Prescription Drug Information: Mirtazapine (Page 5 of 8)

8.2 Lactation

Risk Summary

Data from published literature report the presence of mirtazapine in human milk at low levels with relative infant doses for mirtazapine ranging between 0.6 and 2.8% of the maternal weight-adjusted dose (see Data). No adverse effects on the breastfed infant have been reported in most cases of maternal use of mirtazapine. There are no data on the effects of mirtazapine on milk production.

The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for mirtazapine and any potential adverse effects on the breastfed infant from mirtazapine or from the underlying maternal condition.


In a published pooled analysis of 8 breastfeeding mother-infant pairs, the mean (min, max) total relative infant doses for mirtazapine and its desmethyl metabolite were 1.5% (0.6%, 2.8%) and 0.4% (0.1%, 0.7%) of the maternal weight-adjusted dose (median (min, max) dose of 38 mg (30 mg, 120 mg), respectively). No adverse drug effects were reported for any of the infants.

8.4 Pediatric Use

The safety and effectiveness of mirtazapine tablets have not been established in pediatric patients with MDD. Two placebo-controlled trials in 258 pediatric patients with MDD have been conducted with mirtazapine, and the data were insufficient to establish the safety and effectiveness of mirtazapine tablets in pediatric patients with MDD.

Antidepressants increased the risk of suicidal thoughts and behaviors in pediatric patients [see Boxed Warning and Warnings and Precautions (5.1)].

In an 8-week-long clinical trial in pediatric patients receiving doses between 15 to 45 mg per day, 49% of mirtazapine-treated patients had a weight gain of at least 7%, compared to 5.7% of placebo-treated patients. The mean increase in weight was 4 kg (2 kg SD) for mirtazapine-treated patients versus 1 kg (2 kg SD) for placebo-treated patients [see Warnings and Precautions (5.7)].

8.5 Geriatric Use

Approximately 190 patients ≥ 65 years of age participated in clinical studies with mirtazapine. Mirtazapine tablets are known to be substantially excreted by the kidney (75%), and the risk of decreased clearance of this drug is greater in patients with impaired renal function. Pharmacokinetic studies revealed a decreased clearance of mirtazapine in the elderly [see Clinical Pharmacology (12.3)].

Sedating drugs, including mirtazapine tablets, may cause confusion and over-sedation in the elderly. Elderly patients may be at greater risk of developing hyponatremia. Caution is indicated when administering mirtazapine tablets to elderly patients [see Warnings and Precautions (5.12), (5.15) and Clinical Pharmacology (12.3)]. In general, dose selection for an elderly patient should be conservative, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

8.6 Renal or Hepatic Impairment

The clearance of mirtazapine is reduced in patients with moderate to severe renal or hepatic impairment. Consequently, plasma mirtazapine levels may be increased in these patient groups, compared to levels observed in patients without renal or hepatic impairment. Dosage decrease may be necessary when administering mirtazapine tablets to patients with moderate to severe renal or hepatic impairment [see Warnings and Precautions (5.13), Use in Specific Populations (8.5), and Clinical Pharmacology (12.3)].


Human Experience: In premarketing clinical studies, there were reports of mirtazapine overdose alone or in combination with other pharmacological agents. Signs and symptoms reported in association with overdose included disorientation, drowsiness, impaired memory, and tachycardia.

Based on postmarketing reports, serious outcomes (including fatalities) may occur at dosages higher than the recommended doses, especially with mixed overdoses. In these cases, QT prolongation and Torsades de Pointes have also been reported [see Warnings and Precautions (5.5), Adverse Reactions (6.2), and Drug Interactions (7)].

Overdose Management: No specific antidotes for mirtazapine are known.

Contact Poison Control (1-800-222-1222) for the latest recommendations.


Mirtazapine tablets, USP contain mirtazapine. Mirtazapine has a tetracyclic chemical structure and belongs to the piperazino-azepine group of compounds. It is designated 1,2,3,4,10,14b-Hexahydro-2-methylpyrazino[2,1-a]pyrido[2,3-c]benzazepine and has the molecular formula of C17 H19 N3 . Its molecular weight is 265.36. The structural formula is the following and it is the racemic mixture:

Mirtazapine Structural Formula

Mirtazapine, USP is a white to creamy white crystalline powder which is slightly soluble in water.

