Mirtazapine

(Date of issue: September 2014. Version: 2)

This factsheet has been written for members of the public by the UK Teratology Information Service (UKTIS). UKTIS is a not-for-profit organisation funded by Public Health England on behalf of UK Health Departments. UKTIS has been providing scientific information to health care providers since 1983 on the effects that medicines, recreational drugs and chemicals may have on the developing baby during pregnancy.

What is it?

Mirtazapine (Zispin SolTab®) is an antidepressant that is also used to treat anxiety, and is occasionally used as an anti-sickness medicine.

Is it safe to take mirtazapine in pregnancy?

There is no yes or no answer to this question. When deciding whether or not to take mirtazapine during pregnancy it is important to weigh up how necessary mirtazapine is to your health against the possible risks to you or your baby, some of which will depend on how many weeks pregnant you are. Remaining well is particularly important during pregnancy and while caring for a baby. For some women treatment with mirtazapine in pregnancy may be necessary.

This leaflet summarises the scientific studies relating to the effects of mirtazapine on a baby in the womb. It is advisable to consider this information before taking mirtazapine if you are pregnant. Your doctor is the best person to help you decide what is right for you and your baby.

What if I have already taken mirtazapine during pregnancy?

If you have taken or are taking any medicines it is always a good idea to let your doctor know that you are pregnant so that you can decide together whether you still need the medicines that you are on and, if so, to make sure that you are taking the lowest dose that works and only for as long as you need to.

It is very important that you do not suddenly stop taking mirtazapine as this could be dangerous to you and to your unborn baby if you are already pregnant. Do not make any change to your medication without first talking to your doctor.

Can taking mirtazapine in pregnancy cause birth defects in the baby?

A baby’s body and most internal organs are formed during the first 12 weeks of pregnancy. It is mainly during this time that some medicines are known to cause birth defects.

Only one study has specifically investigated whether there may be a link between mirtazipine use in pregnancy and birth defects in the baby. This study showed that birth defects were no more likely to occur in babies of women who took mirtazapine during pregnancy than in babies born to women who did not take mirtazapine.  Another study which investigated the risk of birth defects among babies born to a mixed group of women who took either mirtazapine or a related antidepressant also showed no increase.

Together, these studies included babies of only around 250 women who took mirtazapine in pregnancy.  More research on this subject is therefore required.

Can taking mirtazapine in pregnancy cause miscarriage?

It is unclear whether mirtazapine use in pregnancy increases the chance of miscarriage. Two studies have investigated this, and while one study suggested that miscarriage was more common in pregnant women taking mirtazapine, the other did not. One of these studies was of only 100 pregnant women taking mirtazapine, and the other did not state how many women taking mirtazapine were included. More information therefore needs to be collected to accurately assess whether there is a link between mirtazipine use in pregnancy and miscarriage.

Can taking mirtazapine in pregnancy cause stillbirth?

No increased risk of stillbirth in pregnant women using mirtazapine was seen in the single study that investigated this. Another study that investigated a mixed group of pregnant women who took either mirtazapine or a related antidepressant also found no link with stillbirth. 

Around 1 in every 200 pregnancies in the general population ends in a stillbirth, and so far, stillbirth rates have been studied in only around 250 pregnant women who took mirtazapine. Information from more women is therefore needed before we can accurately say whether mirtazapine use in pregnancy increases the chance of stillbirth.

Can taking mirtazapine in pregnancy cause preterm birth?

There is currently no strong evidence from the two studies that investigated this that taking mirtazapine in pregnancy causes preterm birth (birth before 37 weeks). Although one study did show that babies exposed to mirtazapine in the womb were more likely to be preterm than those born to women who did not have depression or take any medicines, this same study showed that women on other antidepressants were just as likely to give birth early as women on mirtazapine. This suggests that other factors linked to depression in the mothers may be the reason for the preterm births, rather than the mirtazipine itself. The other study showed no link between use in pregnancy of mirtazapine or related antidepressants and preterm birth.

Because rates of preterm delivery have been studied in only around 250 pregnant women who took mirtazapine, more research into this subject is required.

Can taking mirtazapine in pregnancy cause my baby to be small at birth (low birth weight)?

There is currently no strong evidence from three studies that investigated this that taking mirtazapine during pregnancy causes low birth weight (<2500g), or leads to reduced birth weight in the baby. 

Because the risk of having a low birth weight baby has been studied in fewer than 300 pregnant women who took mirtazapine, more research into this subject is required.

