Summary
Mirtazapine is a benzazepine tetracyclic antidepressant licensed for use in the treatment of depression and mood disorders. It is also used off-licence in the treatment of anxiety and as an antiemetic. Its mechanism of action is related to that of selective-serotonin reuptake inhibitors (SSRIs).
The data regarding exposure to mirtazapine in human pregnancy are limited and comprises around 700 exposures, derived from case reports, case series, cohort studies and registry studies. Use of mirtazapine in pregnancy should therefore be limited to situations where use of another antidepressant, for which there is more pregnancy safety data, is not considered clinically appropriate and where prescriber and patient feel that the benefits of use outweigh any potential risks. Women should be counselled regarding the limited available human pregnancy safety data.
Whilst these data do not suggest a significantly increased risk of congenital malformation they are too limited to exclude any increase in risk. Data regarding risk of spontaneous abortion and preterm delivery are conflicting. Risk of neonatal hypoglycaemia may be increased. Data regarding the risk of neonatal withdrawal is lacking, however there are theoretical concerns with exposure to centrally acting drugs and monitoring for neonatal effects is advised following use near delivery.
There are no epidemiological studies which investigate risk of PPHN following mirtazapine use in pregnancy. However, as mirtazapine increases serotonin concentration and PPHN has been associated with in utero exposure to SSRIs, there are theoretical concerns that in utero mirtazapine exposure may also increase the risk of PPHN in the neonate.
The effects of mirtazapine exposure on neurodevelopment have not been studied. Preliminary findings suggest that children of women with untreated depression, as well as children exposed to antidepressants in utero, have lower IQs and exhibit more behavioural problems than those of non-exposed children of non-depressed women.
Infants exposed to antidepressants in utero should ideally be delivered in a unit with adequate neonatal support and the delivery team made aware of the exposure. In the non-pregnant adult patient mirtazapine has a half life of 20 to 40 hours which is extended in cases of liver or renal impairment. Neonatal clearance of mirtazapine following in utero exposure may be longer, and neonatal postnatal effects may therefore be delayed.
Exposure to mirtazapine at any stage in pregnancy would not usually be regarded as medical grounds for termination of pregnancy or any additional fetal monitoring. However, other risk factors may be present in individual cases which may independently increase the risk of adverse pregnancy outcome. Clinicians are reminded of the importance of consideration of such factors when performing case-specific risk assessments.
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