advantages
Unlike SSRIs, mirtazapine typically causes weight gain and sedation, which can be therapeutic for appropriate patients. It has minimal sexual side effects and low risk of drug interactions, making it suitable for elderly patients or those on multiple medications.
Key Clinical Points
Sedation is greater at lower doses
Antihistaminergic effect dominates at 7.5 to 15 mg while noradrenergic activity increases higher up. Patients troubled by sedation often do better on a higher dose, which is counterintuitive.
Weight gain and appetite stimulation are therapeutic here
What is an adverse effect elsewhere is the reason for choosing mirtazapine in depressed patients with weight loss, poor appetite, or insomnia, particularly older adults.
Minimal sexual dysfunction
5-HT2 and 5-HT3 blockade means sexual side effects and nausea are markedly less common than with SSRIs, which makes it a useful switch when those effects limit treatment.
Agranulocytosis is rare but real
Patients should be told to report fever, sore throat, or other signs of infection rather than waiting for a routine review.
Low interaction burden
Few clinically significant interactions makes it well suited to older patients on multiple medications. Contraindicated within fourteen days of an MAOI.
Prescribing Information
Dosing & Administration
Major Depressive Disorder:
- Initial: 15 mg daily at bedtime
- Increase: May increase at 1-2 week intervals
- Target range: 15-45 mg daily
- Maximum: 45 mg daily
Elderly:
- Initial: 7.5-15 mg daily at bedtime
- Titrate slowly: Based on response and tolerability
Administration:
- Preferably at bedtime due to sedating effects
- Can be taken with or without food
- SolTab formulation dissolves on tongue
Indications
- Major depressive disorder
- Off-label: Insomnia, anxiety, PTSD, appetite stimulation
Contraindications
- MAOI use within 14 days
- Known hypersensitivity to mirtazapine
- Concurrent use with linezolid or methylene blue
Warnings & Precautions
- Boxed Warning: Increased suicidal risk in patients under 25 years
- Agranulocytosis: Rare but serious blood disorder
- Serotonin syndrome: Risk with concurrent serotonergic drugs
- QT prolongation: Monitor in patients with cardiac risk factors
- Weight gain: Significant appetite stimulation and weight gain
- Sedation: Prominent, especially early in treatment
Drug Interactions
- MAOIs: Contraindicated – risk of serotonin syndrome
- CNS depressants: Enhanced sedation
- CYP1A2 inducers: May decrease mirtazapine levels
- CYP3A4 inhibitors: May increase mirtazapine levels
Adverse Reactions
Common (≥5%):
- Somnolence, increased appetite, weight gain, dizziness
Less Common:
- Dry mouth, constipation, confusion, abnormal dreams
Special Populations
- Pregnancy: Category C – use only if benefits outweigh risks
- Elderly: Lower starting dose, monitor for falls risk
- Hepatic impairment: Use with caution, consider dose reduction
- Renal impairment: Use with caution in severe impairment
Frequently Asked Questions
What is Mirtazapine used for?
Mirtazapine is an atypical tetracyclic antidepressant indicated for the treatment of major depressive disorder. It has a unique mechanism of action that combines antidepressant efficacy with beneficial effects on sleep and appetite, making it particularly valuable for patients with depression accompanied by insomnia, weight loss, or anxiety.
What class of drug is Mirtazapine?
Mirtazapine is classified as: Atypical Antidepressant (Tetracyclic).
Sedation is greater at lower doses?
Antihistaminergic effect dominates at 7.5 to 15 mg while noradrenergic activity increases higher up. Patients troubled by sedation often do better on a higher dose, which is counterintuitive.
Weight gain and appetite stimulation are therapeutic here?
What is an adverse effect elsewhere is the reason for choosing mirtazapine in depressed patients with weight loss, poor appetite, or insomnia, particularly older adults.
Minimal sexual dysfunction?
5-HT2 and 5-HT3 blockade means sexual side effects and nausea are markedly less common than with SSRIs, which makes it a useful switch when those effects limit treatment.
Agranulocytosis is rare but real?
Patients should be told to report fever, sore throat, or other signs of infection rather than waiting for a routine review.
Source: FDA prescribing information (DailyMed)