Can taking mirtazapine in pregnancy cause other health problems in the child?

Withdrawal symptoms in the baby at birth (‘neonatal withdrawal’)
Withdrawal symptoms are thought to occur as the newborn baby’s body suddenly has to adapt to no longer getting certain types of medicines through the placenta.

Neonatal withdrawal after mirtazapine use in pregnancy has not been investigated in large studies. However, there are isolated reports of withdrawal in babies exposed to mirtazapine around the time of delivery, and other similar antidepressants are known to carry a risk of neonatal withdrawal. Therefore, close monitoring of your baby for a few days after birth may be advised if you have taken mirtazapine regularly in the weeks before delivery. 

Persistant Pulmonary Hypertension of the Newborn (PPHN)
PPHN or ‘persistent pulmonary hypertension of the newborn’ occurs when a newborn baby’s lungs do not adapt to breathing outside the womb. PPHN only affects around 1 or 2 out of every 1,000 newborn babies in the general population, but can be serious.

There is a single report in the scientific journals of PPHN in a baby exposed to mirtazapine in the womb. Some studies of pregnant women who take antidepressants that work in a similar way to mirtazapine have shown that PPHN is more common in their babies (affecting 1 out of every 100-200 babies).

Because there have been no large studies investigating PPHN in babies exposed to mirtazapine, it is not yet possible to say whether PPHN is any more common in babies who were exposed to mirtazepine in the womb and more research into this subject is required.

Learning and behavioural problems
A baby’s brain continues to develop right up until the end of pregnancy. It is therefore possible that taking certain medicines at any stage of pregnancy could have a lasting effect on a child’s learning or behaviour.

No link with learning and behavioural problems (e.g. autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) is known about in children who were exposed specifically to mirtazapine while in the womb, however, no studies have been carried out to specifically investigate this.

There are ‘one-off’ reports in scientific journals describing normal development in children who were exposed to mirtazapine in the womb. However, none of these children were studied after one year of age and therefore do not know whether they had any difficulties with more complicated activities such as reading and writing. The learning and development of many more children who were exposed to mirtazapine in the womb needs to be studied scientifically before we can say whether use in pregnancy has any effect on the baby’s brain.

Will my baby need extra monitoring?

Most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects as part of their routine antenatal care. Taking mirtazapine in pregnancy would not normally require extra monitoring of your baby. 

If you have taken mirtazapine around the time of delivery your baby may require extra monitoring after birth because of the possible risk of neonatal withdrawal.

Are there any risks to my baby if the father has taken mirtazapine?

We would not expect any increased risk to your baby if the father took mirtazapine before or around the time you became pregnant.

Who can I talk to if I have questions?

If you have any questions regarding the information in this leaflet please discuss them with your health care provider. They can access more detailed medical and scientific information from www.uktis.org.  

General information 

Up to 1 out of every 5 pregnancies ends in a miscarriage, and 1 in 40 babies are born with a birth defect. These are referred to as the background population risks.  They describe the chance of these events happening for any pregnancy before taking factors such as the mother’s health during pregnancy, her lifestyle, medicines she takes and the genetic make up of her and the baby’s father into account.

Medicines use in pregnancy

Most medicines used by the mother will cross the placenta and reach the baby. Sometimes this may have beneficial effects for the baby.  There are, however, some medicines that can harm a baby’s normal development.  How a medicine affects a baby may depend on the stage of pregnancy when the medicine is taken. If you are on regular medication you should discuss these effects with your doctor/health care team before becoming pregnant.

If a new medicine is suggested for you during pregnancy, please ensure the doctor or health care professional treating you is aware of your pregnancy.

When deciding whether or not to use a medicine in pregnancy you need to weigh up how the medicine might improve your and/or your unborn baby’s health against any possible problems that the drug may cause. Our bumps leaflets are written to provide you with a summary of what is known about use of a specific medicine in pregnancy so that you can decide together with your health care provider what is best for you and your baby.   

Every pregnancy is unique. The decision to start, stop, continue or change a prescribed medicine before or during pregnancy should be made in consultation with your health care provider. It is very helpful if you can record all your medication taken in pregnancy in your hand held maternity records.

   

www.medicinesinpregnancy.org

Disclaimer: This information is not intended to replace the individual care and advice of your health care provider. New information is continually becoming available. Whilst every effort will be made to ensure that this information is accurate and up to date at the time of publication, we cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes following decisions made on the basis of this information. We strongly advise that printouts should NOT be kept for any length of time, or for “future reference” as they can rapidly become out of date.

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