Mirtazapine tablets are available for oral administration as scored film-coated tablets containing 15 or 30 mg of mirtazapine, and unscored film-coated tablets containing 45 mg of mirtazapine. Each tablet contains the following inactive ingredients: anhydrous lactose, colloidal silicon dioxide, croscarmellose sodium, FD&C Blue No. 2 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake, hypromellose, magnesium stearate, microcrystalline cellulose, polydextrose, polyethylene glycol, pregelatinized starch (corn), sodium lauryl sulfate, titanium dioxide and triacetin.


12.1 Mechanism of Action

The mechanism of action of mirtazapine for the treatment of major depressive disorder, is unclear. However, its efficacy could be mediated through its activity as an antagonist at central presynaptic α2 -adrenergic inhibitory autoreceptors and heteroreceptors and enhancing central noradrenergic and serotonergic activity.

12.2 Pharmacodynamics

In preclinical studies, mirtazapine acts as an antagonist at α2 -adrenergic inhibitory autoreceptors and heteroreceptors and as an antagonist at serotonin 5-HT2 and 5-HT3 receptors. Mirtazapine has no significant affinity for the 5-HT1A and 5-HT1B receptors.

Mirtazapine also acts as an antagonist of histamine (H1 ) receptors, peripheral α1-adrenergic receptors, and muscarinic receptors. Actions at these receptors may explain some of the other clinical effects of mirtazapine (e.g., its prominent somnolent effects and orthostatic hypotension may be explained by its inhibition of histamine (H1 ) receptors and peripheral α1-adrenergic receptors, respectively).

Cardiac Electrophysiology

The effect of mirtazapine on QTc interval was assessed in healthy subjects. At a dose of 75 mg (1.67 times the maximum recommended dosage), mirtazapine does not prolong the QTc interval to a clinically meaningful extent.

12.3 Pharmacokinetics

Plasma levels of mirtazapine are linearly related to dose over a dose range of 15 to 80 mg (1.78 times the maximum recommended dose). Steady state plasma levels of mirtazapine are attained within 5 days, with about 50% accumulation (accumulation ratio = 1.5). The (–) enantiomer has an elimination half-life that is approximately twice as long as the (+) enantiomer and therefore achieves plasma levels that are about 3 times as high as that of the (+) enantiomer.


Mirtazapine has an absolute bioavailability of about 50% following oral administration. Peak plasma concentrations of mirtazapine are reached within about 2 hours post dose.

Food Effect

The presence of food in the stomach has a minimal effect on both the rate and extent of absorption.


Mirtazapine is approximately 85% bound to plasma proteins over a concentration range of 0.01 to 10 mcg/mL.


Mirtazapine has a half-life of about 20 to 40 hours following oral administration of mirtazapine tablets.


Mirtazapine is extensively metabolized after oral administration. Major pathways of bio-transformation are demethylation and hydroxylation followed by glucuronide conjugation. In vitro data from human liver microsomes indicate that CYP2D6 and CYP1A2 are involved in the formation of the 8-hydroxy metabolite of mirtazapine, whereas CYP3A is considered to be responsible for the formation of the N-desmethyl and N-oxide metabolite. Several unconjugated metabolites possess pharmacological activity but are present in the plasma at very low levels.


Mirtazapine and its metabolites are eliminated predominantly (75%) via urine with 15% in feces.

Specific Populations

Geriatric Patients

Following oral administration of mirtazapine tablets 20 mg/day for 7 days to subjects of varying ages (range 25 to 74 years old), oral clearance of mirtazapine was reduced in the elderly compared to the younger subjects. The clearance in elderly males was 40% lower compared to younger males, while the clearance was 10% lower in elderly females compared to younger females [see Warnings and Precautions (5.15), Use in Specific Populations (8.5)].

Male and Female Patients

The mean elimination half-life of mirtazapine after oral administration ranges from approximately 20 to 40 hours across age and gender subgroups, with females of all ages exhibiting significantly longer elimination half-lives than males (mean half-life of 37 hours for females vs. 26 hours for males).


There have been no clinical studies to evaluate the effect of race on the pharmacokinetics of mirtazapine.

Patients with Renal Impairment

When compared to subjects with normal renal function, total body clearance of mirtazapine was reduced approximately 30% in renal impaired patients with GFR = 11–39 mL/min/1.73 m2 and approximately 50% in renal impaired patients with GFR = < 10 mL/min/1.73 m2) [see Warnings and Precautions (5.15), Use in Specific Populations (8.6)]. provides trustworthy package insert and label information about marketed prescription drugs as submitted by manufacturers to the U.S. Food and Drug Administration. Package information is not reviewed or updated separately by Every individual prescription drug label and package insert entry contains a unique identifier which can be used to secure further details directly from the U.S. National Institutes of Health and/or the FDA.

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