(Date: July 2015. 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 the UK Health Security Agency (UKHSA) 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?

Azathioprine (Imuran®) is used to treat severe eczema that has not responded to other treatments, a number of auto-immune illnesses such as lupus, inflammatory bowel disease, rheumatoid arthritis and psoriasis, and to prevent the rejection of transplanted organs. Another form of the same medicine called mercaptopurine (Puri-Nethol®, Xaluprine®) is used to treat certain forms of leukaemia, and occasionally inflammatory bowel disease.

Is it safe to take azathioprine/mercaptopurine in pregnancy?

When deciding whether to use azathioprine/mercaptopurine during pregnancy it is important to weigh up the potential benefits to your health and wellbeing against any possible risks to you or your baby, some of which may depend on how many weeks pregnant you are. Use of azathioprine/mercaptopurine in pregnancy may sometimes be considered necessary to prevent the rejection of a transplanted organ, or to keep leukaemia or a serious autoimmune illness under control. Your doctor or specialist will be able to help you make decisions about your treatment.

What if I have already taken azathioprine/mercaptopurine during pregnancy?

If you have taken 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 to make sure that you are taking the lowest dose that works.

Can taking azathioprine/mercaptopurine in pregnancy cause my baby to be born with birth defects?

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.

The babies of at least 1,550 pregnant women taking azathioprine/mercaptopurine have been studied. There is currently no suggestion of any link between azathioprine/mercaptopurine use in pregnancy and birth defects in the baby. Ongoing research is required to confirm these findings.

Can taking azathioprine/mercaptopurine in pregnancy cause miscarriage?

Studies of a total of 330 women who took azathioprine/mercaptopurine in pregnancy do not collectively raise alarm that they have a higher chance of miscarriage compared to women not taking azathioprine/mercaptopurine. More women ideally need to be studied to confirm this finding.

Can taking azathioprine/mercaptopurine in pregnancy cause stillbirth?

Studies of a total of around 400 women who took azathioprine/mercaptopurine in pregnancy do not raise concern of a link with stillbirth. More women ideally need to be studied to confirm this finding.

Can taking azathioprine/mercaptopurine in pregnancy cause preterm birth, or my baby to be small at birth (low birth weight)?

Some of the underlying illnesses that azathioprine/mercaptopurine are used to treat may themselves increase the chance of preterm birth (before 37 weeks) and/or of a baby having a low birth weight (less than 2,500g). This complicates investigations of whether azathioprine/mercaptopurine has an effect on these outcomes.

However, studies of over 1,300 pregnant women taking azathioprine/mercaptopurine do not raise concern that these medicines directly cause preterm delivery or reduce the baby’s growth in the womb.

Can taking azathioprine/mercaptopurine in pregnancy cause other health problems in the child?

Low blood count at birth

A well-known side effect of treatment with azathioprine or mercaptopurine is that it can cause low levels of certain types of blood cells that are important to fight infection and for blood clotting. There are some case reports of babies who were exposed to azathioprine/mercaptopurine in the womb being born with low blood counts. So far, no large studies have been carried out to specifically assess the chance that azathioprine/mercaptopurine in pregnancy will affect a baby’s blood cells. If your baby shows any symptoms (such as anaemia, infection, or problems with blood clotting), his/her blood count may be checked after birth.

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.

Three small studies have assessed the learning and behaviour of children exposed to azathioprine/mercaptopurine in the womb. Two of these studies showed no link with learning and behavioural problems up to the age of six years. The third study found that children who had been exposed to azathioprine/mercaptopurine were slightly more likely to require educational support at two years of age but did not assess the children when they were older to see if any differences persisted. Larger scientific studies are needed to accurately study learning and behaviour in children exposed to azathioprine/mercaptopurine in the womb.

Will my baby need extra monitoring during pregnancy?

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 azathioprine/mercaptopurine in pregnancy is not expected to cause problems that would require extra monitoring of your baby. However, women with some of the illnesses that azathioprine/mercaptopurine is used to treat may be more closely monitored during pregnancy to ensure that they remain well throughout, and that their baby is growing and developing as expected.

Are there any risks to my baby if the father has taken azathioprine/mercaptopurine?

Because of the way that these medicines work they may theoretically damage sperm. However, the babies of at least 1,000 men who were using azathioprine/mercaptopurine around the time of conception have been studied and there is currently no suggestion of any link with birth defects, low infant birth weight, or preterm birth. There is not enough scientific data to confirm that other adverse pregnancy outcomes are no more common following use of azathioprine/mercaptopurine by the father.

As a precaution, the manufacturers of these medicines usually advise couples where the male partner takes azathioprine/mercaptopurine to use contraception and wait at least three months after stopping treatment before attempting to conceive. This is because it takes approximately three months for sperm to form. In practice, because there is no evidence of ill effects, if a couple is planning a pregnancy and the man’s medical condition is well-controlled with azathioprine/mercaptopurine, a specialist will be able help them weigh up the potential risks and benefits of continuing the medication so that informed decisions can be made about treatment.

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

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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.

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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