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(Date of issue: May 2016, Version: 3)

This is a UKTIS monograph for use by health care professionals. For case-specific advice please contact UKTIS on 0344 892 0909. To report an exposure please download and complete a pregnancy reporting form. Please encourage all women to complete an online reporting form.


Thiomersal is an organic mercury-based salt which has been available since the early 1930s and is used in trace amounts as an antimicrobial and preservative in biological vaccines. It is important to note that thiomersal is metabolised to ethyl mercury and thiosalicylate and not to methylmercury (a potent neurotoxin). There are few data on the effects of thiomersal or ethylmercury and thiosalicylate in human pregnancy.

Data concerning the risk of congenital malformation following in utero exposure to thiomersal are limited to a single study investigating the use of thiomersal as a topical antimicrobial, not as a vaccine preservative; therefore data are considered insufficient to establish any risk thiomersal may pose to a developing embryo. There is currently no convincing evidence that in utero exposure to thiomersal-containing vaccines is associated with neurodevelopmental problems in the offspring, however the available data are highly limited and it is therefore not possible to state that no increased risk exists.

As with all chemicals, unnecessary exposure to thiomersal in pregnancy should be avoided, however where the maternal and fetal risks posed by failure to vaccinate are considered to outweigh any theoretical risks associated with prenatal exposure to vaccines containing thiomersal, vaccination with thiomersal-containing vaccines should not be withheld during pregnancy if non-thiomersal-containing vaccines are unavailable. The decision to vaccinate should be made in conjunction with the patient.

Exposure to thiomersal-containing vaccines 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.

This document is regularly reviewed and updated. Only use full UKTIS monographs downloaded directly from TOXBASE.org to be sure you are using the most up-to-date version. The summaries of these monographs are openly available on UKTIS.org.

This is a summary of the full UKTIS monograph for health care professionals and should not be used in isolation. The full UKTIS monograph and access to any hyperlinked related documents is available to health care professionals at www.toxbase.org.

If you have a patient with exposure to a drug or chemical and require assistance in making a patient-specific risk assessment, please telephone UKTIS on 0344 892 0909 to discuss the case with a teratology specialist.

If you would like to report a pregnancy to UKTIS please click here to download our pregnancy reporting form. Please encourage all women to complete an online reporting form.

Disclaimer: Every effort has been made to ensure that this monograph was accurate and up-to-date at the time of writing, however it cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes of the measures recommended. The final decision regarding which treatment is used for an individual patient remains the clinical responsibility of the prescriber. This material may be freely reproduced for education and not for profit purposes within the UK National Health Service, however no linking to this website or reproduction by or for commercial organisations is permitted without the express written permission of this service. This document is regularly reviewed and updated. Only use UKTIS monographs downloaded directly from TOXBASE.org or UKTIS.org to ensure you are using the most up-to-date version.