(Date: November 2017. Version: 3)

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?

Metronidazole (Flagyl®) is an antibiotic used to treat a number of infections, including skin infections, dental infections, and bacterial vaginosis. It is also used to eradicate a type of stomach bacteria called Helicobacter pylori, which can cause stomach ulcers.

Is it safe to use metronidazole in pregnancy?

The timely and correct treatment of potentially severe bacterial infections during pregnancy may be crucial to the health of both mother and baby.

When deciding whether or not to take metronidazole during pregnancy it is important to weigh up how necessary this 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. Your doctor is the best person to help you decide what is right for you and your baby.

Can use of metronidazole 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 results of eight studies do not show overall that babies born to women who used metronidazole in early pregnancy have a higher chance of birth defects than babies born to women not using metronidazole.

Can use of metronidazole in pregnancy cause miscarriage?

Two studies of women taking metronidazole in early pregnancy have produced mixed findings. The first small study showed no increased chance of miscarriage following metronidazole use. The second suggested a possible increased chance, although this may have been linked to the types of infection that metronidazole is used to treat increasing the chance of miscarriage, rather than to a specific action of the drug itself. Further research is therefore required to determine whether use of metronidazole in early pregnancy increases the chance of miscarriage.

Can use of metronidazole in pregnancy cause stillbirth?

Neither of the two studies carried out so far have shown that taking metronidazole in pregnancy increases the chance of stillbirth. Further research is required to confirm this finding.

Can use of metronidazole in pregnancy cause preterm birth or my baby to be small at birth (low birth weight)?

Four studies of birth weight and three studies of premature delivery do not show overall that metronidazole use in pregnancy has adverse effects on these outcomes.

Can taking metronidazole 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 studies have investigated learning and development in children exposed in the womb to metronidazole.

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.

Metronidazole use in pregnancy is not expected to cause any problems that would require extra monitoring of your baby.

Are there any risks to my baby if the father has used metronidazole?

We would not expect any increased risk to your baby if the father used metronidazole 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  

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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