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Date of issue: February 2024, Version: 4

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A corresponding patient information leaflet on USE OF AZATHIOPRINE OR MERCAPTOPURINE IN PREGNANCY is available.

Azathioprine (AZA) is a prodrug and is rapidly metabolised to its active metabolite mercaptopurine (6-MP), a purine analogue which interferes with the synthesis of DNA and RNA and has immunosuppressant properties. AZA is licensed to treat various auto-immune disorders and inflammatory conditions, such as inflammatory bowel disease, systemic lupus erythematosus, severe ulcerative colitis, Crohn’s disease, rheumatoid arthritis and psoriasis, and for the prevention of allograft rejection. It is also used off-license to treat severe refractory eczema. 6-MP is licensed to treat acute and chronic leukaemia, as maintenance therapy in acute lymphoblastic and myelogenous leukaemia, and is used off-license to treat severe ulcerative colitis and Crohn’s disease.

The available data do not demonstrate that AZA/6-MP exposure during early pregnancy increases the risk of congenital malformation, preterm delivery, or adversely affects fetal growth. There is also no robust evidence of increased risks of intrauterine death, miscarriage, or altered neurodevelopment following gestational AZA exposure. However, as the available data for many of these outcomes are limited in quantity and sometimes also methodologically, more robust epidemiological data are ideally required.

Neonatal leucopoenia and thrombocytopenia have been reported in a number of case reports following AZA exposure in utero but no epidemiological studies quantifying these risks are available. There is therefore concern that the use of AZA/6-MP during pregnancy could result in immunosuppression in the neonate, leading to an increased risk of infection and adverse effects following administration of live vaccines. A single study has identified an association between AZA exposure and childhood infection. Advice from the Medicines and Healthcare products Regulatory Agency (MHRA) and UK Health Security Agency (UKHSA) states that infants exposed in utero to certain immunosuppressive medications should not receive live vaccines until they are at least 6 months old, although no specific advice relating to AZA exposure was issued.

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 NHS health care professionals who are logged in.

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 to ensure you are using the most up-to-date version.