(Date: September 2014. Version: 1)

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.

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What is it?

Allopurinol (Zyloric®) is a medicine that is prescribed to treat or prevent gout and kidney stones. Because these can also result as a side-effect of chemotherapy, allopurinol is sometimes prescribed alongside chemotherapy medicines. Allopurinol is also used to treat inflammatory bowel disease.

Is it safe to take allopurinol in pregnancy?

There is no yes or no answer to this question. When deciding whether to take allopurinol during pregnancy it is important to weigh up how necessary allopurinol is to your health against any possible risks to you or your baby, some of which may depend on how many weeks pregnant you are. Your doctor is the best person to help you decide what is right for you and your baby.

This leaflet summarises the scientific studies relating to the effects of allopurinol on a baby in the womb.

What if I have already taken allopurinol 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.

We are very keen to collect information from all women who have taken allopurinol in pregnancy. Please ask your doctor to report your pregnancy to UKTIS (using our health care professional reporting form). You will soon be able to register your pregnancy directly by creating a ‘bumps pregnancy record’.

Can taking allopurinol 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.

Only 43 babies born to women who took allopurinol during the first twelve weeks of pregnancy have been written about in scientific journals with respect to whether they had birth defects.  

Forty one of these babies did not have birth defects. However, two of the babies had a similar pattern of birth defects (including cleft lip and palate, and abnormalities of the ear, eye, kidney, brain and genitals). This combination of birth defects can occur as a result of a gene spelling error (mutation). Mutations can run in families but often occur ‘out of the blue’. The two babies mentioned above were not born in the UK and did not have the level of genetic testing that would be done nowadays. We therefore do not know whether the birth defects in these babies were caused by a faulty gene.

However, these two reports also raise the possibility that allopurinol causes birth defects but before we can be sure, many more pregnant women taking allopurinol need to be studied.

If you have been prescribed allopurinol in early pregnancy it is best to discuss any concerns that you have with your doctor, who will be able to help you weigh up these possible risks against the risks to you and your baby of not receiving treatment with allopurinol.

Can taking allopurinol in pregnancy cause miscarriage?

No increased risk of miscarriage was seen in a group of 31 pregnant women taking allopurinol. However, because miscarriage rates have been studied in so few women taking allopurinol, much more research is required to confirm this finding.

Can taking allopurinol in pregnancy cause stillbirth?

Only a handful of women taking allopurinol during pregnancy have been studied with respect to stillbirth rates. The available information does not suggest that taking allopurinol during pregnancy causes stillbirth, but much more research is needed to confirm this.

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

Rates of preterm birth (before 37 weeks) and low birth weight (<2500g) have been studied in a very small number of babies exposed to allopurinol in the womb.

The preterm birth rate was higher than expected in 48 pregnant women taking allopurinol, but this may have been caused by the underlying illnesses in the mothers rather than the allopurinol itself.

The birth weights of 43 babies exposed to allopurinol in pregnancy have been reported. The birth weights of 41 of these babies were normal for the stage of pregnancy at which the baby was born, with only one baby being smaller than expected, and one larger. However, most of the pregnant women studied had stopped taking allopurinol by week 20, so we do not know whether use at the end of pregnancy might affect a baby’s birth weight.

More research is therefore required before we can say whether taking allopurinol in pregnancy causes preterm birth or low birth weight.

Can taking allopurinol in pregnancy cause learning or behavioural problems in the child?

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 to allopurinol while in the womb, however no studies have been carried out to specifically investigate this.

Will my baby need extra monitoring during pregnancy?

As part of their routine antenatal care most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects and to check the baby’s growth.

Because there is a possibility that allopurinol, when taken in the first trimester, increases the chance of birth defects in some babies, your doctor may offer you the option of having more detailed anomaly scans to look for birth defects from as early as around 12 weeks of pregnancy. It is, however, harder to see birth defects at this stage of pregnancy, which is why the main scan for birth defects is generally offered at around 20 weeks of pregnancy.

Additionally, some of the illnesses that allopurinol is used to treat are themselves reasons for a woman to have her pregnancy monitored more closely.

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

We would not expect any increased risk to your baby if the father took allopurinol 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  

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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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