What is Mirtazapine
Mirtazapine is an atypical tetracyclic antidepressant that works by blocking presynaptic α2-adrenergic receptors to enhance both noradrenergic and specific serotonergic neurotransmission, while also blocking 5-HT2, 5-HT3, and histamine H1 receptors. 1, 2
Mechanism of Action
Mirtazapine has a unique pharmacological profile that distinguishes it from other antidepressants:
- Blocks α2-adrenergic autoreceptors and heteroreceptors, increasing norepinephrine and serotonin release 2, 3
- Blocks postsynaptic 5-HT2 and 5-HT3 receptors while leaving 5-HT1A receptors unaffected, resulting in selective enhancement of 5-HT1-mediated transmission 3, 4
- Potent histamine H1 receptor antagonism, which contributes to sedating effects 2
- Does NOT inhibit serotonin or norepinephrine reuptake, unlike SSRIs or SNRIs 2, 4
This mechanism is described as "noradrenergic and specific serotonergic antidepressant" (NaSSA) activity 3, 4.
FDA-Approved Indication
Mirtazapine is FDA-approved for the treatment of major depressive disorder (MDD) in adults. 1
Clinical Applications Beyond Depression
While only FDA-approved for MDD, mirtazapine is used off-label for multiple conditions based on its unique pharmacological properties:
Insomnia
- Recommended as a sedating antidepressant option for insomnia when cognitive behavioral therapy is insufficient 5
- Listed as a third-line option after benzodiazepine receptor agonists and ramelteon in chronic insomnia guidelines 5
- Recent evidence shows mirtazapine significantly reduces insomnia severity in older adults (mean ISI score reduction of -6.5 vs -2.9 for placebo) 6
Cardiovascular Disease Populations
- Shown to be safe in patients with cardiovascular disease, though efficacy for treating depression in this population has not been formally assessed 5
- Offers additional benefits including appetite stimulation and sleep improvement 5
Gastrointestinal Disorders
- Used in irritable bowel syndrome with abdominal pain, showing significant improvements in pain-free days in recent trials 5
- Starting dose 15 mg once daily, titrated to maximum 45 mg daily according to response 5
Appetite Stimulation
- Mirtazapine causes weight gain as a side effect, which can be beneficial in specific populations 5
- In dementia patients with weight loss and comorbid depression, mirtazapine may play a beneficial role (mean weight gain 1.9 kg at 3 months, 2.1 kg at 6 months) 5
- NOT recommended for appetite stimulation in dementia patients without depression 5
Fibromyalgia and Chronic Pain
- Insufficient evidence to recommend for or against mirtazapine for pain and functional status in fibromyalgia 5
Pharmacokinetics
- Bioavailability approximately 50% due to first-pass metabolism 2, 7
- Peak plasma concentrations reached within 2-3 hours 2, 7
- Elimination half-life 20-40 hours, enabling once-daily dosing 2, 4, 7
- 85% protein binding 7
- Metabolized primarily by CYP2D6 and CYP3A4 7
- Pharmacokinetics are enantioselective, with R-(-)-enantiomer having longer half-life (18 hours) than S-(+)-enantiomer (9.9 hours) 7
Dosing
Recommended starting dose is 15 mg once daily at bedtime, with effective range of 15-45 mg/day 5, 4
- Initial dosing: 15 mg/day for 4 days, then 30 mg/day for 10 days 2
- May increase to 45 mg/day if insufficient response 2
- Dose adjustment required in hepatic or renal impairment (30% decrease in clearance with moderate impairment, 50% with severe renal impairment) 7
- Lower doses recommended in elderly patients 5
Adverse Effects Profile
Mirtazapine has a distinct side effect profile compared to SSRIs and tricyclic antidepressants:
Common Adverse Effects
- Somnolence/sedation (19-23%) - significantly more than placebo 5, 8
- Weight gain (10%) and increased appetite (11%) - significantly more than placebo 5, 8
- Dry mouth (25%) 5, 8
- Dizziness 8
Advantages Over Other Antidepressants
- Minimal anticholinergic effects compared to tricyclic antidepressants 5, 2
- Minimal cardiovascular effects - no significant changes in blood pressure or heart rate 5, 2
- Essentially lacks serotonergic side effects such as gastrointestinal symptoms and sexual dysfunction 4
- Decreased risk of headaches compared to placebo 8
Serious Adverse Events
- Rare cases of agranulocytosis and neutropenia have been reported 2
- Very low potential for inducing seizures 5, 4
- Safe in overdose - cases up to 975 mg caused sedation but no cardiovascular, respiratory effects, or seizures 4
Discontinuation Rates
- Adverse events leading to discontinuation occurred in 6 participants on mirtazapine vs 1 on placebo in recent trials 6
- Insufficient evidence on long-term effects of suicides or serious adverse events 8
Efficacy in Depression
- Onset of clinical effect in 2-4 weeks, though sleep and anxiety symptoms may improve within the first week 4
- Faster onset of action than fluoxetine, paroxetine, or sertraline 5
- Equivalent efficacy to tricyclic antidepressants (amitriptyline, clomipramine, doxepin) in short-term trials 5, 2
- 38% of patients do not achieve treatment response and 54% do not achieve remission during 6-12 weeks of treatment 5
Drug Interactions
- Minimal inhibitory effects on CYP isoenzymes, so pharmacokinetics of coadministered drugs are minimally affected 7
- CYP2D6 inhibitors (paroxetine, fluoxetine) cause modest increases in mirtazapine levels (17% and 32% respectively) without clinically relevant consequences 7
- Carbamazepine causes 60% decrease in mirtazapine plasma concentrations through enzyme induction 7
- Additive effects on psychomotor performance with CNS depressants and alcohol 5
Special Populations
Elderly Patients
- Preferred agent for older patients with depression alongside citalopram, escitalopram, sertraline, venlafaxine, and bupropion 5
- Higher plasma concentrations in elderly compared to young adults 7
- Caution advised with downward dosage adjustment 5
Gender Differences
- Females show higher plasma concentrations than males 7
Pregnancy and Nursing
- Not recommended during pregnancy or nursing 5