Mirtazapine is a novel tetracyclic antidepressant, in overdose it frequently has a benign course with mild CNS depression and tachycardia.
Centrally acting alpha-2-adrenergic antagonist that enhances the release of serotonin and noradrenaline. It is also a histamine and serotonin (5-HT2 and 5-HT3) antagonist (therefore causing drowsiness).
- Good oral absorption
- 85% protein bound
- Massive volume of distribution >100L/kg
- Hepatic metabolism, active metabolites which are renally excreted
- Elimination half life is 20-40 hours
- Intubation: Rarely required
- Many patients are asymptomatic.
- Symptoms should occur within 4 hours related to the receptors affected:
- Mild tachycardia
- Drowsiness (in large overdoses)
- CNS depression is more likely if >1000 mg has been ingested but still this is rare. If the patient has significantly reduced GCS another cause should be sought.
- General measures
- Screening: 12 lead ECG, BSL, Paracetamol level
- Not indicated
- Not clinically useful.
- None available.
- Patients who are asymptomatic with a normal (or baseline) 12-lead ECG and vital signs are medically cleared
- Patients with mild sedation are managed supportively on the ward until symptoms resolve.
- Berling I, Isbister GK. Mirtazapine overdose is unlikely to cause major toxicity. Clinical Toxicology 2014; 52:20-24
- Murray L et al. Toxicology Handbook 3rd Edition. Elsevier Australia 2015. ISBN 9780729542241